Vedic basics
Sidereal vs tropical: which zodiac is right?
The tropical zodiac starts at the spring equinox, so it follows the seasons. The sidereal zodiac starts from the fixed stars, so it follows the constellations. They are 24°13′ apart in 2026 and drifting about 1° further every 72 years. Neither is “wrong”: they measure different things. Western astrology uses tropical; Vedic astrology uses sidereal.
The difference in one table
| Tropical | Sidereal | |
|---|---|---|
| Where 0° Aries is | The March equinox | A point fixed against the stars |
| What it tracks | The seasons | The constellations |
| Used by | Most Western astrology | Vedic astrology (Jyotish), and some Western astrologers |
| Signs | 12 equal signs of 30° | 12 equal signs of 30°, plus 27 nakshatras of 13°20′ |
| Moves over time? | Fixed to the equinox; drifts against the stars | Fixed to the stars; drifts against the seasons |
Why they drift apart: precession
The Earth spins on a tilted axis, and that axis slowly circles, like the wobble of a spinning top. One full circle takes about 25,800 years. As it turns, the point where the Sun crosses the equator in March (the equinox) slides backward through the constellations by about 50 arcseconds a year, or 1° every 72 years.
The tropical zodiac moves with that point. The sidereal zodiac stays with the stars. So the gap between them grows every year. By the Lahiri measure the two lined up around the year 292 CE; today the gap is 24°13′.
The ayanamsa: how big is the gap?
The gap is called the ayanamsa. Everyone agrees on how fast it grows; they disagree a little on where it started, which changes the gap by up to a degree or two. The main choices:
- Lahiri (also called Chitra paksha): set so that the star Spica sits at 0° Libra. It was adopted by India’s Calendar Reform Committee in the 1950s and is the most widely used in Vedic astrology. Twenty-Seven uses it.
- Krishnamurti (KP) and Raman: Indian variants. KP is within a few arcminutes of Lahiri; Raman is about 1½° smaller.
- Fagan–Bradley: the main ayanamsa of Western sidereal astrology, about 1° larger than Lahiri.
A one-degree difference rarely changes a sign, but it can move a planet near the edge into the next nakshatra. This is one reason two Vedic sites sometimes disagree.
So which one is right?
Here is the honest answer: nobody has shown that either zodiac predicts anything better. Astrology is a language for time, not a proven science, and Twenty-Seven treats it that way. In its view the planets are clocks, not controllers.
Within that frame, the choice is about what you want to measure:
- If your astrology is about the seasons, the light growing and shrinking through the year, the tropical zodiac is the natural ruler.
- If your astrology is about the stars, the sky you can actually see, the sidereal zodiac is. The Moon in Rohini is the Moon next to the red star Aldebaran, tonight and 2,000 years ago.
Vedic astrology is built on the second. Its 27 nakshatras are named after real star groups, so they only make sense in a zodiac that stays with the stars.
Can I use both?
Yes, as long as you keep them apart. Many people read their Western chart for its own vocabulary and their Vedic chart for the nakshatras and the dasha periods. The mistake is mixing the two: reading sidereal positions with tropical meanings, or the reverse.
How Twenty-Seven computes it
Our chart engine computes the planets from astronomy formulas, subtracts the Lahiri ayanamsa for the moment of birth, and uses whole-sign houses and mean lunar nodes. We test it against Swiss Ephemeris, the standard astronomy library, and it agrees to within about an arcminute.
Questions people ask
What is the difference between sidereal and tropical astrology?
Tropical astrology starts the zodiac at the spring equinox (the seasons); sidereal astrology starts it from the fixed stars. The signs have the same names and sizes, but they are now about 24° apart.
Is sidereal astrology more accurate than tropical?
Neither has been shown to be more accurate. Sidereal matches the visible stars; tropical matches the seasons. Twenty-Seven uses sidereal because the 27 nakshatras are star-based.
What ayanamsa does Vedic astrology use?
Most Vedic astrologers use Lahiri (Chitra paksha), India’s official standard since the 1950s. KP is a close variant.
Why are the two zodiacs about 24 degrees apart?
Precession: the Earth’s axis slowly wobbles, so the equinox slides backward against the stars by about 1° every 72 years. The two zodiacs lined up about 1,700 years ago.
Your chart, measured from the stars
Twenty-Seven computes your sidereal chart and starts with the Moon’s mansion you were born under. Ask it anything about how it reads you.
Twenty-Seven is an AI astrologer, not a person. It makes no predictions: in its view the planets are clocks, not controllers.