What is the difference between the tropical and sidereal zodiacs?
The tropical zodiac starts Aries at the spring equinox and keeps the signs tied to the seasons; Western astrology uses it. The sidereal zodiac ties the signs to the stars, as most Indian astrology does. Earth's slow wobble has pulled the two about 24° apart, so most people's sidereal sign is the one before.
Two ways to measure the same sky
Both zodiacs are twelve slices of 30° along the ecliptic. They only disagree on where to start counting. The tropical zodiac starts at the spring equinox, the moment the Sun crosses the celestial equator heading north. The sidereal zodiac starts from a point fixed among the stars. 'Tropical' comes from the Greek for turning, as in the turning of the seasons; 'sidereal' from the Latin sidus, a star. 1
In the first centuries CE, when Western astrology took its classical shape, the two starting points sat close together. Since then they have drifted apart, because Earth's axis wobbles like a spinning top. The equinox slides backwards against the stars about 50 seconds of arc a year, a full circle in roughly 26,000 years. That drift is called the precession of the equinoxes. 2
What the gap means in practice
Today the difference is about 24°, nearly a whole sign. Someone born with the Sun at 10° Leo in the tropical zodiac has it at about 16° Cancer in the sidereal one. People who look up a Vedic chart for the first time are often surprised to find themselves a sign earlier. Neither answer is a mistake; they are measured from different zeros.
In India, where most astrologers use the sidereal zodiac, the government set up a Calendar Reform Committee in 1952 under the physicist Meghnad Saha, with N. C. Lahiri as its secretary, to produce a national calendar and ephemeris. The correction most Indian astrologers apply today carries Lahiri's name. 3
Western astrologers have argued for the tropical zodiac since Ptolemy, who tied the signs to the seasons: Aries begins spring, Cancer midsummer. On that view a sign means a time of year, not a patch of sky. Sidereal astrologers reply that the signs were named after stars and should stay with them. 4,2
How the machine handles it
Calculated facts only, worked out the same way the machine does it.
- The machine uses the tropical zodiac, measured from the true March equinox of date, the convention of Western astrology and of the Swiss Ephemeris that most chart software uses.
- Your sun sign on the card is the tropical sign. It does not compute a sidereal chart.
- The offset between the two grows by about 1° every 72 years.
Where the idea comes from
Era by era, with sources. More in the Reading Room.
- Ancientc. 450 BCEThe Babylonian zodiac settles into twelve equal zones, anchored to reference stars. 5
- Classical2nd century BCEThe Greek astronomer Hipparchus discusses the slow shift of the equinoxes against the stars. 2
- Classical2nd century CEPtolemy measures the zodiac from the equinoxes and solstices, the choice Western astrology still follows. 4,2
- 1950s1952–1957India's Calendar Reform Committee, with N. C. Lahiri as secretary, recommends a national ephemeris; the first Indian Ephemeris and Nautical Almanac appears in March 1957. 3
- 2010s2016NASA reminds readers that the constellations behind the Sun have shifted since Babylon, and that it never changed anyone's sign. 6
Common misconceptions
“NASA changed the signs and added Ophiuchus.” NASA explained the astronomy of the constellations; it did not change any zodiac. The tropical zodiac has always been twelve equal slices tied to the seasons, whatever stars sit behind them. 6
“The sidereal zodiac is the "real" one and the tropical is out of date.” They answer different questions. One follows the seasons, the other the stars. Astrologers have used both for about two thousand years. 1
Questions people ask
Which zodiac does Vedic astrology use?
The sidereal one, corrected for precession. That is why a Vedic chart usually puts your planets about one sign earlier. 1
Will my Western sign ever change?
No. The tropical signs are tied to the equinox, so the dates stay put within a day or so, however the stars drift. 2
How big is the difference right now?
About 24°, a little under one whole sign, and it grows by about 1° every 72 years.
Which one does the machine use?
The tropical zodiac, like nearly all Western astrology.
Sources
- Sidereal, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .
- Precession, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .
- History of the Positional Astronomy Centre, India Meteorological Department, Positional Astronomy Centre, Kolkata. Read on .
- Ptolemy, Tetrabiblos, Book I, chapters 10–19 (aspects, houses of the planets, triplicities), LacusCurtius, University of Chicago (Loeb Classical Library translation by F. E. Robbins, 1940). Read on .
- Review of Francesca Rochberg, The Heavenly Writing: Divination, Horoscopy, and Astronomy in Mesopotamian Culture (reviewed by Eleanor Robson), BMCR 2005.06.29, Bryn Mawr Classical Review. Read on .
- Constellations and the Calendar, NASA (from NASA Space Place), 20 September 2016. Read on .