Suppose the globe at the centre of the tub (Fig. 31) to represent the sun, and the smaller globes to represent the earth in various positions in its orbit. The surface of the water will then represent the plane of the ecliptic, and the rod projecting from the top of the earth will represent the earth's axis, which is seen to point all the time in the same direction, or to lean the same way. The leaning of the axis from the perpendicular to the surface of the water would cause the earth's equator to be inclined the same amount to the surface of the water, half of the equator being above, and half of it below, the surface. Were the axis of the earth perpendicular to the surface of the water, the earth's equator would coincide with the surface, as is evident from Fig. 32.
Fig. 33.
24. The Equinoxes and Solstices.—The ecliptic and celestial equator, being great circles, bisect each other. Half of the ecliptic is north, and half of it is south, of the equator. The points at which the two circles cross are called the equinoxes. The one at which the sun crosses the equator from south to north is called the vernal equinox, and the one at which it crosses from north to south the autumnal equinox. The points on the ecliptic midway between the equinoxes are called the solstices. The one north of the equator is called the summer solstice, and the one south of the equator the winter solstice. In Fig. 33, EQ is the celestial equator, EcE'c' the ecliptic, V the vernal equinox, A the autumnal equinox, Ec the winter solstice, and E'c' the summer solstice.
Fig. 34.
25. The Inclination of the Ecliptic to the Horizon.—Since the celestial equator is perpendicular to the axis of the heavens, it makes the same angle with it on every side: hence, at any place, the equator makes always the same angle with the horizon, whatever part of it is above the horizon. But, as the ecliptic is oblique to the equator, it makes different angles with the celestial axis on different sides; and hence, at any place, the angle which the ecliptic makes with the horizon varies according to the part which is above the horizon. The two extreme angles for a place more than 23-1/2° north of the equator are shown in Figs. 34 and 35.
The least angle is formed when the vernal equinox is on the eastern horizon, the autumnal on the western horizon, and the winter solstice on the meridian, as in Fig. 34. The angle which the ecliptic then makes with the horizon is equal to the elevation of the equinoctial minus 23-1/2°. In the latitude of New York this angle = 49° - 23-1/2° = 25-1/2°.
Fig. 35.