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What a sidereal birth chart is

A sidereal birth chart maps the sky at your birth using a zodiac measured from the fixed stars. The Western, tropical zodiac is measured from the position of the Sun at the spring equinox. When you compare sidereal vs tropical charts, both look at the exact same sky from two different starting points.

Both systems divide the ecliptic into twelve 30° signs with the same names, from Aries through Pisces. Both zodiacs are valid systems, and neither one replaces the other. Because it's anchored to the stars rather than the seasons, the sidereal zodiac doesn't drift with the wobble of Earth's axis.

Why your signs shift by about one sign

The gap between the Western and the sidereal zodiac is called the ayanamsa, and it grows by about 50 seconds of arc a year. Under the Lahiri standard, the ayanamsa was 23°09′ on January 1, 1950, 23°51′ on January 1, 2000, and is 24°14′ in October 2026. Because of this gap, a planet in the first 24° of a Western sign usually sits in the previous sign in the sidereal zodiac.

In our test of 4,000 random moments from 1950 to 2010, the Vedic Sun sign differed from the Western one on 79.0% of them. The Moon sign differed on 78.4% of the moments in that same test. In our example chart for March 14, 1991, at 4:18 AM in Chicago, the two zodiacs were 23°44′ apart. You'll see the Western Sun at 23°20′ Pisces and Moon at 0°04′ Pisces shift to 29°36′ Aquarius and 6°20′ Aquarius.

Sidereal sign dates

Because of the 24° offset, sidereal zodiac dates fall about three and a half weeks later than Western Sun sign dates. The Sun enters sidereal Aries around April 14 and Capricorn around January 14, and each sign lasts about a month. In 2026 UTC, for example, the Sun enters Taurus on May 15, Leo on August 17, and Scorpio on November 16. The table below gives the exact dates when the Sun enters each sidereal sign in 2026 and 2027.

People often treat sidereal astrology and Vedic astrology as the same term, though they have different scopes. Sidereal refers only to the star-based zodiac used to measure longitudes. Vedic is the whole system, combining the sidereal zodiac with whole-sign houses, 27 lunar segments of 13°20′ called nakshatras, and degree-based significators called karakas.

Lahiri and other sidereal systems

Traditions define the zero point of the sidereal zodiac with slightly different ayanamsa values. This site uses the Lahiri ayanamsa, adopted by India's Calendar Reform Committee in 1955 for the national calendar and the Indian Astronomical Ephemeris.

Other sidereal systems use slightly smaller or larger offsets. On January 1, 2026, the Krishnamurti, or KP, ayanamsa is 5′49″ smaller than Lahiri, and Raman is 1°26′47″ smaller. Fagan–Bradley, the Western sidereal system, is 53′00″ larger than Lahiri on January 1, 2026. Most placements stay in the same sign across these systems, differing only when a planet sits near a sign boundary.

Not 13 signs

Sidereal astrology is sometimes confused with 13-sign astronomy charts that add Ophiuchus between Scorpio and Sagittarius. Sidereal astrology uses 12 equal signs of 30° each, just like the tropical wheel. The 13 constellations with Ophiuchus are unequal star regions used in astronomy and in a different system. This site uses the 12 equal sidereal signs and doesn't use the 13 unequal constellations.

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