Abstract
Most people are familiar with the idea of plotting a graph. This is one example of a coordinate system, the x and y coordinates (abscissa and ordinate) providing a means of specifying the position of a point within the two-dimensional surface occupied by the graph. It is a simple extension of the idea to give completely the position of an object in space using three coordinates, x, y and z (Fig. 4.1).
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Notes
- 1.
The term “great circle” is used to denote any circle drawn on a sphere that divides that sphere into two exactly equal hemispheres. The name originates because great circles are the largest circles that can be drawn onto a sphere. The distance along a great circle that joins two points on the surface of the sphere is the shortest distance between those two points when moving over the surface of the sphere (hence the importance of great circle navigation when moving large distances over the surface of a nearly spherical Earth).
- 2.
The magnetic deviation is the angle between the north needle of a compass and magnetic north. It arises from local magnetic fields such as those from the magnetized steel making up a ship.
- 3.
Occasionally azimuth may be referred to as the bearing, especially for objects on the surface of Earth.
- 4.
Thus at latitudes of ±60°, an aircraft moving west at 14 km every minute (840 km/h, 525 mph) would cause the mean Sun, and so also to a good approximation for a short while, the true Sun, to appear stationary in the sky. At a latitude of ±89.9° you would be able walk westward fast enough to get the same effect.
- 5.
A small irregular motion of Earth’s geographical poles, probably arising from movement of material deep inside Earth.
- 6.
The actual center of the galaxy is now known to be about 4′ away from this point, but this position is still used for the galactic coordinates. Prior to 1958 a rather different definition of galactic latitude and longitude was employed. For a while the old system was distinguished by the use of “I” as a superscript and the new one by the use of “II” as a superscript (i.e. l I and b I or l II and b II). Recently the superscripts have been dropped, since almost all positions now in use are based upon the newer system.
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Kitchin, C.R. (2013). Positions in the Sky. In: Telescopes and Techniques. Undergraduate Lecture Notes in Physics. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-4891-4_4
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DOI: https://doi.org/10.1007/978-1-4614-4891-4_4
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