Page:Popular Astronomy - Airy - 1881.djvu/89

Rh mid- winter; after which, he turns towards the first point of Aries.

Now, upon examining that curve or ecliptic, we find that it is one of those circles which are called "great circles"; they divide the glohe into equal halves. I have stated this as a thing which would be one of rough evidence; that if, by means of the observations I have mentioned, (namely, how long the sun is in passing the meridian after a star, and what is its angle of elevation when it passes the meridian,) if by means of these we mark on a globe the successive places of the sun, we shall find all the successive points in a curve, such as I have described. When I say that it goes on the globe in such a curve, you must understand that there are ways of computing these things; that having got the interval of time between the sun and a star in passing the meridian, and the angular elevation of the sun when it does pass the meridian, it is possible, by computation, to find whether it is going on in such a curve as I have described; but that these computations, though they amount to the same thing as dotting the sun's places down on the globe, are a great deal more accurate. It is thus found that the ecliptic, or apparent path of the sun through the stars, is accurately a great circle, or one which divides the globe into equal halves.

The first point of Aries, as I have said, is one of the places where the ecliptic crosses the equator; it is the point which the sun passes at the beginning of spring; it is not marked by any star, or fixed object of any kind. Though it is an imaginary point, yet, from having a series of places of the sun, defined by the difference of times at which the sun and a bright star passes the meridian, we can tell as exactly where