An astrological age is defined by the constellation of the zodiac in which the vernal equinox lies. Astrological ages change because of a phenomenon known as the precession of the equinoxes. One complete period of this precession is about 25,925 years [2002 AD].
Overview
Earth, in addition to its diurnal rotation upon its axis, has a precessional motion involving a slow periodic shift of the axis itself; approximately one degree every 70 years. This motion, which is caused mostly by the Moon's gravity, gives rise to the precession of the equinoxes in which the Sun's position on the ecliptic at the time of the equinoxes, measured against the background of fixed stars, shifts gradually with time.
In astrology the astrological age is defined by the constellation in which the Sun appears during the vernal equinox. Since each sign of the zodiac subtends (on average) 30 degrees, each astrological age might be thought to last 70 × 30 = 2,100 years. This means the Sun crosses the equator at the vernal equinox moving backwards from year to year at the rate of one degree in seventy-two years, one constellation in about 2156 years, and the whole twelve signs in about 25 868 years (called a Great Sidereal Year).
This is an intellectual division of the circle of the zodiac which coincides with the constellations in the sky only once in about 25 868 years.
The last time when the starting-point in the intellectual zodiac agreed with the zodiacal constellation was in 498 A.D. A year after these points were in exact agreement, the Sun crossed the equator about fifty seconds of space into the constellation Pisces. The year following it was one minute and forty seconds into Pisces, and so it has been creeping backwards ever since, until at the present time the Sun crosses the equator in about nine degrees in the constellation Pisces. It will thus be about 600 years before it actually crosses the celestial equator in the constellation Aquarius.
In simple words, this means that the current astrological Age of Pisces began about 500 A.D., since it was the last time that, astronomically, the vernal equinox occurred in the first point of the constellation Aries leaving it and entering the constellation Pisces (thus at that time the intellectual and natural zodiacs agreed). Nowadays, the vernal equinox occurs astronomically in about nine degrees of the constellation Pisces and it will be only about 2600 A.D. it actually finishes moving backwards through all the 30 degrees of Pisces and enters the constellation Aquarius.
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