Festivals in the sky

Why do most Indian festivals fall on different dates each year? Because many are tied to the Moon's phases, so they follow lunar or luni-solar calendars, while the dates we quote come from the Gregorian (solar) calendar.

For festivals on luni-solar calendars, the Gregorian date moves, but usually by less than a month, because the extra month every few years corrects the gap.

Purely lunar calendars make no correction. So a festival such as Eid-ul-Fitr comes about 11 days earlier each year and, over the years, travels right through all the seasons.

A few festivals are different again. Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh and Puthandu follow a solar sidereal calendar (based on the stars), so they fall on almost the same Gregorian date every year.

Long ago these festivals were tied to a solstice or an equinox. But the sidereal year is slightly longer than the year of the seasons, so their dates slowly drift. This is caused by a slow wobble of the Earth's axis, like the wobble of a spinning top. Makar Sankranti moves one day later about every 71 years.

Even in one year the date can differ across India: many festival dates depend on the Moon's exact phase at sunrise, and the Sun rises earlier in the east of the country than in the west. To keep things uniform, the Positional Astronomy Centre publishes the Rashtriya Panchang each year, calculating the Moon's and Sun's positions for a central place in India and announcing festival dates in advance, so holidays can be declared.