Precession and the Tropical and Sidereal Zodiacs: Where Does 0° Aries Begin?

‘Two thousand years ago, the equinox Sun was in Aries, but today’s equinox Sun is against Pisces.’ This observation often becomes a challenge to astrology: if your Aries is not the Aries in the sky, is the system wrong?

It is a useful question, and the answer involves an astronomical motion so slow you cannot notice it within a lifetime: precession. Understanding it also explains why Western and Indian astrology assign different signs to the same planet.

In Zodiac Signs and Ecliptic Longitude, you learned that the tropical zodiac defines 0° Aries at the March equinox. Now we examine precession and the distinction between the two zodiacs in depth.