ABSTRACT
Astronomy is the science of vast distances, of the ultra-large taking in as it does the furthermost corners of all existence. For long ages, however, it was based solely on what the eye could see and, although this method of observation works efficiently in many cases, for example in the charting of the constellations and the movements of the Sun, Moon and planets across the heavens, it can often lead us into serious error. This is perhaps nowhere more obvious than in the illusion, which persisted from the earliest times to the end of the Middle Ages, that the stars were all at the same distance from us, mere specks of light on an inverted bowl. It is not difficult to understand how this illusion came about. The human eye is adapted to estimating the relative distances of objects only so long as they are comparatively near. When the distance becomes too great, this ability to differentiate between nearby objects and more distant ones breaks down. Where the stars are concerned, a further problem also arises. Prolonged observation with the naked eye reveals no change in their relative positions; all appear to be moving at a constant rate across the sky owing to the axial rotation of the Earth. The belief that the various bodies of the solar system are closer to us than the stars came from their obvious motions against the background of fixed stellar points, but even here it is only comparatively recently that any accurate determinations of distance were made.
