Surya Siddhanta and Drishya Ganita Panchang

Two ways of computing the panchang

Drishya Ganita computes the Sun and the Moon where they are actually seen in the sky, the way this site computes every panchang. Every eclipse falls within minutes of the moment it gives for Purnima or Amavasya.

The Surya Siddhanta is the classical text many printed Telugu and other traditional panchangs still follow. Its times can differ from the sky by a few minutes near Purnima and Amavasya, and by several hours in the middle of the paksha, so the tithi or nakshatra at sunrise can differ on some days.

Both columns use the same sunrise, for the city shown. Times are in the city's own time zone.

Why the times differ: the Moon's motion

The Moon does not move at an even speed. Modern astronomy corrects its position for several effects. The Surya Siddhanta applies only one of them.

The Moon moves about half a degree an hour, so one degree is about two hours. Together the missing corrections move the Moon by up to about 3 degrees, which moves a tithi or nakshatra end by up to about 6 hours.

At Purnima and Amavasya the variation is zero, and the evection and the main correction add up to about 5 degrees, the size the Surya Siddhanta uses. So the text comes close to the sky there, which is why its eclipse times work, and it drifts by hours in the middle of the paksha.

The Sun: the Surya Siddhanta's one correction for the Sun is close, but its zodiac starts from a slightly different point from this site's, so the Sun's rashi entry (Sankramana) can differ by several hours, sometimes half a day.