A tithi in a Hindu calendar is a lunar day. Amavasya (No moon or new moon) happens when the longitudes of the sun and the moon are equal. A Thithi is calculated when the moon moves 12 degrees eastwards from the sun. This is the first Tithi and is known as Pratipada Thithi during Shukla Paksha or waxing phase of moon. When the moon moves 180 degrees it is Purnima or full moon and when the moon moves 360 degrees it is again Amavasya. There are 15 Tithis in a fortnight. The Length of a Tithi Since the motions of the sun and the moon are always varying in speed, the length of a Tithi constantly alters. Some times a tithi can be as long as 26 hours and as less as 21 hours 34 minutes and 24 seconds. This means the time a tithi begins varies with the general time followed in a country. Same Tithi on two days and Two Tithis on one day Sometimes a tithi might begin and end on the same day. Thus there will be two tithis on a day. For example – you might see a Tritiya and