Solar months, and the twelve sankrantis

Most Indian festivals are set by the moon. But Tamil Nadu, Kerala, Odisha, West Bengal, Assam and Punjab run their everyday months on the sun.

Here is how that works. The sun's yearly path is divided into twelve signs, the same twelve as Aries to Pisces. Each time the sun crosses out of one sign and into the next, a new month begins. That crossing is called a sankranti, which means a going across, and there are twelve of them in a year.

The signs are not all equally quick to cross, because the sun does not travel at an even pace against the stars. So the months come out unequal, between about twenty-nine and a half days and about thirty-one and a half.

Then comes the awkward part. A crossing happens at a particular minute, and a minute is not a day. Somebody has to decide which calendar day the new month starts on, and the regions decided differently. Take the crossing into Aries at 09:39 on the morning of Tuesday 14 April, and put it to each rule in turn:

- Odisha: the month starts on the day of the crossing, whenever in that day it falls. So the month opened on the 14th. - Punjab: the month starts on the crossing day itself. The 14th again. - Tamil Nadu: the month starts that day if the crossing came before sunset. It came about nine hours before sunset, so the 14th. - Kerala: the month starts that day if the crossing came before a point about three fifths of the way through the daylight. That point fell in the early afternoon, so the 14th once more. - Bengal: the month starts the morning after the crossing, and the crossing day counts as the last day of the old month. So the month opened on the 15th.

The names differ too. On the same day in late April, the month is Chithirai in Tamil Nadu, Medam in Kerala, Baisakha in Odisha, Boishakh in West Bengal, Bohag in Assam and Vaisakh in Punjab. Kerala uses the sign's own name. The others use the old month names, and Tamil Nadu starts that list of names one step earlier than everybody else.

One crossing out of the twelve is known everywhere: Makar Sankranti, in the middle of January. It carries the harvest festivals, and unlike the moon-based festivals it lands on nearly the same date every year, which is a large part of why everyone remembers it.