Sidereal vs tropical zodiac explained
Two zodiacs are in use. The tropical zodiac starts Aries at the spring equinox. The sidereal zodiac, used in Jyotish, is measured against fixed stars. The two differ by the ayanamsa, currently about 24 degrees.
Two reference points
The tropical zodiac is anchored to the seasons. Zero degrees Aries is the point where the Sun crosses the celestial equator in spring. The sidereal zodiac is anchored to the stars, and Jyotish uses it.
Why the two drift apart
Earth's axis wobbles slowly, a motion called precession. The equinox point therefore moves backward along the zodiac by about one degree every 72 years, and completes a full cycle in roughly 25,800 years. A tropical sign and the sidereal sign with the same name no longer cover the same stars.
How the conversion works
Sidereal longitude equals tropical longitude minus the ayanamsa. A planet at 10 degrees Aries tropical, with an ayanamsa of 24 degrees, has a sidereal longitude of 346 degrees, which is 16 degrees Pisces. The conversion does not change a planet's speed or direction; it changes only the reference.
What this site does
The tools use the sidereal zodiac. Lahiri is the default ayanamsa, and the panchang, birth chart and checks tools also offer Raman and KP. Each tool states which ayanamsa it used. For the formula, see the method page.
A worked example with a planet
Suppose a planet is at 10 degrees of the tropical sign Aries. That is 10 degrees from the spring equinox point. With an ayanamsa of 24 degrees, its sidereal longitude is 10 minus 24, which is negative, so we add 360 to get 346 degrees. Sidereal 346 degrees is 16 degrees into Pisces, since Pisces covers 330 to 360 degrees. The planet is therefore in Pisces in the sidereal chart and in Aries in the tropical chart. The distance between the two sign names is one full sign, which is the ayanamsa rounded to the nearest sign. The position of the planet in the sky has not moved. Only the reference line has changed.
Why the choice matters for interpretation
Many readers ask which zodiac is correct. The honest answer is that both are internally consistent measurements. Tropical astrology ties the signs to the seasons, which is useful for weather and agriculture. Sidereal astrology ties the signs to the fixed stars, which is the frame of the classical Jyotish texts. A reader who mixes the two may take a sign's qualities from one system and its boundaries from the other. That leads to confusion. The safe approach is to fix the zodiac first, say which one is used, and read every rule within it. This site uses the sidereal zodiac throughout, with the choice of ayanamsa shown on each tool.
Sources and checks
When you read about the two zodiacs, check which one an author means. Many books use the word zodiac without qualifying it, and the sign names are the same in both systems, which makes it easy to mix them. A good test is to look for the ayanamsa in the text. If it is absent, the author is probably using the tropical zodiac, especially if the book is aimed at a general audience. Jyotish texts from the classical period assume the sidereal frame and rarely state it, because for their readers it was the only zodiac. Stating it clearly, as this site does, prevents the confusion that comes from silent assumptions.