Uttarayan, Dakshinayan and the Indian National Calendar

For thousands of years people in India watched the sky and built calendars. They didn't know the Earth goes round the Sun and had no modern instruments, yet by years of careful watching they found the patterns and worked out that a year is about 365 days.

Here's one pattern. The Sun doesn't always rise exactly in the east. In summer it rises a little north of east; in winter a little south. The furthest points come at the solstices, around 21 June and 21 December.

This repeats every year and goes with the changing seasons.

The Taittirīya Saṁhitā (verse 6.5.3) records it: Thus the Sun moves southwards for six months and northwards for six months. People also tracked equinoxes and solstices by which stars rose at sunset; the Surya Siddhanta noted that the star pattern Makar (Capricorn) was behind the Sun around the winter solstice in those ancient times.

Today India has its own national calendar, used alongside the Gregorian calendar for official purposes. It is a solar calendar of 365 days.

Its year begins on 22 March, the day after the spring equinox. Its months have 30 or 31 days (never 28), with names from the traditional Indian calendars. In an ordinary year the 2nd to 6th months have 31 days and the rest have 30.

In a leap year an extra day goes to Chaitra, the first month, and the year begins on 21 March instead.

In 1952 the Government of India set up a Calendar Reform Committee to study all the calendars used in the country and recommend one accurate, uniform calendar for all of India. Its calendar was adopted from 21 March 1956 (1 Chaitra 1878 Saka) and follows the general principles of the Surya Siddhanta. The committee was chaired by Meghnad Saha, the astrophysicist who studied stars and their temperatures and developed the Saha equation; the Saha Institute of Nuclear Physics in Kolkata is named after him.