EQUINOX, in astronomy, is that time of the year when the day and night are equal: the length of the day is then 12 hours; the sun is ascending six hours, and descending the same time. This is the case twice a year, in spring and in autumn, when the sun is on the equator. When the sun is in this situation the horizon of every place is divided into two equal parts by the circle bounding light and darkness, generally. The vernal equinox is on 21 March, and marks the beginning of spring; the autumnal is generally on 23 September, which is considered the commencement of au tumn; at all other times the length of the day and of the night are unequal, and their differ ence is the greater the more we approach either pole, and in the same latitude it is everywhere the same. On the equator this inequality en tirely vanishes; there, during the day, which is equal to the night, the sun always ascends six hours and descends six hours. In the opposite hemisphere of our earth the inequality of the days increases in proportion to the latitude: the days increase there while they diminish with us, and vice versa. The points where the ecliptic intersects the equator are called equi noctial points. The vernal equinoctial point was formerly at the entrance of the constella tion of Aries; hence the next 30 degrees of the ecliptic, reckoned eastward, have been called Aries; but this point long ago deserted the constellation of Aries, and now stands in Pisces; for it is found by observation that the equinoctial points, and all the other points of the ecliptic, are continually moving backward or westward; which retrograde motion of the equinoctial points is what is called the precession of the equinoxes. This retrograde motion is quite analogous to the revolution of a gyroscope weighted at one end and balanced in the middle. It is due to the gravitational pull exerted by the sun and moon on the equa torial protuberances of the earth, which is a spheroid, not a sphere. It appears from the
result of calculations that the path of the poles of the equator is nearly a circle, the poles of which coincide with those of the ecliptic, and that the pole will move along that circle so slowly as to accomplish the whole revolution in about 25,800 years. The diameter of this circle is equal to twice the inclination of the ecliptic to the equator, or about 47°. Now, as the ecliptic is a fixed circle in the heavens, hut the equator, which must be equidistant from the poles, moves with the poles, therefore the equator must be constantly changing its inter section with the ecliptic. And from the best observations it appears that the equator cuts the ecliptic every year 50.25" more to the west ward than it did the year before; hence the sun's arrival at the equinoctial point precedes its arrival at the same fixed point of the heavens every year by 20 minutes 23 seconds of time, or by an arc of 5025". Thus, by little and little, these equinoctial points will cut the ecliptic more and more to the westward, till, after 25,800 years, they return to the same point. The pre cession of the equinoxes is not absolutely uni form, for the forces inducing the precession de pend on the position of the sun and moon with reference to the earth. Thus at the vernal and autumnal equinoxes the sun is in the plane of the equator, and can cause no precession. Another important factor in causing the pre cession to vary is the fact that the plane of the moon's orbit is somewhat inclined to the eclip tic. Besides causing the component of the moon's attraction in the plane of the ecliptic to vary, this inclination introduces a component perpendicular thereto. This is an important factor in causing the path of the pole of the equator to be a wavy curve rather than a pre cise circle. This wavy motion is known as nutation. See DAY; EcLurric; EQUATOR; GYROSCOPE.