Fig. 544.—Sun’s motion.
On the 21st of March, this path brings the sun on the “ equinoctial” line mentioned at the close of the last chapter. Day and night are then of equal duration as the arcs are equal. So this is the Vernal (or spring) Equinox. Some weeks after the sun is at midday higher up at S´, so the diurnal arc being longer, the day is longer, (Z is the zenith, Z´ is the nadir, P P´ is the celestial axis). From that time he descends again towards the equinoctial to the autumnal equinox, and so on, the diurnal arc becoming smaller and smaller until the winter solstice is reached (S).
Fig. 545.—The ecliptic.
From what has been previously said, it is evident that the sun has a twofold apparent motion—viz., a circular motion obliquely ascending from the horizon, which is explained by the rotation of the earth, and by our position, o, to the earth’s axis, p p´, and also by a rising and setting motion between the solstitial points, S and S´, which causes the inequality of the days and nights. Independently of the daily motion of the sun, we observe that at the summer solstice, on the 21st of June, at midday, the sun is at S´, and one half year later—viz., on the 21st of December, at midnight, the sun is at s, from which he arrives again in the space of half a year at S´; so we are able to represent this annual motion of the sun by a circle, the diameter of which is the line, S´ s. This circle is called the Ecliptic.
The plane of the ecliptic, S´ s, cuts the plane of the equinoctial, A Q, at an angle of 23½°, and the axis of the ecliptic, S″´ s″, makes the same angle with the axis of the heavens, P P. The two parallel circles, S´ s´ and S s, include a zone extending to both sides of the equinoctial, and beyond which the sun never passes. These circles are called the Tropics, from τρέπω, I turn, because the sun turns back at these points, and again approaches the equinoctial. The parallel circles, S″ s″, and S″´ s″´, described by the poles of the ecliptic, S″´ s″, about the celestial poles, P P, are called the arctic and antarctic circles.
Whenever the sun crosses the equinoctial, there is the equinox; but the points of intersection are not invariably the same every year. There is a gradual westerly movement, so it is a little behind its former crossing place every year. (See diagram, fig. 547.)
Fig. 546.—The Seasons.