Sidereal and tropicaltwo ways to anchor the same road

The Earth spins like a top, and like a top it wobbles. Its axis traces a very slow circle against the stars, making one full turn in about 25,800 years. This is precession, and it was first measured around 130 BCE by the Greek astronomer Hipparchus, who compared his own observations of the star Spica with older Babylonian records.

The effect is small each year, about 50 seconds of arc, which adds up to one degree every 72 years. It has a surprising consequence. The point where the Sun crosses the celestial equator in March, the vernal equinox, slides slowly backward through the stars. Two thousand years ago it was in the constellation Aries. Today it is in Pisces.

A zodiac needs a zero point. There are two natural choices.

Both zodiacs coincided about 1,700 years ago, in roughly the third to fifth century CE, depending on the choice of fixed star. Since then they have separated by a bit more than 24 degrees. That gap has a name.

The gap between the two zodiacs is the ayanamsa. In 2026 it is a little over 24 degrees, and it grows by about 50 seconds each year. To find a planet’s sidereal position, subtract the ayanamsa from its tropical longitude.

The ayanamsa is not a single agreed number. It depends on where the sidereal zero is placed, and several choices are in use. The one adopted by the Government of India’s Calendar Reform Committee in 1955 is called Lahiri, or Chitrapaksha, after the star Spica (Citrā) that anchors it. Other widely used choices include Raman and Krishnamurti. They differ by up to roughly a degree and a half.

That difference matters in practice. A planet near the end of a sign, or a Moon near the edge of a nakshatra or pada, can fall in a different sign or pada under another ayanamsa. A careful reading names which one it used.

Because the gap is about 24 degrees, most people have a different Sun sign in the two systems. The Sun spends about 30 days in each sign, so a gap of 24 days means that someone born in the first three weeks of a Western sign usually has their Sun in the previous sign in Jyotisha, and someone born later in the month has it in the same sign.

In Jyotisha, the sign people usually mean when they say “my rāśi” is not the Sun sign at all. It is the sign the Moon occupied at birth, the janma rāśi. The Moon moves through a sign in about two and a half days, so it is the more sensitive position and the one that sets the dasha periods.

The question is a natural one, and the answer depends on what each zodiac is claiming. A tropical zodiac says, in effect, “the signs are a calendar of the seasons, and the interpretation grows from that.” A sidereal zodiac says, “the signs are a division of the stars that actually stand behind the planets.” These are different assumptions and they lead to different readings, so neither can be proven wrong by the other.

The fair summary is that astronomy has no say here. Both systems are mathematically correct about what they measure. What each claims for its signs is a matter of tradition, not of measurement. This site follows the sidereal frame because Jyotisha does, and it names that choice whenever a position is shown.