"Age of Aquarius" is an astrological term denoting either the current or forthcoming astrological age, depending on the method of calculation. Astrologers maintain that an astrological age is a product of the earth's slow precessional rotation and lasts for 2,160 years, on average (26,000-year period of precession / 12 zodiac signs = 2,160 years).
There are various methods of calculating the length of an astrological age. In sun-sign astrology, the first sign is Aries, followed by Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces, whereupon the cycle returns to Aries and through the zodiacal signs again. Astrological ages, however, proceed in the opposite direction ("retrograde" in astronomy). Therefore, the Age of Aquarius follows the Age of Pisces.
The approximate 2,150 years for each age corresponds to the average time it takes for the vernal equinox to move from one constellation of the zodiac into the next. This can be computed by dividing the earth's 25,800-year gyroscopic precession period by twelve, the number of zodiac constellations used by astrologers. According to different astrologers' calculations, approximate dates for entering the Age of Aquarius range from AD 1447 (Terry MacKinnell) to AD 3597 (John Addey).