Abstract
In Chap. 2 we briefly explored Wratten filters for visual observation. In this chapter we take each of the planets in turn and see how the observer can use these filters to enhance each body to the maximum.
Keywords
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In Chap. 2 we briefly explored Wratten filters for visual observation. In this chapter we take each of the planets in turn and see how the observer can use these filters to enhance each body to the maximum.
Obviously the amount of detail that can be seen on a planet depends on atmospheric seeing and the size of the telescope in use. For observers using small telescopes in the 50–80 mm region, the best planets to observe will be either the closest or brightest. This covers Mercury, the gas giants Jupiter and Saturn and the bright planet Venus. Quite a lot of detail can be seen under steady seeing of both of the gas giants no matter what the aperture, though increased magnification and loss of contrast can result with a small telescope and high magnification. Just experiment until you can see the maximum detail with the best magnification and filters. With Venus, several filters may be used depending on the phase of the planet and the size and magnification of the telescope.
Notice that Mars is not included in the above list. Mars as a planet is not the easiest on which to glimpse detail, and the need for filters is at its greatest here to separate the subtle features of its surface. It is also a very small planet at a great distance and so demands on a larger aperture to ensure that the light transmission lost by using a filter is no hindrance to the observer. Apertures of 200 mm and above are recommended to get high magnification filters views of the Martian surface, though of course the observer can try and see general features with any aperture.
With regard to planetary observations, it is easier to either “blink” the filter across the eyepiece or place the eyepiece carefully on to the eyepiece cup so that it can be quickly removed and substituted for another filter without having the bother of screwing the filter to the eyepiece and then laboriously removing it every time. Observers who wear eyeglasses may find that this enables a little longer eye relief, but it depends on the quality of the eyepiece.
The following sections will deal with general views of the planets through various filters and provide a table at the end of each section that sums up the filters and features for each planet.
Mercury: The Innermost Planet
Mercury is the most neglected of the planets. Sometimes brighter than Sirius, the brightest naked-eye star, Mercury is regarded by most amateur astronomers as being too close to the Sun to be an easy target, but it can be seen and appreciated nevertheless with some planning and preparation.
Mercury was once thought of as the smallest planet, the hottest planet—and also the coldest planet, due to the mistaken belief, based on the work of astronomers such as G. Schiaparelli and E.M. Antoniadi, that it always kept one face pointed away from the Sun. Several science fiction stories were set around this weird property, notably Hal Clement’s excellent “Hot Planet.” We now know that Triton, the large satellite of Neptune, at −235 °C, is the coldest body ever imaged in the Solar System, and Venus, with its extreme greenhouse effect, is the hottest. A distinction that Mercury will never lose, however, is that of the innermost and the fastest planet, orbiting the Sun in 88 days.
This is partly what makes it so elusive to observers. It is an “inferior planet” (closer to the Sun than Earth is), like Venus, and goes through the same order of major events: superior conjunction on the far side of the Sun, greatest eastern elongation in the evening sky, inferior conjunction on the near side of the Sun, greatest western elongation in the morning sky and then back to superior conjunction. But, whereas Venus can travel out to 47° from the Sun and set 4 h after (or rise 4 h before) the Sun at its best apparitions, Mercury can manage only 28°, usually setting a few minutes after the end of evening twilight (or rise a few minutes before the start of morning twilight). Mercury can never really be seen properly in a fully dark sky. This signals the planet as one that definitely requires the use of filters to increase its contrast against the twilight sky.
Mercury has a much lower surface brightness than Venus, and it always appears closer to the Sun and thus in a brighter area of sky. The observer will have to observe on very clear, haze-free days, when the Sun has no more than a small aureole of scattered light around it. The greater proximity to the Sun also means that you will have to be even more careful about not getting the blinding solar disk in your field of view, especially if you try scanning with setting circles. Actually, you are quite likely to find Mercury in daytime by using a GOTO mount accurately set.
Another observational problem is that the apparitions of Mercury are not all equally good for observers at a given latitude. Greatest elongations of Mercury vary between 17° and 28°. For an observer at mid-northern latitudes on Earth, the minimum greatest elongation of Mercury could be the year’s best. What matters is the time of year because it determines the angle of the ecliptic as it intersects the horizon. At mid-northern latitudes, the angle of the ecliptic is steep around spring equinox in the dusk sky and around the autumn equinox in the dawn sky. So the best evening apparitions of Mercury thus tend to be those near the spring equinox; the best morning apparitions are those near the autumn equinox. At such times, a planet’s elongation can be almost vertical at sunset; its angular altitude is greater and thus displays the planet to advantage.
A good apparition features Mercury as easily visible and setting over 1.5 h after the Sun (or rising this long before the Sun) around the time of greatest elongation. This means that Mercury can be seen about 2 weeks before and 1 week after a greatest evening elongation at a good apparition—so swift is this planet in its orbit around the Sun.
Filters for Observing Mercury
Actually, the surface brightness of Mercury is great enough that observers do not want to see it through the telescope against a fully dark sky. However, if you wait until after sunset to find Mercury in the twilight sky, the planet is so low that “seeing” is usually bad. One possible solution to this problem is to observe Mercury after sunset often enough to catch the few occasions when “seeing” is reasonably good even down low in the sky. Another is to observe Mercury high in broad daylight—which is achievable using a good telescope, one equipped with either good setting circles or a GOTO mount, but Mercury’s features tend to get blanked out due to the excessive sunlight. To overcome this it is possible to use a Wratten No. 25 red filter, which then darkens the background sky and banishes the brightness of Mercury, enabling some surface features to be glimpsed under high powers.
Under such a filter, Mercury seems to take on more of the recorded pinkish tinge it generally displays. Figure 4.1 displays Mercury’s general appearance as seen through a Wratten No. 80A blue filter. However, check the color of Mercury yourself and see what you think. If Mercury is higher in the sky and seen against a darker backdrop, then try observing it with a Wratten No. 12 yellow filter. This could separate some of the surface features by contrast and enable some darker “mare”-type areas to become visible in instances of good seeing. However, beware! A lot of this surface detail is in the eye of the beholder. Maps drawn by eminent astronomers such as E.M. Antoniadi at the beginning of the twentieth century turned out to have no recognizable connection to the maps made by Mariner 10 in 1973 and certainly no connection to the surface as imaged by NASA’s MESSENGER spacecraft. Most of these surface features could be just areas of different contrast connected by hand–eye and brain coordination to display something else.
Mercury can be seen for more nights and more apparitions than the average amateur astronomer thinks. Trying to see Mercury at a fairly poor apparition, besides being a chance to test and sharpen your skills, might actually pay off in showing you features of a face of Mercury very rarely seen, especially if filters are used. Using a combination of Wratten No. 15 deep yellow or deep red Wratten No. 25 or even a light blue No. 82A on the bright twinkling star-like object at low elevation may make some features swim into view. It is imperative that the filters also be coupled to a high magnification eyepiece on a fairly large aperture telescope to gain the most from the clashing dark filter/twilight sky.
With these filters, the observer should immediately notice that Mercury does show phases. You will find observing Mercury when it is still almost fully lit far more difficult than doing so with Venus, for example, because Mercury then is still at such a small elongation. Observers who wish to see an almost “full” Mercury will be disappointed to find that under such illumination not only is the planet just above or below the Sun (depending on node), but that it is also on the other side of the Sun and is a much smaller object in angular diameter. If this type of observing is attempted then a Wratten No. 25 or even the deep violet Wratten No. 37 may show the planet, though any surface detail may well be lost.
Shortly after superior conjunction in the evening (or not long before superior conjunction in the morning), Mercury appears brightest and is therefore best set for observing with filters. However, Mercury’s increase in apparent diameter as it approaches us on our respective orbits cannot make up for its dwindling phase for long. Mercury’s phase shrinks rapidly in the week or two before inferior conjunction, and its magnitude dims into insignificance.
One time when filters of a different kind may be used is during transits of Mercury across the solar disc. White light filters or a Hα filter may be used to advantage at such times, though it must be pointed out that transits are relatively rare. These usually happen in November or May, and the following table gives the time in UTC of the next transits, but check on local times and visibility online.
Date | Start time | Mid transit | End time | Duration |
---|---|---|---|---|
May 9, 2016 | 11:12 | 14:57 | 18:42 | 7 h 30 min |
November 11, 2019 | 12:35 | 15:20 | 18:04 | 5 h 29 min |
November 13, 2032 | 06:41 | 08:54 | 11:07 | 4 h 26 min |
November 7, 2039 | 07:17 | 08:46 | 10:15 | 2 h 58 min |
Under such conditions the materials covered in the chapter dealing with solar filters will be useful. For those equipped with small telescopes, a simple Baader Astro filter paper will suffice to see the tiny dot cross the surface of the Sun, as can be seen from using such a filter during the May 2003 transit (Fig. 4.2).
So to sum up, the filters that are most useful to observe the planet Mercury with are shown in the following table.
Wratten No | Color | Features |
---|---|---|
No. 15 | Deep yellow | Contrast features and separation of sky |
No. 25 | Deep red | Contrast features and darkening of sky |
No. 80A | Blue | Contrast features and darkening of sky |
Can the same filters be applied to the planet Venus, as it, too, is an inferior planet and displays many of the uncertain contrast features of Mercury?
Venus and Wratten Filters
Venus is the brightest planet visible to the naked eye and approaches Earth closer than any other planet. Through a telescope, Venus can also subtend a large visual angle and appear even larger than Jupiter; although at only 9 arc seconds at superior conjunction, it cannot be seen due to its proximity to the Sun. While orbiting in its closest approach to Earth, however, its slim crescent can grow to 68 arc seconds. Yet despite this, Venus is often a difficult planet to observe telescopically and usually shows less detail than Jupiter, Saturn or Mars. In fact, it is so difficult to discern detail on the surface that most astronomers, amateur or professional, do not bother to give it more than a cursory glance. This is partly due to its great brilliance; the planet usually shines between magnitude −3.6 and −4.4, making it the third brightest object in the sky after the Sun and Moon in apparent magnitude. To separate any of the dusky features on its bright disc, filters are a must.
Most of the difficulties inherent in getting a good telescopic view of Venus involves its proximity to the Sun. Venus is never more than 47° from the Sun in the sky. Also, for a large fraction of the time, it lingers in the solar glare and makes observation difficult. But the greatest problem for telescopic observers is viewing Venus when it is high enough to avoid the poor “seeing” we encounter when we look through a large proportion of Earth’s atmosphere. To overcome this, Venus can be observed high in the sky in the middle of the day, and many observers feel that this is the best time to look at it. This is possibly due to the well-known visual effect that the “surface” brightness of Venus is very great, but so is that of the bright sky in the same field of view; thus the brightness tends to cancel out, giving the planet a lower contrast and thus rendering faint detail better. But finding even an object as brilliant as Venus in broad daylight is usually very difficult for those who do not know how to go about it and requires some work even for the veteran observer. Such an exercise is rendered easy with a telescope fitted with effective setting circles or a GOTO mount.
There is an additional problem with seeing detail on Venus. The planet is eternally shrouded in cloud, revealing very few features. This cloud, which is mostly made of carbon dioxide, sulfuric acid and water vapor, prevented all attempts to learn what conditions were like on this world until the advent of radar and satellite technology. Until then, Venus was considered Earth’s twin, not only in size but in surface conditions, too, being compared to Earth’s early geological periods such as the Carboniferous. In fact, many flights of fancy were penned about Venusian conditions, one of the more famous being Tennyson’s quote from “Locksley Hall, Sixty Years After”:
Venus, smiling downward on this Earthlier Earth than ours,
Closer on the Sun; perhaps a world of never fading flowers,
Hesper; whom the poet called the bringer home of all good things,
All good things move in Hesper, perfect peoples, perfect kings,
Hesper – Venus – were we native to that splendour, or in Mars,
We should see the globe we groan in, fairest of their evening stars.
Could we dream of wars and carnage, craft and madness, lust and spite,
Roaring London, raving Paris, in that peaceful point of light?
Might we not glancing heavenward on a star so silver fair,
Yearn and clasp the hands and murmur “would to God that we were there.”
Given what we now know of the surface, few would like to visit the planet and wonder about our “fair Earth” from its perspective. The yellow–white cloud cover of Venus that we see through the telescope is a layer of sulfuric acid haze very high above the crushingly heavy atmosphere of the planet. The huge amount of carbon dioxide in the atmosphere produces a “greenhouse effect,” creating surface temperatures of around 475 K. Adding to the planet’s mystique, each day on Venus lasts slightly longer than the planet’s year (the Venusian day is 247 Earth-days long, while the Venusian year is 224 Earth-days long), resulting in Venus spinning backward, or having retrograde axial motion. The origin of this strange rotation is one of our most basic questions still unanswered regarding the nature of this hellish world, although theories such as huge impacts and tidal distortion during the formation process are extant. Whatever history may record, Venus certainly is not an identical twin of Earth! Tennyson’s romantic summation above provides an insight into the professional and public perception of the planet at the end of the nineteenth century.
From the observational point of view, Venusian apparitions fall into two classes: those in which Venus is the evening star at greatest eastern elongation and those in which Venus is the morning star and is thus at western elongation. The Greeks and several other early cultures considered these apparitions to be separate entities, with the result that Venus had two names—Phosphorus and Hesperus—which denoted these differing appearances.
Filters for Observing Venus
Low in the dusk after coming out from superior conjunction, observers can see Venus as a round little disc hardly more than 10 arc seconds across. Venus then goes through an almost full set of phases as seen from Earth, from almost full or “gibbous” after superior conjunction and then dwindling until it becomes “new” or invisible to us at inferior conjunction. During this time from full to new, the size of the Venusian disc increases to a 68″ maximum at slim crescent, and we can see at a glance through the telescope almost just where Venus is in its orbital journey in relation to Earth. It is recommended that amateur astronomers make some observations of Venus through every phase it displays if possible. All too many observers will wait until Venus nears its critical half-lit phase to start watching, and it is quite possible that some interesting sights are being missed.
One of the advantages of observing Venus, as mentioned earlier, is that it can be seen in the daylight. Once its position is known setting circles or GOTO mounts will find the planet which looks like a bright star in a low power eyepiece. Figure 4.3 reveals Venus as seen shining in a daytime sky. The violet No. 47 filter shades the vivid blue sky and allows the bright planet to shine out, but also adds contrast to some of the surface features—which in this case are nothing more than clouds. Markings on Venus are difficult to see, and many hours of patient observation and experience are required to see them well, but the added contrast of the violet filter does make a difference to the view.
Using a Wratten No. 15 deep yellow filter has been recommended by various observers, as the filter brings out irregular deformations of the terminator that are very striking around the time when Venus is half lit, near the time when it is at greatest elongation, which is coincidentally the time when it is highest in the sky at sunset or before sunrise. These deformations are of course due to the varying heights of the cloud cover in the atmosphere and can be seen through a modest telescope with high magnifications under good seeing conditions. Figure 4.4 demonstrates Venus seen through such a filter.
An intriguing facet of observation is that Venus ought to appear half lit exactly at greatest elongation, but usually it appears so at least several days before in the evening apparition. This dichotomy is known as the Schroter effect, after Johann Schroter, the man who first noticed over 200 years ago this lag between the phases of Venus and its elongation timings. To see the maximum contrast in the Venusian clouds at this time either a deep red Wratten No. 25 or even a violet No. 37 filter can be used, but again, very like Mercury, the observer has to be extremely wary that any contrasting features are real rather than imaginary.
However, observers of the past noticed that Venus had a 4-day atmospheric rotation, which was called into question when its axial rotation was found to be 247 days. The Magellan spacecraft however set the record straight in the early 1990s when it discovered that despite the long axial rotation, the upper atmosphere had a 4-day rotation period! Something in the cloud decks had been noticed with the assistance of a filter. Figure 4.5 shows the crescent planet in the twilight assisted by a Wratten No. 80A filter.
Try and see if you can spot any contrast features on the cloud tops of the planet. These will most probably be slight shadings in hue or its irregular terminator. Spacecraft have imaged a polar hood in the clouds of Venus that has a slightly different color to the clouds elsewhere so it may be possible to see this under the right conditions with the correct filter. Trial and error with Wratten blue, yellow, green and red filters may provide such opportunities.
A rare event that gives the observer a glimpse of the planet in full is that of a Venusian transit. This intriguing phenomenon has inspired and aided astronomers to make some crucial discoveries. Venus, being inside the orbit of Earth, presented historical opportunities to glean information on the size of the Sun and the distance to our nearest star. As the orbital element of the planet is known fairly accurately, the timing from when the planet first touches the Sun’s disc to the time when it exits the disc enables the observer to calculate the solar diameter. Measurements of the transit from different locations will also give an accurate distance to the Sun.
Sadly, transits of Venus are generally very rare. Two occur separated by 8 years, but then there is a gap of either 121.5 or 105.5 years before the next pair! The last pair was in 1874 and 1882; the last pair of transits was on June 8, 2004, and June 6, 2012. The next pair will be in 2117 and 2125. The astronomical importance of a transit is now significantly diminished simply because the Earth–Sun distance can be more accurately achieved using radar, but such technology did not exist in the eighteenth century, when the need to find the Earth–Sun distance became a major political race between Britain, France, Spain, Holland and Italy. Sir Edmund Halley first suggested measuring the transits of the inner planets across the Sun’s disc as a way of determining the distance to the Sun.
Halley was then the Astronomer Royal at Greenwich Observatory and involved in settling the problem of longitude. This, and determining the Earth–Sun distance, were probably the two largest problems in eighteenth-century astronomy. Needless to say, the race included some spectacular debacles. Jean-Baptiste Chappe d’Auteroche of the French Royal Academy of Sciences went to observe the 1761 transit from Russia, but was attacked by mobs who thought his experiment would somehow interfere with the Sun. His countryman Alexandre Guy Pingre was rained out when he attempted to observe the transit from Madagascar, and was subsequently captured by the British, but took refuge against his bad luck in liquor. Worse was to follow for Guillaume Le Gentil; attempting a trip to Pondicherry in India to observe the transit, he was becalmed in the Indian Ocean on the big day as British troops had routed the French garrison at Pondicherry. The French and British eventually made a concordat and returned the town to the French, and Le Gentil was able to build his observatory, but he had missed the transit.
Undaunted, he waited for the next transit 8 years later, only to have the Sun obscured by clouds on the day! Returning home in misery, he contracted dysentery, was shipwrecked, but eventually made it back to Paris 11 years after he had set out, only to find he had been declared dead and his estate looted! Few astronomers have had to put up with so much.
Figure 4.6 shows Venus in transit across the Sun in 2004. The 2012 event was widely photographed by legions of observers through solar filters ranging from Mylar and Baader film to Hα and Calcium II filters. This photograph was taken by the author using a Canon 350D SLR through an 80-mm telescope fitted with a Baader Astro film filter. Exposure was 1/350th of a second at f6.2 and ISO 800, and the image was then processed and colored in Photoshop™.
Hopefully, you will not have to suffer such rigors as the astronomers above if you go out and observe this fantastic planet and ponder its physical characteristics. Using filters in the Wratten system will enable you to see some of the depths of cloud in the upper atmosphere of the planet, but careful observation is required. Try testing various Wratten filters across the range from red to blue on the planet under moderate to high magnification and attempt to discern any features that can be followed over a period of a few hours if possible. As Venus hangs in the sky for several hours after sunset, it presents more of an opportunity for filter observation than its inferior counterpart Mercury.
The table below provides a handy guide to the types of filters that will aid the visual observation of Venus.
Wratten No | Color | Features |
---|---|---|
No. 15 | Deep yellow | Contrast features, possible cloud layers, separation of sky |
No. 25 | Deep red | Contrast features and darkening of sky, clouds. |
No. 80A | Blue | Contrast features, possible cloud layers, darkening of sky |
No. 47 | Violet | Contrast features and darkening of sky |
Mars: The Red Planet
The most fascinating world beyond our own is the planet Mars. No other planet holds such potential for human exploration in the next 25–50 years. Mars appears to be Earthlike enough to make its colonization seem almost inevitable. Mars draws, demands our attention. Of all the planets of our Solar System not one has the stature of Mars in science, myth or fantasy. Mars is a magical name, heavy with allusions, aspirations and grand dreams unfulfilled; it is a repository of creativity and desire from which spring new insights, innovations and technologies.
For naked-eye observers, Mars is also the planet with the most distinctive color—a deep orange so impressive that it has inspired most cultures to call it “the Red Planet.” For telescopic observers, Mars is the only terrestrial planet that shows real surface features, and its apparition can last an appreciable time, enabling observers to familiarize themselves with the surface.
More than any other planet, Mars is also an enigma; surface conditions are never really good for any length of time, and, even when visible, it appears tiny and rather bland through the telescope. In fact, the periods when it swells large enough to have a wealth of details visible on its disc occur only at infrequent intervals and only at certain of its oppositions. It is therefore easier to discuss an average apparition—one in which it comes neither extremely close to us nor misses us by a great distance.
Mars orbits at about 1.5 times as far as Earth from the Sun, taking almost twice as long as Earth to complete an orbit. It requires about 780 days for Earth to catch up to Mars a second time, or about 780 days between one opposition of Mars and the next. This synodic period is far larger than that of any other planet and determines the way Mars tends to be seen through telescopes—distant, small and disappointing 1 year, close and relatively large the next. An apparition of Mars lasts far longer than that of any other planet, as does its period of invisibility near the Sun between apparitions, thus adding to the mystery surrounding the planet and the frustration of observers.
This legendary propensity to be awkward has left its mark on the human psyche. Fantastic tales have entertained humankind since the dawn of civilization, but an avid and ready audience has always been available for any story that features the planet Mars. There are not many in western culture who have not heard of H. G. Wells and his famous War of the Worlds, a novel that introduced the warlike Martians to the public consciousness. Science fiction stories have featured regular references to Mars as a place likely to be inhabited by alien life, or at least by ourselves 1 day. Such tales drew upon the then current astronomical theories of Lowell, Schiaparelli and Flammarion, which gave such stories verisimilitude, and, interestingly, drove the development of some modern technologies.
The young American scientist, Robert Hutchings Goddard, was entranced by the possibility of spaceflight to or from Mars. Reading a serialization of Wells’ War of the Worlds, he began to ponder the existence of life on other worlds and how to contact such beings. By 1926 he had launched the first liquid-fueled rocket and developed the theories that would propel humans out of their Earthly home. Similarly, the SF influence of Kurd Lasswitz’s On Two Planets (1897) motivated such rocket pioneers as Hermann Oberth, Arthur Rudolph and Werner von Braun. In Russia, imaginative rocket fiction from the pen of Konstantin Tsiolkovsky evolved into reality by the late 1930s.
The work of these rocket pioneers was paralleled by science fiction writers such as Edgar Rice Burroughs, who wrote a series of books, starting with A Princess of Mars. Burroughs based his ideas of the Red Planet on the lurid “canal” theories of the astronomer Percival Lowell, creating a world that was dying as the planet dried up and the canals emptied, ultimately to be fought over by the green-, yellow-, red- and black-skinned inhabitants. Burroughs stories inspired not only a host of imitators but a generation of scientists and technicians who would later become involved in robotic exploration of the surface. Mars in popular culture was given an additional boost by the pulp magazines of the late 1920–1940s, wherein dramatic tales of survival, warfare and intrigue were played out on or over the Martian landscape. Stanley Weinbaum’s seminal Martian Odyssey in 1935 cemented the human relationship to Mars by creating a fascinating, ecologically rigorous world populated by totally un-human, yet intelligent, Martians (tweels), while radio plays and cinema shorts such as Flash Gordon kept the possibility of Martian life (good or evil) in the public eye.
It is little wonder that the public is so fascinated with the Red Planet, yet at the same time it can be disappointed by the usual telescopic views! For the observer equipped with a moderately large telescope, observing Mars is almost as fascinating as its cultural and literary connections. As with any superior planet, an apparition of Mars begins with its first sighting low in the morning twilight after conjunction with the Sun. The reason why the sighting occurs many weeks after conjunction, compared to just a few weeks for Jupiter and Saturn, is twofold. First of all, Mars on the far side of its orbit from us appears decidedly dimmer than Saturn and much less luminous than Jupiter. It is a small planet and needs to come closer to Earth before any appreciable details can be seen. To be appreciated, it also requires a suitable elevation away from the glow of the bright twilight. Additionally, delays in the apparition sighting are due to the eastward orbital speed of Mars, which is so much greater than that of the other superior planets. It takes a while to catch up, which results in oppositions at approximately 2-year periods.
When Mars does begin to get reasonably high in a dark sky before dawn, notice how it keeps almost the same altitude week after week while the constellations are carried far more rapidly westward by Earth’s own (eastward) motion in orbit around the Sun. The brightness of Mars is only about second magnitude at this stage—quite dim—and consequently many a novice may have trouble spotting it among what may be as many as a dozen or so stars as bright or brighter in this general section of the heavens.
Because Mars is so much closer to Earth than Jupiter or Saturn, its retrograde loop against the starry background as Earth begins to overtake it is larger than any of the other superior planets, yet lasts for a shorter time. When you see the Red Planet halt and start to go westward with respect to the background stars, you know that opposition is only about a month away. Observing the planet regularly will enable you to determine the retrograde motion and the likely opposition times.
When you set your telescope on the correct second-magnitude object and see a planetary disk, you may be surprised to discover it is tiny, only about 4 arc seconds in width. Most are disappointed with their first telescopic view of the planet, and even at a good opposition, the disc is still small and shows few details. Observers immediately notice the ochre color of the disk, but at this stage there are no visible features in small telescopes. When Mars is about 60º from the Sun, its disk has grown to about 10 s, the minimum size at which useful observations of the planet may be possible. At this point observers can discover whether their telescopes and levels of experience can lead to seeing any features on Mars at this stage, even on nights of good seeing.
An average opposition of Mars has the planet rivaling or surpassing the brightness of the brightest star and swelling to about 25 arc seconds in width, or about five times Mars’s apparent width when it is near conjunction. This is just about the threshold at which the few details on the planet begin to proliferate into numerous features in medium-sized telescopes. One of the best oppositions was the recent apparition in 2003 where Mars was the closest it had been for 60,000 years! It will never look the same at any opposition, however, because of the inevitable seasonal changes that have enticed so many astronomers to spend a lifetime observing this planet of mystery, speculation and awe. Included here is a map of the general surface features of Mars so that the observer can make comparisons to the eyepiece image. There are several applications for smartphones which also show detailed maps of Mars in real time and this author would recommend them. They can be downloaded, generally free, from any Apple or android online store (Fig. 4.7).
Mars reaches quadrature at 90º west of the Sun. It is then brightest around dawn or early evening and shows a remarkably wide shadowed edge from phase effect. By this stage, Mars has improved to roughly 0 magnitude and is on average about 10 arc seconds across. Medium-sized telescopes in the 200 mm+ range now begin to show a few features to the careful observer under good seeing conditions. A few of Mars’s darker markings can be spotted. The most prominent is often the triangular Syrtis Major, located not far north of the Martian equator. It is a difficult feature to miss at this stage if it is near the central meridian of Mars, facing Earth, at the hour you are looking. A white glint of a polar icecap is also likely to be spotted, as are other hazy features of light and dark variation across the surface.
The appearance of even these most prominent of Martian features is greatly dependent on the current seasonal conditions on Mars. Some of the seasonal effects, such as the melting of polar icecaps, can be predicted fairly well in general. Others, like the famous dust storms, are more irregular in occurrence and varying in scope so that no one can be sure of when or even whether they will appear. Once in a while, a planet-wide dust storm can hide all the Martian features for many weeks. Thus, you can never tell just what you will see when you gaze through a telescope at this surprising world. Considering its variable pattern of storms, seasonal changes and polar cap growth and shrinkage, it is unsurprising that many thought Mars a likely abode for life.
Observing Mars with Filters
Probably the best filter to use on Mars is the Wratten No. 25 red, which gives maximum contrast to almost all of its surface features. Obviously with such a deep colored filter a telescope of 150 mm and above will be the most effective, as the planet will also demand high magnification to discern any surface features at all. With the No. 25 filter, the areas such as the Planitia deserts and darker areas such as Syrtis Major become more defined, and the filter also separates out the polar caps from the overall ochre hue of the planet.
However, the planet’s glare and general fuzziness due to Earth’s turbulent atmosphere, and the atmosphere of Mars combined with its occasional low elevation in our skies, render the image less than spectacular. During such times the filter above can be substituted for the Wratten No. 21 orange filter, which does make a great difference by reducing contrast, blocking blue light and allowing fainter detail to be glimpsed as can be seen in Fig. 4.8 here.
Many observers have also noticed good contrast with the use of the Wratten No. 15 deep yellow filter, which darkens areas of less reflectivity on the Martian surface and thus make large areas of the southern hemisphere and northern areas such as Acidalia stand out. The No. 58 green filter has also been used by many Martian observers to darken red and blue features, and it has been recorded that use of this filter also enhances frost patches, surface fogs, and features in areas close to the poles of the planet.
Mars has enormous seasonal variations across its disc. Observers of the past noticed a wave of “darkening” that crossed the Martian surface during the onset of northern or southern summers. Mars’s axial tilt is very similar to Earth’s at 23º, so these differences in appearance were interpreted to be the growth of vegetation. The orange No. 21 filter will bring out the darker features associated with this phenomenon, but close and regular inspection of the planet is recommended. Some observers have noted a distinct difference during such times of summer onset by using a yellow or light green filter in the Wratten Nos. 8–11 range, but it is recommended that such filters be used only once some experience of observing Mars has been gained. Others have noticed that the darker areas under such phenomena are rendered more visible and separated from the planet background if the deep red Wratten No. 25 is used.
As Mars has a thin atmosphere, occasional clouds can be seen near the large volcanic areas of the equator such as Olympus Mons and the Valles Marineris. Observers recommend the use of the blue Wratten No. 80A at such times to increase the contrast between cloud features and the surface, though obviously sharp eyes and some experience would be needed to discern such details. Nevertheless use a wide range of filters to examine the planet, as one never knows what could become visible. Using Wratten filters on a multi-faceted gem of a planet such as Mars may enhance features that were previously unseen by ordinary visual observation.
It is also worth considering that such filters are appropriate for CCD or webcam imaging of the planet. Some observers imaging with DMK cameras recommend the use of the Wratten No. 21 orange filter to increase the contrast in light or dark areas and to bring out any polar features such as icecaps, etc. Nonetheless, the table below gives a ready guide to the types of filters to use while observing Mars.
Wratten No | Color | Features |
---|---|---|
No. 25 | Deep red | Contrast in surface features, enhances fine surface details, dust cloud boundaries, and polar cap boundaries. |
No. 21 | Orange | Increased contrast between light and dark features, penetrates hazes and most clouds, and limited detection of dust clouds. |
No. 58 | Green | Darkens red and blue features; enhances frost patches, surface fogs and clouds |
No. 80A | Blue | Shows atmospheric clouds, limb hazes, equatorial cloud layers, polar cloud hoods, and darkens reddish features |
No. 47 | Violet | Reveals clouds, limb haze and darkens reddish features |
Jupiter: King of the Planets
Jupiter is not only the largest of the planets in true size, but it is also the one whose combination of size and distance usually makes it appear largest as seen from Earth. On the infrequent occasions when Venus gets close enough to loom larger than Jupiter, only a small fraction of Venus’s disk is lit. Jupiter is always a full on disc and is simply unmistakable and a wonderful sight even in the smallest telescope. Even a pair of binoculars, steadily held, will show the starry spots of the Galilean moons. Jupiter is also a very bright planet, second only to Venus from our Earthly viewpoint and can reach up to magnitude −2.6 and so again adds difficulties in contrast to the eye between the bright disc and the dark sky background. Filters therefore become extremely helpful in reducing the planet’s glare and increasing the contrast between features on its surface.
Firstly, it is advisable to become acquainted with the largest features one can observe on the planet—or, rather, in the planet’s clouds, for the only parts of Jupiter we ever see are its outer cloudy layers. The diagram that follows later will be invaluable in aiding you to observe the planet, even though at first glance you may not be able to see the underlying complexity shown in the diagram.
The most prominent features on Jupiter to see and record are the dark bands running parallel across Jupiter’s bright face. Even a small telescope of barely passable quality can show a few of them. When Galileo looked at the planet in 1610, he recognized that the Jovian system was different; an observation that heralded the beginning of our modern scientific age. Galileo’s telescope was a mere 1 in. in diameter, yet produced a revolution unlike nothing prior; so, too, may your perceptions of this system be overturned by the view through a modest ‘scope.
The Jovian system has attracted planetary observers in droves, as there is such a wealth of spectacular and changing detail on the cloudtops. Jupiter has figured largely in astrology as the king of the planets, an appellation that has attracted artists and composers, Mozart and Holst dedicating a symphony, or parts of, to the planet. In fiction Jupiter has been the focus of several famous works by Arthur C. Clarke. His 1972 short story “A Meeting with Medusa” introduced the idea of sentient life forms on gas giant planets, while his seminal 2001, A Space Odyssey has all the action of humanity’s first meeting with extraterrestrial intelligence take place in the vicinity of the Jupiter system. In recent years, intense speculation has abounded in astrobiology circles as to the likelihood of life emerging on the moon Europa, which is now known to harbors a satellite-wide ocean beneath its icy crust—another idea pioneered by Clarke in 2010 Odyssey Two.
The Galilean moons are a very prominent feature. Outward from the planet they are: Io, Europa, Ganymede and Callisto. However, as they revolve around the planet, they do not maintain this outward configuration, and it is necessary to examine a monthly astronomy magazine or software database every day to get their positions. Occasionally, they transit the planet, their shadows produce dark eclipse shadows on the cloud belts or they disappear as they are occulted by Jupiter. The dance of these satellites can become a source of endless fascination, and the whole system is an obvious target for budding astrophotographers.
After the moons, the planet’s most obvious features are the dark bands that cross it. These bands are called belts. The lighter stripes between them are known as zones. The belts and zones are cloud features that are stretched out into planet-encircling parallel bands by Jupiter’s rotation, which is the most rapid of any Solar System planet’s rotation (less than 10 h per single rotation). A look at the belts and zones thus immediately tells you which way the rotational axis of Jupiter is aligned (perpendicular to the belts and zones) and therefore where the polar regions are located. For ease of identification the following map of the Jovian belt and zone system may be useful (Fig. 4.9).
You can also tell where the poles are and where the equator of Jupiter is by another property of the planet that is quite noticeable in small telescopes—its shape. Jupiter is decidedly oblate. This means its diameter measured from pole to pole is less than its equatorial diameter. Jupiter’s equatorial diameter is about 6 % larger than its polar diameter. It is relatively easy to spot this feature.
The most prominent of Jupiter’s belts are usually the South Equatorial Belt and North Equatorial Belt, located to either side of the planet’s equator. They appear as a brownish or reddish series of lines bounding the whiter equatorial zone and are very prominent even in small ‘scopes. Seeing such detail requires a little patience and application rather than a quick glance through the eyepiece. Numerous belts and zones, as well as many other features, may be seen by a good observer with a medium-sized or large telescope. Beginners should content themselves with first identifying the prominent belts and zones and watching the dance of the satellites as they wheel around the planet.
Novices can also note what colors they glimpse in the belts and zones. These may at first seem merely gray and yellow–white, respectively. But further examination (or a larger telescope) should reveal at least a hint of brown (you might even say a slight ruddiness) in the belts and distinctive yellow hues in the zones. Remember, however, that even on nights of good seeing color estimates should really be tried only when the planet is fairly high in the sky.
On the subject of color, the famous Great Red Spot (GRS) of Jupiter is often very pale in color and often difficult to see, belying its epithet. It is located near the edge of the South Equatorial Belt but can be very difficult to spot in small instruments. Most of the time the GRS hollow is visible as a “bite” in the cloud belts rather than the spot itself. The whole feature is visible with some dedication and patience even through a modest instrument. The planet is an endless source of wonder and its ever-changing configuration should be viewed whenever it is above the horizon.
Filters for Observing Jupiter
Jupiter is an amazingly detailed object, and as its size varies from 30 to 52 arc seconds during its apparitions it subtends the largest body of any of the superior planets. As noted earlier, Jupiter is also a very bright planet. Using nothing but the naked eye, most observers can gain enough detail to satisfy their requirements, but many astronomers also find that blinking the orange No. 21 filter into the eyepiece does render the planet literally in a different light. Subtle shading and detail can be seen flashing into view as the wide bandpass enhances the reddish areas such as the Great Red Spot on the planet and the equatorial and temperate belts as can be seen from Fig. 4.10 here. How much of this detail can be seen by a keen eyed observer without a filter? The filter also sharpens the boundaries between the belts and zones and increases the contrast in the bright zones by lowering their light transmission, enabling some faint details to be seen under higher powers.
A full range of filters can be used on Jupiter, though the color correction filters do not often give the same blaze of details that the blocking filters do. For example, the effect of the No. 38A filter can be seen in Fig. 4.11, where the delicate cloud bands of Jupiter show up in marked contrast to a very glare-filled view through an unfiltered eyepiece. The red banding is a little more enhanced, but so are the white zones between the belts. It is easy to see how the areas of shear between the belts and zones and the melding between them reveal more subtle detail than in the orange No. 21 filter, as seen previously. Use of the dark blue Wratten No. 38 filter really does work, and the substantial reduction in glare allows the astronomer to work without tiring the eye, and since the planet is large, high magnifications even on relatively small telescopes can be used.
In fact, the filter that does all of the above in enhancing the belts and zones and revealing marvelous detail in the cloud tops under steady atmospheric seeing is the Wratten No. 58 green filter. Although this filter colors the planet in a rather surreal hue, the detail visible in the separation areas between bright zones and the darker belts is fantastic. Shear areas are brought out in sharp relief, and the reddish areas of the belts become broken into a maze of detail under high magnification, with white high clouds floating above the main belts and the limb of the planet easily visible as broken into an irregular sawtooth pattern. The belts and zones represent areas of different pressure and height in the cloud layers of Jupiter. The GRS and its hollow become far more visible and the poles themselves a little more detailed.
Some observers notice that the use of the Wratten No. 15 yellow filter deepens the detail noticeable on festoons of cloud close to the poles, while the Wratten No. 25 deep red filter enhances the view of several of the giant oval storms visible close to the equatorial belts. It is thus obvious that the full range of longpass filters are appropriate to use on Jupiter, bringing out the wealth of detail on this giant planet. Below is a quick guide to the filters and features that are generally visible with their use.
Wratten No | Color | Features |
---|---|---|
No. 38 | Dark blue | Enhances the contrast within the bright zones and sharpens boundaries of faint cloud currents |
No. 21 | Orange | Contrast sharpened between belts and zone boundaries |
No. 58 | Green | Increased contrast in belts and zones and polar areas better defined. More detail enhanced in belts |
No. 15 | Yellow | Contrast sharpened between belts and zone boundaries |
No. 25 | Deep red | Increased contrast in bright zones and better resolution of white oval storms |
Saturn: The Ringed Planet
The two most striking sights in a telescope for most first-time viewers are the Moon and the planet Saturn. With good seeing and a properly collimated telescope (even a rather small one), the body and rings of Saturn are beheld with a sharpness that photos cannot convey. The impression one is left with when viewing the planet for the first time are varied, but inevitably, most people go to the front of the ‘scope in order to look for the photograph that the astronomer has placed there to fool them! Such is the glory of Saturn through the telescope. Even veteran observers never grow quite tired of this basic sight, even if the presentation angle of the rings never changed. Nevertheless, dedicated observers find an interest beyond the aesthetic enjoyment of the study in illuminated geometry or statuary of light, shadow, and shapes that is Saturn.
As a superior planet, Saturn behaves in the sky much like Jupiter—first dawn appearance after conjunction with the Sun, west quadrature, start of retrograde motion, opposition (rising at sunset), retrograde motion, east quadrature, and last dusk appearance just before conjunction with the Sun. But Saturn is far slower rotating than Jupiter—so slow that we can find it practical to talk about where Saturn will be and what Saturn will be doing over the course of not just a year but a whole decade. Saturn takes 29 years to go around the Sun and thus can be found in the same constellation for 2 consecutive years before moving on.
This slow motion across the sky led ancient people to associate the planet with the onset of old age—a fact rendered artistically by Holst in his Planets suite. This slow movement was also held to be responsible for slow yet unstoppable growth of any inhabitants, as imagined by Voltaire in his satirical Micromegas, detailing the visit of Sirian and Saturnian giants to Earth.
The unlikelihood that a gas giant could be inhabited made little difference to the literary or scientific establishment of the eighteenth century. At that time people did not know the composition of the planets, and it was therefore in keeping with then current speculation that such worlds were like Earth. No less an astronomer than William Herschel considered all the planets, including Saturn, to be inhabited.
The color of the planet is quite striking; it almost always appears as a steady gold point of light of between about magnitude 0 and 1, which is comparable in brightness with a number of the brighter stars; indeed at first glance it can be confused with one of the stellar beacons of the night, its baleful hue being alluded to by several ancient poets, including Homer.
A look through the telescope confirms the identity of the planet in spectacular fashion. Even binoculars or finderscopes can show that this image is elongated. A very small telescope and ×30 or ×40 magnification shows the rings as being tiny but definite. A slightly larger telescope and more magnification reveal that these are in fact rings in the plural. You may not see the thin black line that separates the two most prominent rings, the A and B rings, but the greater brightness and lightness of the inner (B ring) of the two make it stand out.
What are the rings of Saturn made of? Galileo’s telescope was so poor that he thought they looked like “ears.” About 50 years later, in 1655, Christiaan Huygens was the first to see that the rings encircle the planet without touching. But it was a matter of speculation as to what the rings were constructed from. Were they solid or were they gaseous or icy? The matter was eventually settled by the Scottish physicist James Clarke Maxwell in the mid-nineteenth century when he found by simple Newtonian physics that the rings must be composed of individual particles—unimaginably large numbers of them—orbiting Saturn at whatever speed is appropriate for a given particle’s distance from the planet following Kepler’s laws of planetary motion, though on a small scale. Scientific speculation as to their origin abounds, though there is little doubt that these particles are bits of ice and rock, probably the kinds of materials that aggregated to construct the planet and its moons in the early Solar System but became part of an extended equatorial system due to high angular momentum.
The rings are fascinating. One of the most intriguing features of observing Saturn is the ever changing angle of its rings. As the planet is tilted on its axis, the rings present themselves at various degrees. For several years the northern aspect will be visible, and then the rings will thin to an almost invisible line before the tilt brings the southern aspect of the rings into view. The telescopic view of the rings varies over a long time, although some of the changes leading up to the edge-on appearance of the rings happen in a matter of months or even weeks or days.
What basically happens during this phenomenon is that the angle from which we view the rings changes. The rings stay tilted at the same angle with respect to Saturn and its orbit, but Earth’s vantage point shifts to give us anything from about a 27° to a 0° angle of the rings with respect to the horizontal. In other words, the rings can be fairly well tilted (“open”), showing us a good view across the vast expanse of their top or bottom (north face or south face), or the rings can be presented to us perfectly edge-on or “edgewise” from a sideways view. The marvelous thing is that, although Saturn’s two most prominent rings span across a distance of about 300,000 km in the other two dimensions, their thickness is as little as 50 km. The planet and its ring system can be seen here in Fig. 4.12, as seen through a neutral density filter, and its majesty and wonder are visible at a glance.
Although its superb ring system usually steals the show, the globe of the planet can be very interesting, too. As with the other gas giant planets, with Saturn we look at only the planet’s clouded atmosphere. The Voyager and Cassini spacecraft passing Saturn found that it is not merely the planet’s distance that makes its cloud features appear far fewer and subtler than those of Jupiter. The features really are far fewer and subtler due to on overlying layer of haze. There is also less activity out at Saturn; the sunlight here is less than 1 % of that impinging on our atmosphere, so solar heating plays a smaller part in the weather features of the planet. However, Saturn has an internal heat source that makes its weather systems as dynamic as Jupiter’s. Fortunately, there are times when features rise up through the haze so that some rare but marvelous detail is sometimes glimpsed by veteran observers, such as the huge white storms that happen every 30 years or so.
Another aspect of Saturn’s globe that is always visible, but mostly overlooked, is its oblateness. This is not surprising when we consider that Saturn is the only planet with an average density much less than that of water; and that it spins almost as rapidly as Jupiter. Saturn’s diameter through its poles is about 10 % less than through its equator—roughly one entire Earth-diameter different! The reason that even novices do not notice the oblateness more often can be blamed on the rings. Besides distracting us from evaluation of the planet’s shape, perhaps they also lead us to subconsciously think that our perception of the planet’s oblateness is an optical illusion induced by the influence of seeing the rings elongated in the planet’s equatorial plane. In any case, when the rings are near edgewise and almost vanished, a look at Saturn’s globe immediately shows how significantly oblate it really is.
The satellites of Saturn are very varied. Usually visible as an eighth magnitude starry point is the giant moon Titan, shrouded in a methane and nitrogen haze and containing the possibility of pre-biotic chemistry on its surface, a feature that was investigated by the Huygen’s probe in 2004. Fainter satellites such as Tethys, Enceladus and Dione are available to larger instruments, while Iapetus is of note due to its variable magnitude, one half of the moon being covered with a dark layer of compounds, possibly the result of a cosmic collision or outgassing from its surface.
Even if you cannot make out any of the belts or zones of Saturn on a given night, however, there are several basic sights to look for. Two are the shadow of the planet on the rings and the shadow of the rings on the planet. These sights are best visible around the time of the quadratures, for, although Saturn itself is too similarly distant from both Earth and the Sun to really show a phase effect, the rings are large and elongated enough to make it possible to see some of the planet’s shadow on them and some of their shadow on it.
Filters for Observing Saturn
In the same manner as Jupiter, the planet Saturn responds well to the longpass filters that block light of particular wavelengths. However, due to the atmospheric layers of aerosol haze, features on Saturn are much more subtle, and require observers to concentrate their efforts in times of good seeing. Additionally, Saturn is smaller than Jupiter and is almost twice as far away from Earth as the king of the planets so its disc will require high magnification and telescopes in the 150 mm+ range to bring out the best of the details.
Once again the Wratten No. 58 green filter gives the best rendering of surface detail on the planet, as it differentiates the belts and zones quite well and reveals some features on the rings, too. The filter does not provide the wealth of details we saw on Jupiter, as Saturn has belts and zones that are smaller and smoother than its giant counterpart, yet they can be brought out from the overall yellow color of the planet nonetheless.
With the No. 58 filter the belts are noticeable as evenly distributed, thinner, with less range than Jupiter’s and with all of the belts almost a uniform hue. Some observers report seeing some definition in the zone areas of the clouds with a Wratten No. 15 yellow filter, and some of this definition can be seen here in Fig. 4.13, which was taken with a Canon DSLR mounted on a 100-mm telescope at f10 and an exposure of 1/250 s at an ISO of 800, coupled to the No. 15 Wratten filter and processed in Photoshop™.
Visual observation of the planet reveals that the brighter zones appear off-white or even slate-gray or yellowish at times. However, in contrast, Saturn’s belts exhibit bluish-gray, brown and reddish colors easily seen using the same filters as for Jupiter. Brighter patches sometimes appear on this Ringed Planet and are best seen by using the Wratten No. 11 yellow green or the No. 58, as detailed above.
The rings are a slightly different matter to the belts and zones of the planet. Many observers recommend the use of the Wratten No. 11 yellow green filter to bring out the details in the rings and enhance the Cassini division between the A and B rings whilst others have recommended the use of the No. 47 Violet filter to bring out subtle features in the rings and to see the Enke division in the A ring in better detail. This filter has also been claimed to render the belts and zones in greater detail on the surface of the planet too. Still other observers report seeing details to good effect by using the red Wratten No. 25 filter, so it seems obvious that these longpass filters are very versatile, and the detail they can reveal depends on our atmospherics, visual acuity, and the phase of Saturn and its ring system angle.
Saturn is a planet of unique beauty that continues to draw the attention of observer after observer. To gain the most from your observing, below is a table for quick reference for filtered observation of the ringed planet.
Wratten No | Color | Features |
---|---|---|
No. 11 | Yellow green | Contrast on the rings and belt zones. The Cassini division |
No. 21 | Orange | Increased contrast in some belt and zone areas |
No. 15 | Yellow | Zone definition and some separation in the rings |
No. 25 | Red | Zone and cloud features, penetration of some haze layers |
No. 47 | Violet | Ring details and some enhanced features in the clouds |
No. 58 | Green | Bright cloud patches and storms, some enhanced detail in the rings |
The Outer Darkness: Uranus and Neptune
It seems incredible to us living in the twenty-first century that ancient humans knew nothing about the existence of the three planets beyond Saturn. Pondering this, we can acknowledge the surprise of astronomers who discovered these worlds floating in the darkness at the edge of the Solar System. The story of their discovery is a tale of scientific insight, buffoonery and observational skill that highlights the nature of any human endeavor.
Uranus was discovered by William Herschel on March 31, 1781. Herschel was observing a bright disc-like object in the constellation of Gemini from the garden of his home in Bath, England. After watching it for several hours and noticing a slight movement against the background stars he recorded his observation and sent it to the Royal Society, believing the object was a comet. Calculations suggested that the object was in fact in an almost circular orbit around the Sun with a period of 84 years. The body was not a comet; rather it was a new planet, the first to be discovered in the age of science and one that doubled the size of the Solar System overnight when the calculations revealed a distance of almost 2 billion miles from the Sun. Herschel became famous overnight and went on to greater fame as the originator of the general catalogs in use by deep sky observers, the discoverer of binary star systems and the discoverer of radiation beyond the visible spectrum.
As a planet, Uranus is not a good target for an observer. It just reaches naked-eye visibility at magnitude 5.7 but is barely bright enough to be noticed, a fact that caused confusion and consternation when a search of historical documents found that the first Astronomer Royal, John Flamsteed recorded it as the star 34 Tauri in 1678 in his first sweeps of the sky, and saw it six times afterward without realizing its true nature! Similarly, Tobias Mayer and the French astronomer Le Monnier also spotted Uranus but failed to note it. If such luminaries can make such a mistake, then average observers can be forgiven if they consider Uranus a less than pleasing object.
Telescopically, the object does show a small disc shining at about sixth magnitude at opposition. Under good seeing conditions it can exhibit a slight green color, which is due to a high haze of methane smog in its upper atmosphere. Of its satellites and ring system, the modestly equipped amateur will see nothing, and will have to content himself or herself with observing the slow movement of the planet against the stars over the course of the night or of several nights. So small is the disc of Uranus that it can be mistaken for a star if given a cursory glance, so once the field containing the planet is identified, sweep around it using a higher power eyepiece until the object becomes obvious.
The planet is an ice giant, although in comparison with planets such as Saturn and Jupiter the term “giant” can be rendered moot. Uranus is only five times larger than Earth, so it is hardly surprising that it is a difficult visual object given its size and extreme distance at almost 3 billion km. Any astronomy magazine or software program will enable the observer to ascertain co-ordinates to find this elusive object. We shall cover observing this planet with filters together with its neighbor Neptune.
Neptune is an elusive planet that requires a telescope to be seen properly, although binoculars will reveal the planet as a star-like object, lacking resolution. Discovered in 1846, Neptune has the great distinction of having been discovered “mathematically” before being seen visually, and the tale of its discovery is worthy of note.
Shortly after the discovery of Uranus by Herschel, observers noticed that it did not follow its predicted pattern around the Sun. Astronomers hastily calculated that Uranus was slowing down in its orbit due to the gravitational pull of an unseen body beyond it. Telescopes searched the heavens in vain, but nothing was found, and the problem was deemed irresolvable due to the intricacies of the mathematics and the lack of observational data. A young graduate at Cambridge, John Couch Adams, undertook the calculations as to the mass, orbit, size and position of this unseen planet—a task that took him 5 years.
Once the position was determined, Adams went to see the Astronomer Royal, George Bidell Airey, at Greenwich, hoping that Airey would initiate a search for the planet. However, Airey was a singularly pedantic and unimaginative individual, a good administrator, but a lackluster astronomer who dismissed Adams with excuses. Adams returned to Cambridge, where he tried in vain to interest James Challis, the director of the Cambridge Observatory, to hunt for his new planet. Unknown to Adams, Urbain Jean Joseph le Verrier in Paris had also calculated the position of the new planet, and run into similar difficulties with observers who dismissed his calculations. Stung by their obtuseness, Le Verrier sent his notes to the Berlin observatory under the directorship of Johann Enke. The night the Germans received the notes, the observatory’s young observer, Heinrich D’Arrest, under the supervision of Enke and the chief observer Johann Galle, stepped up to the telescope, turned it to le Verrier’s calculated position and within minutes noted the planet! Neptune had been found and another body added to the solar system. The resultant cataclysm over the true “discoverer” embroiled scientists and astronomers alike for some time.
As with Uranus, Neptune had inadvertently been noticed before. A thorough search through historical records reveals that Galileo had recorded the object in 1612 as a bright star in the same telescopic field of view as Jupiter, and Jerome Lalande had noted the planet as a star in 1795. So attuned to not observing anything out of the ordinary were astronomers at the time that Neptune actually appears as a star on the celestial atlas of K.L. Harding in 1822!
Despite the furor of its discovery, Neptune is even more insignificant visually than Uranus. The planet is 4.5 times the size of Earth, yet is over 4 billion km from the Sun, a lonely world skirting the darkness of deep space. Visually, it shines at eighth magnitude, the only major planet of the Solar System that absolutely requires a telescope, and reveals a small disc shining with a pale blue hue. Its major moon Triton, an object with the coldest surface in the Solar System at a temperature of −235 °C, cannot be seen with small amateur telescopes, and the whole system is rather a disappointment, although finding the faint disc is an achievement in itself.
Being rather remote worlds, little was known scientifically about either of these planets until the flyby of the spacecraft Voyager in 1986 (Uranus) and 1989 (Neptune). The spacecraft revealed them to be astonishingly different worlds, more ice giants than gas giants. Due to their isolation, few writers have attempted to situate stories on or around such worlds, with the notable exception of Olaf Stapledon, who in 1930 penned one of the most famous SF tales of all time, Last and First Men. The story details the evolution and eventual destruction of humankind, but contains the interesting idea that 1 day people would learn how to modify themselves so that they could live in the inhospitable environment of the outer Solar System as the Sun expands to become a red giant star. Stapledon’s “last” men, the nineteenth of their race, now live on Neptune, rising and falling in the harsh atmosphere almost like fish in the sea. It is an interesting idea in exploring extreme life environments, one which many scientists are beginning to ponder.
Until summer 2006, Pluto, one of the outer members of the Solar System, was a planet. However, a meeting of the International Astronomical Union in Prague demoted Pluto, making it part of a group of “dwarf planets,” including Ceres, in the inner Asteroid Belt, Quaoar and Eris in the Kuiper Belt, a system of icy and rocky debris at the outer edge of the Solar System beyond Neptune. These Kuiper Belt objects are extremely difficult to observe, and the task is not something that many amateurs undertake. Nevertheless, some amateurs will attempt to spot Pluto during its opposition, though the use of filters of any kind will be very limiting, as Pluto generally shines at magnitude 13.8 at its brightest.
Like Neptune, Pluto was discovered after a “mathematical” search for an object that perturbed the orbits of Uranus and Neptune. Not much hope was held of ever finding a planet this remote from the Sun, and the problem remained until in 1929, Lowell Observatory in Arizona hired a talented young amateur astronomer by the name of Clyde Tombaugh to perform a photographic search. Tombaugh struggled with comparisons of photographic plates, eventually comparing the positions of over one million stars until finally, in February 1930, he found the small body that astronomers had largely forgotten. At an average distance of 8 billion km from the Sun and a diameter of 3,000 km, Pluto is tiny and insignificant visually and requires a telescope of at least 250-mm aperture to view it. Finder charts are printed in astronomical magazines and online for those interested in pursuing this distant object.
Filters for Observing Uranus and Neptune
The presence of high haze layers of methane in both atmospheres and the small angular sizes of each body make observing these planets difficult at best. Although Uranus and Neptune are still fairly bright at magnitudes 5.7 and 8.1, respectively, they subtend tiny discs even at opposition. Uranus is just 4 arc seconds in diameter, and Neptune is 2.1 arc seconds in diameter. Visually, this is about the size of the Great Red Spot and some of the oval storms on Jupiter!
As both planets are predominantly green and blue green, then the use of the yellow Wratten No. 8 or No. 15 may make some difference to your observing, or, if using a telescope of 300-mm aperture try the Wratten No. 25 deep red to see if any cloud features swim into view. However, when one considers that Uranus as observed by the Voyager spacecraft was unremittingly bland and featureless and Neptune revealed subtle cloud belts but is a tiny visual object then any filter use that reduces light transmission is going to affect the image more than the filter can compensate for.
Though it is not recommended here, observers with large aperture telescopes are welcome to try a high magnification combination of the following filters to attempt any resolution of features, with the proviso that tiny Neptune is the better target.
Wratten No | Color | Features |
---|---|---|
No. 15 | Yellow | Possible high ice clouds on Neptune, deepening of planetary color |
No. 25 | Green | Possible high ice clouds on Neptune, deepening of planetary color |
Comets: Visitors from Deep Space
The sight of a bright comet, its tail streaming across the heavens, is an incredible though rare sight. However, when one considers that annually there are about 40 comets visible to astronomers, by what standard do we determine how rare these events are? Simply put—any comet that is actually visible to the public or is mentioned in the national press is a rare event!
Most comets are decidedly uninteresting, even seen through a powerful telescope, and few and far between are the beautiful comets like Hale-Bopp or comet PanSTARRS 2013 to name two recent ones that delighted sky watchers.
From the point of view of the novice, comet hunting is a highly intensive and time-consuming search for the ephemeral and undistinguished. Comets vary greatly in brightness, size and visibility. One must have an intimate knowledge of the night sky and an almost photographic memory to remember the positions of thousands of faint stars in order to distinguish if any newcomer appears among them. From this point of view, comet searching is left to the dedicated amateur, or more commonly to automated telescopes involved in monitoring the activity of near-Earth objects.
Nevertheless, it can be instructive to scan the heavens from time to time just in case. Keeping a weekly eye on the comet newsletters on websites such as www.harvard.edu is a good indication of forthcoming spectacular comets that would merit observation. Coupled to this, understanding the ephemerides and locating the comet on a map and in the skies is an excellent practical demonstration of one’s skills.
Comets are highly important, as they contain pristine elements and molecules from the formation of the Solar System. They are in fact time capsules, enabling astronomers to gauge what sorts of materials were in the solar nebula out of which the Sun and planets formed and how such materials have evolved in the 5 billion years since. Recent scientific research focuses on these exotic materials, as many comets not only contain water in the form of ice but also pre-biotic molecules that could have rained down on Earth and the other planets of our Solar System and contributed to the rise of life. Indeed, some have gone as far as to speculate that comets may well contain either bacteria or viruses in addition to pre-biotic matter. This is an interesting scenario that at present has little supporting evidence but nevertheless has stirred the astronomical community. If one is fortunate enough to observe a comet, perhaps it would be good to ponder the make-up of debris that it leaves behind.
This debris is frequently seen as meteor showers and some are fixed at particular times of the year such as the Geminids in December and the Perseids in August. These meteors take their name from the constellations they appear to “radiate” from, and the two mentioned here are the best of the year. Not to be forgotten are the Leonids in November and the Orionids in October, the stream associated with comet Halley. However, meteors are not a subject for filtered observing and are best seen with the naked eye.
In ancient times, comets were thought to be the harbingers of doom, ill fated, with a rather baleful aspect. The philosopher Aristotle believed comets to be foul exhalations in the atmosphere, all part of the degradation of an Earth at the rank center of the universe and removed from heavenly perfection. Subsequently, astrologers foretold the rise and fall of kingdoms by the appearance of such bodies, and many cultures occupied their astrologers and astronomers in watching the skies for the appearance of these bad-omened celestial objects in order to forewarn others. The Chinese have records going back to 200 bc, detailing observations of “hairy stars,” and the most famous of all comets—Halley—features on the Bayeaux tapestry of 1066, interpreted by the court of William the Conqueror as bad luck for Harold Godwinson the English king. Comets, by their rarity and high station, only presaged bad luck for the rulers of the day as Shakespeare acknowledged in Julius Ceasar (Act II, Scene II):
When beggars die, no comets are seen,
The heavens themselves blaze forth the death of princes
The famous astronomer Tycho Brahe began to set the scientific record straight when his observations of the bright comet of 1577 enabled him to discover that comets were outside of the atmosphere. This discovery made the Solar System a dynamic and evolving place and shattered the ideal of immutability that Aristotle demanded of the heavens. Following on several historical observations, Sir Edmund Halley performed calculations based on Newton’s law of gravity that tied the observations to a single comet returning on a prescribed orbit. Comets became part of the predictable, mechanistic universe despite their rather disastrous aspect, and modern science has continued to dispel the comet myth.
Comets have a few features that are worth noting. The bright head, or “coma,” of the comet is a cloud of icy and gaseous material immediately surrounding the nucleus, which is generally a very small rocky/icy body just a few km in size. The coma can be over 1 million km in diameter, and streaming behind it will be a lustrous tail (if we are fortunate) that is divided into two components, the dust tail and the ion tail. The dust tail lives up to its name as the physical silicate material streaming away from the comet in the solar wind, and the ion tail is the ionized gaseous material that reacts to changes in the solar magnetosphere and can disconnect and re-grow as the comet heads around the Sun. Figure 4.14 shows comet PanSTARRS 2012 as a typical example of such a body.
Observing comets can be done with either binoculars or a small telescope, depending on the brightness and size of the comet. Comets, due to their very nature, are unpredictable, and their quoted magnitudes on ephemerides may not be what the observer actually witnesses. This is due to their nature—comets are basically icy snowballs of varied sizes interspersed with rock and dust. The current scientific model of a “dirty snowball” was first proposed by Fred Whipple in the late 1950s and has stood the test of time and scrutiny; Halley was intensively investigated in 1985/1986, the European probe Giotto discovered that the nucleus was in fact a dirty, potato-shaped chunk of icy debris orbiting the Sun and spewing out gases at unpredictable rates. The melting of any cometary nucleus of indeterminate size thus releases gases and dust at an unspecified rate, relating directly to the erratic nature of foretelling cometary magnitudes. Comet ISON of 2013 is a case in point here where it was portrayed in the media as the “comet of the century,” only to break up and disappear as it rounded the Sun!
These heavenly visitors are purported to come from an area on the periphery of the Solar System, about 40,000 astronomical units away from the Sun. This area, a spherical capsule around the Sun, is now known as the Oort Cloud, after the Dutch astronomer Jan Oort, who postulated the existence of a body of comets at great distance surrounding our parent star. These bodies would be formed at the earliest time in our Solar System’s history, while the solar nebula was still largely spherical and the Sun had yet to form. Although we still have no direct evidence for the Oort Cloud, the high inclination of cometary orbits, so different from the smooth plane of the planets, has convinced most astronomers of its reality. The recent discovery of a half-way house between the planets and the Oort Cloud, the Kuiper Belt, has only added to the increasing amount of evidence in favor of the Oort Cloud hypothesis.
It is thought that the short-period comets (up to 250 years) come from the Kuiper Belt, while the long period comets come from the Oort Cloud itself. Periodic comets have rather varied timescales, the shortest being that of Comet Encke at 3.3 years, Halley at 76 years and Swift-Tuttle at 120 years. Comet McNaught-Russell, featured above, will never return to the Solar System, and neither will comet NEAT, since both have hyperbolic orbits rather than elliptical ones and will escape the Sun’s gravity and wander the galaxy. The fabulous comet Hale-Bopp of 1997 will return in 4,200 years’ time. Clearly comet watching is a rather long and laborious process!
Filters for Comet Observation
Observe a comet carefully over as long a period as possible to see any changing features, which are generally observed visually without filter help. Nonetheless there is a “comet” filter in the market that may be a useful tool for observing.
The filter is a narrowband one rather than the colored correction or longpass filters we have so far been discussing. Comet filters have a narrow bandpass of just 25 nm that is centered around 510 nm in the green part of the spectrum and essentially isolates the oxygen component of the comet tail in addition to that of cyanogen lines in the spectrum at 511 and 514 nm. This filter, usually known as the Swann band filter, after the Scottish physicist William Swann who identified several spectral features of hydrocarbon fuels in 1856. This filter picks out the gaseous components of the tail, and any comet observer will know from experience how difficult it can be to see structure and detail in cometary tails. This filter therefore fills a vital niche in the market, and its use is recommended by such observers as Howard Brewington and David Levy.
Comet hunting requires dedication beyond the pale and lots of patience. Sweeping the horizon after sunset and working your way upward in an overlapping pattern takes some practice, but once one is confident enough, the search just may be worthwhile. If the comet filter above can add to your observing experience, then it is a valuable addition to your filter toolkit. Whenever one of these visitors from the outer Solar System deigns to visit us, it is worthwhile to capture the apparition by DSLR camera or web-cam as the above filter will also aid enhancement in imaging.
A bright comet is an extremely rewarding sight, one that is not easily forgotten by those fortunate enough to see one of these celestial visitors. The table below gives the details of a useful comet filter.
Filter | Bandpass | Features |
---|---|---|
Swann band | 501–526 nm | Gaseous components of comet tails and coma |
Conclusion
It is hoped that the above information gives the reader the impetus to go and use Wratten filters to explore the Solar System. These filters are generally inexpensive and are a great addition to observing through any telescope and will reveal previously unseen details if used carefully.
However, there is one Solar System object that we have not covered so far—the Sun. Observing our nearest star requires completely different filters to those described here and so it is to a description of the types and uses of solar filters that we now turn.
Further Reading
Benton J (2005) Saturn and how to observe it. Springer, New York
Grego P (2012) Mars and how to observe it. Springer, New York
McNally J (2012) Jupiter and how to observe it. Springer, New York
North G (2012) Observing the solar system. The modern observers guide. Cambridge University Press, Cambridge, UK
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Griffiths, M. (2015). Observing the Solar System with Filters. In: Choosing and Using Astronomical Filters. The Patrick Moore Practical Astronomy Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-1044-1_4
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