The earth's orbit lies in the plane of the ecliptic; but it extends only an inappreciable distance from the sun towards the celestial sphere. [Illustration: Fig. 31.] 23. _The Obliquity of the Ecliptic._--The ecliptic is inclined to the celestial equator by an angle of about 23-1/2°. This inclination is called the _obliquity of the ecliptic_. The obliquity of the ecliptic is due to the deviation of the earth's axis from a perpendicular to the plane of its orbit. The axis of a rotating body tends to maintain the same direction; and, as the earth revolves around the sun, its axis points all the time in nearly the same direction. The earth's axis deviates about 23-1/2° from the perpendicular to its orbit; and, as the earth's equator is at right angles to its axis, it will deviate about 23-1/2° from the plane of the ecliptic. The celestial equator has the same direction as the terrestrial equator, since the axis of the heavens has the same direction as the axis of the earth. [Illustration: Fig. 32.] 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. [Illustration: 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. [Illustration: Fig. 34.]