These are the points where the sun crosses the celestial equator, which he necessarily does twice in his yearly circuit of the zodiac, at two opposite points, distant from each other in space 180°, and in time six months. The point where the sun crosses in spring, coming north, is called the vernal or spring equinox; and the other, where he crosses six months afterward, going south, is called the autumnal equinox. At these periods the days and nights are equal, and that is the reason why they are called equinoctial points, from two Latin words, aquus, equal, and nox, night. These two points are in the signs Aries and Libra (—), and are so marked on the diagram of the zodiac.
The relative positions of the equinoctial and solstitial points and the celestial equator will be better understood from the following diagram. Imagine a hoop lying horizontally, and within this another hoop teaching the first, and with one side elevated above the other, as represented in the diagram: The horizontal hoop, marked A B, is the equator; the other, and around which the signs of the zodiac are displayed, is the ecliptic, or apparent path of the sun. The earth is in the center, with its equator on the same plane with the celestial equator. The equator of the earth is marked e e. The line f f is on the same plane as the ecliptic. The two other lines, one above and one below the equator of the earth, and parallel to it, are the tropics of Cancer and Capricorn, parallel with the same lines extended in the heavens, and marked as the tropics.
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