The difference between the tropical and sidereal zodiacs, and how that relates to an age.
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The zodiac is the band of constellations that lay behind the Sun from our perspective as we orbit it; it is by definition the plane of the solar system. The Sun never moves out of the zodiac, and the stars/constellations never change with respect to one another (at least not on time scales relevant to human civilization).
In astrology, they divide the zodiac into twelve sections, each with its own sign which are named after the twelve constellations in the zodiac (actually there are thirteen constellations but one is ignored).
“Tropical” astrology uses 12 equal divisions for the zodiac, like a clock, and the “midnight” on the clock is always one of the two points where the zodiac crosses the equator. This point is called the “vernal equinox” and the Sun reaches this point around March 21 every year. In tropical astrology, this is the beginning of Aries.
The trick is, this equinox point is not fixed. It drifts over time, because the Earth is slowly wobbling in a circle like a top that’s losing its momentum. The poles gradually point to other places over time (in other words, Polaris has only been the actual North Star for a couple of centuries, and in a few more centuries it won’t be the North Star any more). Since the poles are pointing to different places, the equator is also aligned differently, and so the equinox moves down the zodiac over time.
“Sidereal” astrology accounts for this. Instead of dividing the zodiac into twelve fixed signs, it fixes the signs to the actual constellations they’re named after. When the Sun is in the constellation Aries, that is for sidereal astrologers the actual sign of Aries. When astrology was first thought up, the sidereal Aries and the tropical Aries were identical. But since the equinox has drifted down the zodiac over time, they are now a month apart…the Sun is actually in Aries from April 15 to May 15.
Effectively, tropical astrology becomes more and more inaccurate over time, because its zodiac is fixed to the calendar, while sidereal astrology accounts for the drift of the sky.
The “age” in astrology refers to which constellation the vernal equinox is in. When the thing was invented, it was the “first point of Aries” and it was the Age of Aries. For the last 2000 years or so it has been in Pisces, so we are in the Age of Pisces. In another couple of centuries it will have drifted into Aquarius.
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