# Lunar Day, Calculation of Tithi, Phases of the Moon by 12 degrees, Hindu Calendar

1. May 29, 2012

### petrix108

Dear Physics/Astronomy Forum Folks:

I have been recently reading, studying about an interesting feature of the Hindu Calendar, called the tithi, which is basically a lunar day, or a difference of 12 degrees between the Sun and the Moon. Now the angular speed of the Moon in relation to the Sun, which determine the cycle of lunar phases, is not constant or even linear. My question is: does anyone know how to find the exact mathematical relationship (a trigonometric formula maybe?) using which one could calculate the exact time within a day, when a certain tithi starts and ends. I've read on some other forum that it's got to do with the difference between the hour angle or right ascension of the Moon and the Sun. So given, for instance the exact time and day of the New moon or the Full Moon, how does one calculate when the exact time is that 12 degrees have passed between Sun and Moon? What is the formula for calculating tithis?