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).
Astrologers do not agree on when the Aquarian age will start or even if it has already started. Nicholas Campion in The Book of World Horoscopes lists various references from mainly astrological sources for the start of the Age of Aquarius. Based on Campion’s research, most published materials on the subject state that the Age of Aquarius arrived in the 20th century (29 claims), with the 24th century in second place with twelve claimants.
Astrological ages exist as a result of precession of the equinoxes. The slow wobble of the earth’s spin axis on the celestial sphere is independent of the diurnal rotation of the Earth on its own axis and the annual revolution of the earth around the sun. Traditionally this 25,800-year-long cycle is calibrated, for the purposes of determining astrological ages, by the location of the sun in one of the twelve zodiac constellations at the vernal equinox, which corresponds to the moment the sun rises above the celestial equator, marking the start of spring in the Northern Hemisphere each year. Roughly every 2,150 years the sun’s position at the time of the vernal equinox will have moved into a new zodiacal constellation. However zodiacal constellations are not uniform in size, leading some astrologers to believe that the corresponding ages should also vary in duration. This however is a contentious issue among astrologers.
In 1929 the International Astronomical Union defined the edges of the 88 official constellations. The edge established between Pisces and Aquarius officially locates the beginning of the Aquarian Age around AD 2600. Many astrologers dispute this approach because of the varying sizes and overlap between the zodiacal constellations