Jupiter and Tai Sui: Why Your Zodiac Year Keeps Falling Behind Jupiter

Your Chinese zodiac year (the year of your own animal, which comes round every twelve years) and "offending Tai Sui" are both about the year's Earthly Branch, and the year branch originally came from Jupiter. Jupiter circles the sky in about twelve years, and Western astrology calls its return to your birth position a Jupiter return, which also happens around 12, 24 and 36. But "about" matters: one Jupiter orbit is not 12 years but about 11.86, so each twelve-year cycle falls a little further behind Jupiter.

Tai Sui came from Jupiter

Ancient Chinese astronomers saw that Jupiter took about twelve years to go round the sky, so they divided the sky into twelve stations and used the one Jupiter occupied to name the year; that is why Jupiter was called the Year Star. The problem was that Jupiter moves west to east, the opposite of the direction the twelve Earthly Branches were counted, so they imagined a counterpart moving the other way, Tai Sui (太歲), and gave it one branch per year.

The stem-and-branch year count, the zodiac animals, your zodiac year and offending Tai Sui all grew out of this "Tai Sui calendar". So the original meaning of your zodiac year is roughly "Tai Sui has come back to where it was in your birth year", and Tai Sui was Jupiter's shadow to begin with.

11.86 years vs. 12

Jupiter actually orbits the Sun in about 11.86 years, some 50 days short of twelve. Our engine tracked Jupiter daily from 1900 to 2100:

Jupiter's longitude calculated daily for 1900–2100 by the birth chart engine (astronomy-engine).

In other words, every eighty-some years Jupiter gets a whole station ahead of the Tai Sui count, which the ancients called the Year Star "skipping a station". This made years named by Jupiter's real position increasingly messy, and the calendar switched to simply counting stems and branches in order without looking at the sky, as it still does today. Today's year branch has nothing to do with Jupiter; only the names and the idea remain.

Where is Jupiter in your zodiac years?

Put the two together: on each birthday at 12, 24, 36 and so on, how far is Jupiter from where it was at birth? We took a birthday every seven days from 1900 to 1990 (4,749 in all):

Jupiter's longitude calculated daily by the birth chart engine; distance is the shorter arc between the two.

At 12, Jupiter is almost back where it started, still in the same sign for 85.9% of people; at 60 it is on average 21.7° away; by 84 it is off by nearly a whole sign, still in the original sign for only 5.9%. Your zodiac year and your Jupiter return coincide when you are young and separate as you age.

Same animal, different Jupiter signs

You can see it from the other side too: if the year branch still followed Jupiter, everyone born under the same animal would have Jupiter in the same sign. Here is Jupiter at Start of Spring each year from 1900 to 2099:

Jupiter's longitude one hour after each year's Start of Spring (solar-term times from lunar.js), calculated by the birth chart engine.

Rat years in the early 1900s had Jupiter in Sagittarius; by the 2090s they have it around Aquarius. That is the station-skipping adding up.

Jupiter returns and zodiac years: what each is about

Western astrology sees Jupiter as growth, expansion and opportunity; a Jupiter return closes one growth cycle and opens the next, and is often used to look at turning points around 12, 24 and 36. The Chinese folk zodiac year leans toward caution: the year shares your birth year's branch, and tradition says it brings more change and calls for care; for details see Tai Sui.

Both start from "roughly every twelve years", the trace of the same planet; but one follows Jupiter's real position and the other a year branch long since cut loose from Jupiter, so the older you get, the further apart they fall. The calculator above works out the actual age of each of your Jupiter returns.

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