We have, therefore, by the aid of the stars, determined the position of the earth’s axis, and by this latter we can assign to the equator its proper place. For if pp´ be the earth’s axis, aq´ is the greatest circle drawn round the earth, equally distant from both poles, and the plane of which cuts the earth’s axis at right angles.
Furthermore, let us suppose the plane of the equator to be extended till it reach the celestial concave; we thus find the place of the celestial equator, A Q, or equinoctial, as it is generally termed in opposition to the equator, which always means the terrestrial equator. The equinoctial divides the heavens into the northern and southern hemispheres. We cannot actually describe the equinoctial and make it visible, but we can imagine its line of direction by observing those stars through which it passes.
We are now in a condition to assign to an observer different stations in relation to the earth’s axis on the earth’s surface, which will essentially modify the aspects under which celestial phenomena are represented. One of these stations may be supposed to be at one of the two poles, for example, at p, or at any one point of the equator, as at a, or, finally, on any portion of the surface of the earth which lies between the pole and the equator, as, for example, o.
Fig 543.—Great Nebulæ in Orion.
CHAPTER XXXVI
THE SUN.
MOTION OF THE SUN—THE SEASONS—CHARACTER OF THE SUN—SUNSPOTS—ZODIACAL LIGHT.
Suppose that we rise early in the morning we shall, as the reader will say see the sun rise—that is, he appears to us to rise as the earth rotates. By the accompanying diagram (fig. 544) we can understand how Sol makes his appearance, and how he comes up again; not, it will be observed, after the manner stated by the Irishman, who declared that the sun “went down, and ran round during the night when nobody was looking.” The earth rotates from west to east, and so the sun appears to move from east to west. If we look at the diagram we shall see that after rising at O, the sun advances towards the meridian in an oblique arc to A, the highest or culminating point—midday. He then returns, descending to W; this path is the diurnal arc. At Q similarly, during his passage in the nocturnal arc, he reaches the lowest or inferior culmination. HH´ is the meridian.