Equinox

Equinox, in Astronomy, is the time when 'the sun apparently crosses the plane of the equator. It is sometimes understood as the equinoctial point, or point where this crossing is effected. It occurs twice a year - on the 21st of March and the 22nd of September; the first is called the vernal equinox, and the second the autumnal. At these times, the sun being directly over the equator, the days and nights are of equal length; at the vernal equinox the days are becoming longer than the nights, and at the autumnal equinox the days are-becoming shorter. The vernal equinoctial point is known as the first point in Aries, and is important in star measurements. The line joining the two equinoctial points is called the line of the equinoxes, and the fact of the gradual change in position of this line is of much importance in astronomy. The earth's axis remains at a constant angle to the plane of the sun's apparent motion, but does not remain constant in direction. Just as a spinning top may wobble, with its mass-centre motionless and the extremities of its axis describing circles, so that this axis is for some time at the same angle to the ground, so the earth's axis wobbles at a constant angle to the plane of the ecliptic. This affects the line of the equinoxes, and causes it to slowly travel round; hence the term precession of the equinoxes in describing this irregularity of the earth's motion, The lengths of summer and winter depend on the position of the equinoctial points. If we divide the year into two parts, calling that portion summer between vernal and autumnal equinoxes, and the rest of the year winter, we may say that our winter in the northern hemisphere is eight clays shorter than summer. But with a different position of the line of equinoxes an extreme winter thirty-three days longer than the corresponding summer may occur, or an extreme summer thirty-three days longer than the corresponding winter. Such extremes are 10,500 years apart, but they represent widely different conditions of climate; the first extreme gives us a glacial epoch and the second a genial epoch. Our present condition is intermediate, in the northern hemisphere tending to a genial epoch but in the southern tending to a glacial epoch. [Earth, Precession, Nutation.]