Sidereal and tropical: what the ayanamsa is, and why Lahiri

Two ways to start the circle

Every zodiac needs a starting point, a zero degree from which longitudes are counted. There are two natural choices.

The tropical zodiac starts from the vernal equinox, the point where the Sun crosses the celestial equator heading north, around 20–21 March each year. Its signs are tied to the seasons. Western astrology uses it.

The sidereal zodiac (Sanskrit nirayana, "without motion") starts from a fixed point among the stars. Its signs are tied to the constellations and to the Nakṣatras. Indian astronomy and Jyotish use it.

In the early centuries CE the two starting points were close together. They are not now.

Precession

The Earth's axis wobbles slowly like a spinning top, tracing a circle in about 25,800 years. As it does, the equinox point slides backwards along the ecliptic against the stars by about 50.3 seconds of arc a year, roughly one degree every seventy-two years. This is the precession of the equinoxes. Hipparchus described it in the second century BCE. Indian astronomers discussed a related motion, ayana-chalana, and some texts model it as an oscillation (trepidation) rather than a continuous movement†.

The angular distance between the tropical zero point and the sidereal zero point at any moment is the Ayanāṃśa, literally the "portion of the ayana (solstitial motion)". It is about 24 degrees today and grows by about one degree every seventy-two years.

So a planet at 10° Taurus in the tropical zodiac is at roughly 16° Aries in the sidereal zodiac. Most of a person's planets, and often the Lagna (ascendant), fall in a different sign in the two systems.

Why there is more than one ayanamsa

Precession itself is well measured. The disagreement is about the starting point: in which year were the two zodiacs aligned, or equivalently, which star marks the sidereal reference? Different choices give ayanamsas that differ by up to a degree or more. Some of the better known:

• Lahiri (Chitrapaksha): places the star Chitra (Spica) at exactly 180°, the start of Tula (Libra). This puts the beginning of Mesha directly opposite Spica.
• Raman: proposed by B. V. Raman, smaller than Lahiri by about one and a half degrees.
• Krishnamurti (KP): used in the Krishnamurti Paddhati system, very close to Lahiri.
• Surya Siddhanta: the classical treatment in the Sūrya Siddhānta, with its own model, used by some traditional almanacs.

None of these is a matter of measurement error. They are different conventions.

How Lahiri became the Indian standard

After independence the Government of India set up a Calendar Reform Committee in 1952 under the chairmanship of the physicist Meghnad Saha, with N. C. Lahiri among its members. Indian almanacs at that time followed many different methods, and festival dates could vary from one region to another. The committee's report of 1955 recommended a unified national calendar and a standard ayanamsa for religious calendars. The value adopted was based on Chitrapaksha reasoning, with Spica fixing the sidereal frame, and it came to be known as the Lahiri ayanamsa. The Indian Astronomical Ephemeris, prepared by the Positional Astronomy Centre (now in Kolkata), publishes it.

That official standing is why the Lahiri ayanamsa is used by default in most Indian software and many printed almanacs. It is a choice of convention made by a public scientific body, which is what we look for in a default.

What depends on the ayanamsa, and what does not

The ayanamsa shifts every sidereal longitude by the same amount. So:

• The Tithi does not depend on it. A tithi is the difference between Moon and Sun, and the shift cancels.
• The weekday and sunrise do not depend on it.
• The nakshatra, the sign of each graha, the sign of the lagna, and the house a graha falls in can all depend on it, especially for positions within a degree or so of a boundary.
• The Vimshottari dasha balance at birth depends on it, because it depends on how far the Moon has travelled in its nakshatra.
• The dates of solar ingresses (sankrantis) such as Makara Sankranti depend on it. That is why Makara Sankranti falls around 14–15 January, while the tropical winter solstice falls around 21–22 December.

How we use it

Our tools default to the Lahiri ayanamsa and state that they do so. Where a reader's printed chart uses another convention, the difference of a degree or so is often enough to explain a sign or nakshatra that does not match. We regard this as a matter of convention, not of one system being right and another wrong.

Further reading

The report of the Calendar Reform Committee (1955) is available through the Government of India. For historical background on how Indian astronomers handled precession, see David Pingree's surveys of Indian astronomy and the notes to K. S. Shukla and K. V. Sarma's edition of the Aryabhatiya.

Try it

Open the Kundli tool and compare a chart near a sign boundary. The Panchang shows today's ayanamsa value, and the Calendar converter shows how solar months follow the sidereal zodiac.

† Marked dates or readings are debated among scholars or still being confirmed by our faculty.