A Tithi is one of the 30 phases of the Moon. 65-90% of Hindu festivals are based on them.
Check out its usage in Hindu traditions. How is it calculated ? It is unrelated to the 24 hour day.
The Moon revolves around the Earth. The Sun is always a big looming entity!
As the Moon moves, only a certain amount of its surface is visible to us. That amount varies from 0 to half of its surface. And it is visible to us due to the sunlight reflecting from it.
When the moon is in the direction of the Sun (A in figure) no part of it is visible. That Tithi is Amavasya (No Moon/ Dark Moon).
At the other extreme of its revolving path, the Moon, is opposite to the Sun (P in figure). That Tithi is Poornima (Full Moon).
Between these two extremes are the stages when some part of the moon is illuminated. But is less than the light during Poornima.
Those in-between stages are divided into 14 each, on either side of the extremes.
So, we have (Amavasya & Poornima) 2 + 14 + 14 = 30 Tithis.
The moment after Amavasya to Poornima is Shukla Paksha. (Moon's move from 0° to 180° in figure). That's the half when the Moon's Shukla (brightness) increases.
The moment after Poornima to Amavasya is Krishna Paksha. (Moon's move from 180° to 0°). That's the half when the Moon's Krishna (darkness) increases.
One Tithi is when the Moon moves 12 degrees. So, the Prathama (first) after Amavasya is when the Moon moves from 0 degrees to 12 degrees relative to the Sun.
And the Dvitiya (second) is when the Moon is between 12 and 24 degrees from the Sun. This goes on till the fifteenth (which is Poornima) when the Moon is between 168 and 180 degrees.
Similarly we have fifteen more from the Prathama after Poornima (Krishna Paksha Prathama) to Amavasya at 12 degree intervals.
The calculation is a bit more involved since both Sun and Earth also change their positions.
A Tithi is independent of a Solar day.
The time taken for the Moon to go around the Earth is 29.53 Solar days. And we have 30 Tithis.
Which implies that the average length of one will be less than a day. Turns out that, that length varies from 19 hours to 26 hours.
Why this variation ? The path taken by the Moon is not a perfect circle. So, the time taken to complete each 12 degrees, isn't equal.
A Solar day is 24 hours. And is solely dependent on Earth rotating on its own axis. While, Tithi is dependent on three entities - Earth, Moon, Sun.
Although, its length happens to be around 24 hours, it is separate from a Solar day.
Death anniversaries are defined by it. The month, Paksha, and Tithi of the day of death is used to observe them.
People perform Shraddha to ones parents/ ancestors based on this month-paksha-tithi combination.
Birthdays of prominent people of the traditions are also observed based on this.
Birthdays of Krishna (Ashtami) and Rama (Navami) are based on Tithi. So is that of Adi Shankaracharya and Ramanujacharya (both Panchami).
About 65-90% of Annual Hindu Festivals are based on it. For traditions that use Sauramaana calendar, that percent is nearer the 65% mark. And for the other traditions, it is more like 90%.
My favorite is Prathama (also called Pratipad). Both Shukla Paksha one and the Krishna Paksha one.
That's when I get to do the Sthaalipaakam rite. A rite that one does every Prathama as part of the daily Agni Upaasana (worship).
What is your favorite ? Share your thoughts on it, in the comments section below.
I used to wonder why Sthaalipaakam was to be done on Prathama. Why not Dvitiya or Ekadashi? Turns out that Agni is the Devata of Prathama.
Every Tithi has a Devata connected with it. Amavasya is special for Pitr (ancestors). Chaturthi for Ganesha. Likewise, Prathama for Agni.
It can be called a Lunar day. It is the most important parameter of the day, with majority of festivals dependent on it. But, it is independent of a Solar day. That's Tithi for you.
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