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 Chaturthi on the same day
Sometimes a tithi might overlap into the next day and then there will be two same tithis on two consecutive days. For example – it will be Tritiya on Sunday and Monday.
Days with two Beginning or End points of tithis and no beginning or end points of tithis are considered inauspicious.
The thirty Tithis in a month are:
- Pratipada, Padya or Parthami, the first day after Amavasi
- Dwitiya
- Tritiya
- Chaturthi
- Panchami
- Shasti
- Saptami
- Ashtami
- Navami
- Dasami
- Ekadashi
- Dwadashi
- Trayodashi
- Chaturdashi
- Purnima or full moon
- Partipada
- Dvitiya
- Trithiya
- Chaturti
- Panchami
- Sashti
- Saptami
- Ashtami
- Navami
- Dasami
- Ekadashi
- Dwadashi
- Triodashi
- Chaturdashi
- Amavasya or No Moon or New Moon