Hindu calendar guide

Tithi explained

तिथि

A lunar day, calculated from each 12 degree increase in Moon-Sun angular separation.

Tithi explains why Hindu dates move through the civil calendar: the calendar follows the changing angular relationship between the Moon and the Sun.

Key 1

One tithi is one 12 degree step of angular separation between the Moon and the Sun.

Key 2

There are 30 tithis in a lunar month: 15 in Shukla paksha and 15 in Krishna paksha.

Key 3

A tithi can begin or end at any clock time, so the tithi at sunrise often decides the Hindu date.

The lunar day is not a 24-hour day

A civil day changes at midnight. A tithi changes when the Moon has moved another 12 degrees away from the Sun. Because the Moon speed varies, a tithi can be shorter or longer than a civil day.

This is why you may see a tithi start at 10:38, end at 14:21, or span two sunrises. The Hindu date is following the Moon-Sun geometry, not the wall clock.

Paksha: waxing and waning halves

Shukla paksha is the bright fortnight, from Amavasya toward Purnima. Krishna paksha is the dark fortnight, from Purnima toward Amavasya.

The names repeat in both halves: Pratipada, Dwitiya, Tritiya, and so on. Shukla Chaturthi and Krishna Chaturthi are different moments in the lunar cycle even though both are the fourth tithi of a paksha.

How festivals use tithi

Many festival rules are written as a lunar month, paksha, tithi, and observance moment. Raksha Bandhan is Shravana Shukla Purnima. Diwali Lakshmi Puja is tied to Kartika Amavasya during pradosha.

The moment matters. A tithi that exists in the morning may not be the right tithi for an evening observance, and a tithi that starts after sunrise may still matter for a night observance.

Worked tithi calculation

Tithi comes from the angular separation between the Moon and the Sun, measured along the ecliptic.

Formulatithi number = floor(((Moon longitude - Sun longitude + 360) mod 360) / 12) + 1

1Example 1: if Moon-Sun separation is 49 degrees, divide 49 by 12. The result is 4 remainder 1, so the active tithi is number 5: Panchami.
2Example 2: if Moon-Sun separation is 187 degrees, divide 187 by 12. The result is 15 remainder 7, so the active tithi is number 16 overall, which is Krishna Pratipada.
3Numbers 1 to 15 belong to Shukla paksha, ending in Purnima. Numbers 16 to 30 belong to Krishna paksha, ending in Amavasya.

The simple formula hides serious astronomy: the hard part is calculating accurate solar and lunar longitudes for the place and time being tested.

Practical examples

Use these as working patterns when reading Panchang details or planning around Hindu dates.

Example

Why Chaturthi may not match the printed date

Your calendar shows Chaturthi, but the tithi changes in the afternoon.

1.Check whether Chaturthi was active at sunrise.
2.Check the tithi end time.
3.For a specific vrata or puja, check whether the tradition uses sunrise, moonrise, or another observance moment.

The printed civil date is only a container. The relevant tithi window decides the observance.

Example

Planning Purnima observance

You want to plan a Purnima puja for family members in different countries.

1.Check Purnima tithi start and end times in each city.
2.Compare the local sunrise and evening windows.
3.Use the rule for that observance instead of assuming a single global date.

Different countries can correctly observe on different civil dates because the Moon-Sun angle crosses the threshold at different local times.

The beauty of tithi

Tithi is one of the clearest examples of Hindu timekeeping turning sky motion into lived rhythm. The date is not arbitrary; it is a measured relationship between the two brightest bodies in the sky.

That is why tithi feels both devotional and mathematical. It lets ritual life follow a celestial cycle that can be observed, computed, and explained.

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