We now know that the earth rotates on its axis, but this idea was of course quite unknown to these early peoples. Since the earth rotates, with stars infinitely removed surrounding it on all sides, the apparent movements of the stars will depend very much upon the position we happen to occupy on the earth. An observer at the North Pole of the earth, for instance, would see the stars moving round in circles parallel to the horizon ([Fig. 2]). No star could therefore either rise or set—one half of the heavens would be always visible above his horizon, and the other half invisible. An observer at the South Pole would of course see that half of the stars invisible to the observer at the northern one.
If the observer be on the equator, the movements of the stars will appear to be as indicated in this diagram ([Fig. 3])—that is, all the stars will rise and set, and each star will be, in turn, twelve hours above the horizon, and the same time below it. But if we consider the position of an observer in a middle latitude, say at Stonehenge, we find that some stars will always be above the horizon, some always below—that is, they will neither rise nor set. All other stars will both rise and set, but some of them will be above the horizon for a long time and below for a short time, whereas others will be a very short time above the horizon and a long time below it, each star completing a circle in a day ([Fig. 4]).
Wherever we are upon the earth we always imagine that we are on the top of it. The idea held by all the early peoples was that the surface of the earth near them was an extended plain: they imagined that the land that they knew and just the surrounding lands were really in the centre of the extended plain. Plato, for instance, was content to think the Mediterranean and Greece upon the top of a cube, and Anaximander placed the same region at the top of a cylinder.
By the use of a terrestrial globe we can best study the conditions of observation at the poles of the earth, the equator and some place in middle latitude. The wooden horizon of the globe is parallel to the horizon of a place at the top of the globe, which horizon we can represent by a wafer. By inclining the axis of the globe and watching the movement of the wafer as the globe is turned round, we can get a very concrete idea of the different relations of the observer’s horizon to the apparent paths of the stars in different latitudes.
We have next to deal with the astronomical relations of the horizon of any place, in connection with the observation of the sun and stars at the times of rising or setting, when of course they are on or near the horizon; and in order to bring this matter nearer to the ancient monuments, we will study this question for both Thebes and Stonehenge. We may take the latitude of Thebes as 25°, Stonehenge as 51°, and we will begin with Thebes.
To consider an observer on the Nile at Thebes and to adjust things properly we must rectify a celestial globe to the latitude of 25° N., or, in other words, incline the axis of the globe at that angle to the wooden horizon.
Since all the stars which pass between the North Pole and the horizon cannot set, all their apparent movements will take place above the horizon. All the stars between the horizon and the South Pole will never rise. Hence, stars within the distance of 25° from the North Pole will never set at Thebes, and those stars within 25° of the South Pole will never be visible there. At any place the latitude and the elevation of the pole are the same. It so happens that many of those places with which archæologists have to do in studying the history of early peoples—Chaldæa, Egypt, Babylonia, &c.—are in low middle latitudes, therefore we have to deal with bodies in the skies which do set and bodies which do not, and the elevation of the pole is neither very great nor very small. But although in each different latitude the inclination of the equator to the horizon as well as the elevation of the pole will vary, there will be a strict relationship between the inclination of the equator at each place and the elevation of the pole. Except at the poles themselves the equator will cut the horizon due east and due west; therefore every celestial body to the north of the celestial equator which rises and sets will cut the horizon between the east and west point and the north point; those bodies which do not rise will of course not cut the horizon at all.
The stars near the equator, and the sun, in such a latitude as that of Thebes, will appear to rise or set at no very considerable angle from the vertical; but when we deal with stars very near to the north or south points of the horizon they will seem to skim along the horizon instead of rising directly.
We now pass on to Stonehenge. To represent the new condition the axis of the globe will now require to be inclined 51° to the horizon. The number of northern stars which do not set and of southern stars which do not rise will be much greater than at Thebes. The most northern and southern stars visible will in their movement hug the horizon more closely than was observed under the Thebes condition.
The sun, both at Thebes and Stonehenge, since it moves among the stars from 231⁄2° N. to 231⁄2° S. each year, will change its place of rising and setting at different times of the year.