After knowing and singing this song since age 6 I finally learned what it means in both New Age and Astrophysical sense

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en.wikipedia.org/wiki/Axial_precession
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www.answers.com/topic/aquarian-age-1

An age of universal brotherhood and enlightenment that many believe humanity is presently entering. It is often identified with the New Age. While astrologers have used it as a technical term for many years, it was popularized by the Broadway musical Hair with the song “This Is the Dawning of the Age of Aquarius.” Astrologers believe that the Earth enters a new zodiacal age every 2,160 years, a figure calculated by the changing position of the Sun at the spring equinox each year. Among astrologers and occultists, there is no consensus on the date for the beginning of the Age of Aquarius. According to some calculations, the Aquarian Age began as early as the seventeenth century, while others place it as late as the twenty-first century.