Sidereal astrology, explained with your own chart

Sidereal astrology is a way of casting a birth chart that fixes the twelve zodiac signs to the background of the stars. The word sidereal comes from the Latin for star. When a sidereal astrologer says your Sun is in a sign, they mean the Sun was in front of that stretch of sky as seen from Earth on the day you were born.

The usual Western method, called tropical astrology, does something different. It ties the signs to the seasons and starts the zodiac at the March equinox. Sidereal astrology starts it at a fixed point among the stars instead. Both systems cut the sky into twelve equal signs of 30 degrees, so the difference is only where the count begins. That one choice is why your sidereal Sun, Moon and Rising can be different from the signs you grew up with.

The ayanamsa is the angle between the two zodiacs. Earth's axis wobbles very slowly, completing one turn in about 26,000 years, and as it does the March equinox slides backwards against the stars. The tropical zodiac slides with it and the sidereal zodiac does not. By the Lahiri reckoning the two lined up around the third century of the common era, and the gap has been growing ever since: it is about 24.2 degrees in 2026, and it widens by roughly 1 degree every 72 years.

To turn a tropical position into a sidereal one, subtract the ayanamsa. To go the other way, add it. Different traditions pick slightly different anchor stars, so the ayanamsa has several versions that differ by a fraction of a degree to a degree or so. This page uses Lahiri, the standard in India.

Roughly speaking, every planet moves back by one sign for people whose tropical degree is under about 24, and stays put for the rest. Because of that, about four in five placements in a typical chart change sign. A few things are worth knowing before you compare readings.

• The Sun sign usually changes if you were born in the first 24 days or so of a tropical sign.
• The Moon sign matters more in sidereal practice and is often read before the Sun.
• The Rising sign shifts too, and with it the whole layout of houses, so topics like career or partnership can look different.
• The element, such as fire or water, often changes with the sign, which can change the tone of a description.

Interpretation is a separate matter from measurement. Two astrologers using the same sidereal chart can still read it differently, and the same is true in the tropical world.

These results come from the same engine behind the tool, using the Lahiri ayanamsa. Each pair shows the tropical position first and the sidereal one second.

The first birth shows the typical case, with all three moving back one sign. The Sun at 10 degrees of Capricorn becomes 16 degrees of Sagittarius once the 23.86 degree ayanamsa is subtracted.

The second shows the opposite. The Sun is late in Leo, so subtracting 23.66 degrees leaves it early in the same sign, and the Moon stays in Libra too. In the third, the Sun at 24 degrees of Taurus lands just past the start of Taurus and stays, while the Moon and Rising both step back a sign.