Page:Popular Astronomy - Airy - 1881.djvu/88

74 North Pole in summer, and further from it in winter. These are the general facts deduced from our observations. But, supposing we do not trust to them; supposing we make use of the Transit Instrument, and the Mural Circle. By the former we observe how long the sun is passing the meridian after a star; we find it is later and later every day; that every time the sun comes to the meridian, he has travelled (with respect to the stars) towards the left or towards the east, in such a manner, that, in the apparent diurnal motion, or passage, he is later and later, with respect to the stars, every day. Suppose that with the Mural Circle we observe the altitude of the sun every day when he passes the meridian; we find that he is nearer to the North Pole in summer than in winter. Thus, the sun travels to the left, and at the same time changes his distance from the North Pole.

Now, putting these things together, if we were to dot on the globe a succession of observed places of the sun, we should find they would follow each other in a curve, like that marked on Figure 24, (see Frontispiece,) which represents two views of a celestial globe, on opposite sides. This curve is called the ecliptic. It is convenient now to refer our description of this curve to the equator, which is the great circle on the globe equally distant from both Poles. Now, we find that the sun's path or ecliptic, crosses the equator at two points. One of these is called the First Point of Aries, and this is the sun's apparent place at the beginning of spring; from this point the sun appears to approach nearer and nearer to the North Pole until midsummer. He then appears to recede from the North Pole, and crosses the equator in the first point of Libra, at the beginning of autumn, and approaches the South Pole till