The Difference in Appearance between Stars at Greater and Lesser Distances

Abstract

The following points should be noted here: (1) Just as there is a difference between light and brightness, so do illuminating and lighted bodies differ from one another. Illuminating bodies like the sun and fixed stars have light in themselves. Lighted bodies, however, have no light in themselves but receive it from another source, like the planets or their satellites. Consequently one can easily understand why the wanderers have a less bright light encompassing them than the fixed stars. There is indeed a great difference between the illuminating sphere of the sun or a fixed star which is diffused through its own virtue in the ether and the bright sphere of a planet which is merely a reflection. (2) It is characteristic of a lighted and illuminating body that its diameter does not decrease as its distance increases to the same degree as that of an opaque body. For a torch is visible when an opaque body similar in size and at a comparable distance is not. Thus a light and bright body like this, removed at an immeasurably greater distance is preserved in its own size. In other words, light and brightness make far distant bodies seem proportionately much greater than they ought to appear according to optic and geometric principles. The reason for this is that the illuminating and bright sphere is also contracted, with the result that that which previously was otherwise invisible, becomes visible at a distance. (For the more constricted the virtue of a sphere of light or brightness becomes, the more vivid and visible it is; and whatever is more vivid and visible, this is easier to perceive and is more apparent to the eyes.) Consequently although the stars which are visible to us despite their great distances away, ought to seem smaller, they are not so reduced in size as geometric or optic principles demand. Their bodies are joined to the illuminating and bright sphere encompassing them and thus they necessarily appear far greater than they actually are.