7. The equinox is the time when the sun enters the equinoctial points, - viz., March 21 and Sept. 22; the former being called the vernal or spring equinox, and the latter the autumnal: therefore the sun is exactly on the equator twice a year.
But, in consequence of the gradual change in the obliquity of the axis, the obliquity of the ecliptic is brought about. The time of the equinox is found each year to have advanced a few minutes (twenty-five minutes seven seconds). It is this advance which is called the precession of the equinoxes from the Latin proecedere, to march before,” derived from proe, signifying “before,” and cedere, signifying “to go away.”
These few minutes, after a long time, make years. The result is that the equinox of the springtime, which now arrives in March, will arrive in a given time in February, then in January, then in December; and then the month of December will have the temperature of March, and March that of June, and so on in succession, until, returning to the month of March, it will be found as at present, which will be 25,868 years from now. Then it will recommence the same revolution indefinitely. *
* The procession of the equinoxes leads to another change, that which has been brought to pass in the signs of the Zodiac. The earth going around the sun in a year, according as it advances, the sun finds itself every month opposite a new constellation; these are twelve in number; viz., Taurus, Aries, Pisces, Aquarius, Capricorn, Sagittarius, Scorpio, Libra, Virgo, Leo, Cancer, Gemini. These are the signs of the Zodiac they form a circle in the plan of the terrestrial equator. According to the month of the birth of an individual, they say that he was born under such a sign: whence the prognostications of astrology. But, in consequence of the precession of the equinoxes, it happens that the months do not correspond to constellations as they did two thousand years ago. For instance, a person born in July is not now born in the sign Leo, but in that of cancer: thus falls the superstition attaching to signs (chap. V, n° 12).