I want to know about the calculation of years of Pitris (manes) according to scriptures.
1 Pitri year = How many human years?
1 Pitri day = How many human days?
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One day of Pitris (day + night) is equal to 30 days of Earth/humans. This chapter in the Shanti Parva of the Mahabharata states:
Thirty days and nights are called a month, and twelve months are called a year. Persons conversant with mathematical science say that a year is made up of two ayanas (dependent on sun's motion), viz., the northern and the southern. The sun makes the day and the night for the world of man. The night is for the sleep of all living creatures, and the day is for the doing of action. A month of human beings is equal to a day and night of the Pitris. That division (as regards the Pitris) consists in this: the lighted fortnight (of men) is their day which is for the doing of acts; and the dark fortnight is their night for sleep. A year (of human beings) is equal to a day and night of the gods. The division (as regards the gods) consists in this: the half year for which the sun travels from the vernal to the autumnal equinox is the day of the deities, and the half year for which the sun travels from the latter to the former is their night. (Mahabharata Shanti Parva Section CCXXXI)
As clear from the above passage:
30 days of Humans = 1 day of Pitris
Here, on earth in the 30 days, we have 15 days as a bright fortnight of the moon (also called as Shukla Paksha) during those times it is day time of Pitris. During the remaining 15 days of the dark fortnight of the moon (also called Krishna Paksha), it is nighttime of Pitris. By this same calculation, One year of Pitris would be equivalent to 30 years on Earth.