The Tamil calendar, 12 solar months, 60 Jovian years

A solar calendar anchored to the stars

The Tamil calendar is a solar calendar, months follow the Sun, not the Moon. But it uses the sidereal zodiac, anchored to the actual star positions, not the tropical zodiac used by the Gregorian calendar. This makes it unique: solar like the Gregorian, but star-anchored like the Indian tradition.

Each Tamil month corresponds to one zodiac sign (rashi). Chithirai begins when the Sun enters Mesha (Aries). Vaikasi begins when the Sun enters Rishabha (Taurus). And so on through all 12 signs. Because the sidereal zodiac is about 24° offset from the tropical one, Chithirai starts in mid-April, not at the spring equinox like the Western Aries.

The 12 Tamil months

Note the pattern: summer months (Chithirai–Aadi) last about 31–32 days, while winter months (Aippasi–Maasi) last about 29–30 days. This is Kepler's second law at work, the Sun moves faster across the sky when Earth is closest to the Sun (perihelion in January), so the Sun traverses the winter signs more quickly.

Sankranti, the moment a month is born

The instant the Sun crosses the boundary into a new zodiac sign is called sankranti. The month begins on the sankranti day, but with an important rule: if sankranti falls after sunset, the month starts the following day.

This "sunset rule" is specific to Tamil Nadu and Kerala. North Indian calendars use midnight as the cutoff, and the Kannada calendar uses sunrise. This is why Tamil and North Indian calendars can show different month starts for the same sankranti instant.

The 60-year Jovian cycle

Each Tamil year carries a name from a cycle of 60 names called samvatsara (Jovian cycle). The cycle repeats infinitely, it has no beginning and no end. Jupiter takes about 12 years to orbit the zodiac, and 5 Jupiter orbits make 60 years, hence the cycle's length.

The Sun sets the date, the Moon sets the holy day

The Tamil calendar has an elegant division of labour: the Sun determines the date (month and day), while the Moon determines the holy day (via tithi and nakshatra). When you say "today is Chithirai 15", that is the solar calendar. When you say "today is Ekadasi", that is the tithi system, a lunar observation.

This distinguishes the Tamil calendar from North Indian calendars (like Vikram Samvat), which use tithi as the date system itself. In Tamil Nadu, the solar calendar is the framework, and the lunar elements fill the framework with religious significance.

Certain festivals, however, are tied directly to the solar calendar: Thai Pongal always falls on Thai 1 (the day the Sun enters Makara), and Tamil New Year (Puthandu) always falls on Chithirai 1. These are solar festivals, not lunar ones.

How the Tamil date is calculated

The calculation requires three steps:

1. Find the Sun's sidereal position: Sun's tropical longitude (NOAA algorithm) minus Lahiri ayanamsa. Divide by 30° to find the current zodiac sign and therefore the Tamil month.
2. Find the month's start day: step backwards day by day and find the day the Sun changed signs (sankranti). Remember the sunset rule.
3. Count the days from start to today, that is the Tamil date.

The year's name is calculated separately: take the Gregorian year, adjust for the fact that Chithirai starts in April (before April, you are still in the previous year), and look up the 60-name samvatsara cycle.

Why the Tamil date can vary by city

Because the sankranti rule compares the sankranti instant with the local sunset, two cities with different sunset times can begin the month on different days. A city with a late sunset may still catch sankranti before sunset, while a city with an early sunset pushes it to the next day.

In practice this rarely happens within Tamil Nadu, but it can happen for diaspora users in northern countries, where sunset varies dramatically with the season. Our calculator handles this correctly for each city.