The Vernal Equinox, where the Sun crosses the Celestial
Equator (red line) on the first day of Spring (March 20/21).
Credit: Space.com/Starry Night

"Can you tell me about the upcoming beginning of the Age of Aquarius?" said the voice on the phone. "I heard that it starts this Saturday…"

Now, I get a lot of phone calls and emails from people with astronomy and sky related questions. Very often it's something like, "What was that thing that tried to land in my front yard yesterday evening?!" or, "Is it true that Mars will be closer to the Earth this weekend than it has been in a gazillion years?" I've even had one or two asking if it's true that the world is ending in 2012.

Okay, I'm embellishing a bit. Those are all very good questions, and I do my best to provide a science-based answer — like, "Venus tried to land in your yard," or "The Mars extra-close encounter happened in 2003… and it had only been less than a century since the previous time," or, "We'll just have to wait for 2012 to roll around to find out…"

As for the Age of Aquarius question, that got me to wondering. I've always regarded this issue as astrology-related more than astronomy, but I also realized there are physical underpinnings to the definition. So I fired up Google and clarified some of the details for myself. The first thing I learned is that, among astrologers at least, there is little agreement on precisely when the Age of Aquarius is supposed to begin (or if it's already begun). Different astrologers at different times and from different parts of the world have tried to define this, resulting in multiple schools of thought on the subject.

But from a purely astronomical standpoint, the delineation of these Ages is based on a natural physical cycle, just as a year is defined by Earth's motion around the Sun and a day is defined by Earth's rotation on its axis.

An astrological Age (aka "Great Year") is determined by the position of the Vernal Equinox — at least by one of the schools of thought… The Vernal Equinox is that point in the sky occupied by the Sun when it crosses the Celestial Equator heading into the Northern Hemisphere. So, you can think of the Vernal Equinox as a distinct point on the sky (and it's easy to locate on the first day of Spring: Just look at the Sun — I take that back: DON'T look at the Sun!)

But the position of the Vernal Equinox shifts over time due to a cycle of change in the orientation of the Earth's rotation. The Earth spins like a top, but also like a top it undergoes a gyrating motion, called precession. One complete gyration takes about 26,000 years — so all of the points in the sky defined by Earth's spinning (the celestial poles and equator, and, yes, the Vernal Equinox) move around the sky over 26,000 years.

At this moment, the Vernal Equinox is in the constellation Pisces — at least, within the region of the sky defined by modern astronomers as encompassing all the stars of Pisces. So, if one were to acknowledge the constellation boundaries according to modern astronomers, then one would say that we are in the Age of Pisces still (and, by the same definition of constellation boundaries, the Vernal Equinox will remain in Pisces until about the year 2600, when it will cross the border into Aquarius.)

However, there is little agreement among different groups of astrologers on where one constellation ends and another begins–and to my knowledge none of them have adopted the modern astronomical boundaries.

So, when does the Age of Aquarius begin? Depends on who you talk to….

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