Two zodiacs, two starting points
The tropical zodiac begins at the spring equinox, the moment the Sun crosses the celestial equator going north. The sidereal zodiac begins against the fixed stars.
Both are twelve thirty-degree divisions. What differs is where zero degrees of Aries is, and the answer has been drifting apart for centuries.
Why they drifted
Earth's axis wobbles, completing a slow circle every twenty-six thousand years. That is precession, and it moves the equinox backwards against the stars at about one degree every seventy-two years.
The two zodiacs coincided roughly seventeen centuries ago. The accumulated gap is now around twenty-four degrees, which is most of a sign, and it grows by a degree every human lifetime.
What changes in your chart
Every position moves back by the same amount, so most people land one sign earlier. Someone at five degrees of Leo tropically is at eleven degrees of Cancer sidereally.
Because the shift is uniform, no aspect changes. The distance between your Sun and Moon is identical in both charts, so the aspect list is the same list. Only the sign labels move.
The comparison table in the result shows both side by side and counts how many bodies change sign. For most charts that is nearly all of them.
The ayanamsa
The gap between the two zodiacs is called the ayanamsa, and its exact value depends on where you decide the sidereal zodiac starts, which is not settled.
| Ayanamsa | Used by |
|---|---|
| Lahiri | Indian government standard; most Vedic practice |
| Fagan-Bradley | Western sidereal astrology |
| Raman, Krishnamurti | Particular Indian schools |
They differ by up to about a degree, which rarely changes a sign but can change a nakshatra near a boundary. This calculator uses Lahiri and says so. Additional ayanamsas are planned and not yet implemented.
Which one is correct
Neither, in any checkable sense. They measure from different starting points and each is internally consistent.
The argument for tropical is that it tracks the seasons, which is what the sign names describe: Aries begins at spring, Cancer at midsummer. The argument for sidereal is that it tracks the constellations the signs were named after.
Both arguments are sound and they cannot both be the point of the system. That is the actual disagreement, and it is philosophical rather than astronomical.
Sidereal is not the same as Vedic
A sidereal chart changes the zodiac and leaves everything else as you set it. Vedic practice changes three things at once: the zodiac, the house system, which is always whole sign, and the starting point of the reading, which is the Moon rather than the Sun.
If you want Jyotish, the Vedic calculator applies all three and adds the nakshatras. This page is for Western sidereal work, where the houses and the reading method stay as you choose them.
What to do with the result
Read the comparison rather than the sidereal chart alone. The interesting question is which bodies changed sign and whether the sidereal placement describes you better, which is a question only you can answer.
Most people find one of the two more recognisable and cannot say why. That is not evidence for either system, and it is the honest state of the argument.
Common questions
What is the difference between tropical and sidereal?
Where zero degrees of Aries sits. Tropical starts at the spring equinox, sidereal against the fixed stars. They have drifted about twenty-four degrees apart.
Why is my sidereal sign different?
Because the accumulated gap is most of a sign, so most people land one sign earlier. The shift is uniform, so every position moves by the same amount.
Do my aspects change?
No. Because everything shifts equally, every aspect and orb survives untouched. Only the sign labels move.
Which ayanamsa do you use?
Lahiri, the most widely used. Others differ by up to about a degree, which rarely changes a sign but can change a nakshatra near a boundary.
Which zodiac is correct?
Neither, in any checkable sense. Tropical tracks the seasons the sign names describe; sidereal tracks the constellations they were named after. The disagreement is philosophical.
Is sidereal the same as Vedic?
No. A sidereal chart changes the zodiac only. Vedic practice changes the zodiac, uses whole sign houses, and reads from the Moon rather than the Sun.