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Sidereal Birth Chart Calculator

Compare the same birth sky in tropical and sidereal zodiac coordinates. The chart uses an approximate Lahiri ayanamsha, with Whole Sign houses when birth time and place are known.

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How the comparison works

The tropical zodiac starts at the moving vernal equinox. A sidereal zodiac holds a chosen reference orientation while the equinox slowly shifts through precession. The angular difference is called the ayanamsha. For each planet, this page subtracts an approximate Lahiri ayanamsha of the birth date from its apparent geocentric tropical longitude.

Lahiri is one of several sidereal conventions; another ayanamsha can put a point in a different sign near a boundary. Our approximation uses a published Lahiri value at J2000, IAU 2006 general precession, and the Astronomy Engine nutation term. Positions are shown to 0.1° because this is an approximate model. The birth-chart engine covers 1800–2100. Unknown birth time uses a local-noon sample and omits the Moon and time-dependent chart points.

Reading the sidereal comparison

Tropical and sidereal positions

The calculation begins with the apparent geocentric tropical longitude of each planet. It then subtracts a date-specific approximation of the Lahiri ayanamsha, which accounts for precession and a nutation term, to show an estimated sidereal longitude. The two columns describe one sky at the same instant. They do not represent different birth moments or different physical planet locations.

Birth time and houses

A recorded local time, birthplace coordinates, and the historical UTC offset let the calculator find the Ascendant and assign Whole Sign houses from its sidereal sign. If the birth time is unknown, the page uses a local-noon sample, omits the Moon and angles, and does not assign houses. A precise house placement cannot be recovered from a date alone.

Which sidereal convention?

Lahiri is one named sidereal convention. Other ayanamshas can give different degrees, and a point near a sign boundary can even change sign. This page rounds the derived result to one tenth of a degree and labels its precision as approximate. Check the original source note before comparing the result to a professional ephemeris.

Open the expanded sidereal view →