Jain tithi calculation method

In Jain Dharma, tithi means lunar day. It is decided by the position of the Moon relative to the Sun, not by the simple 24‑hour civil date.

1. Basic astronomical idea (same for Digambar & Shwetambar)

• Think of a circle of 360° around the Earth.
• The angular distance between Sun and Moon is measured in this circle.
• This circle is divided into 30 equal parts of 12° each.
• The time it takes for the Moon–Sun angle to increase by 12° is one tithi. ( jainpedia.org)

So:

• 0–12° = Prathama (1st tithi)
• 12–24° = Dwitiya (2nd tithi)
• … and so on up to 30th tithi.

Because the Moon’s speed is not constant, one tithi is not always 24 hours. It can be less or more than a day, so sometimes:

• a tithi is skipped (kṣaya tithi – “lost” tithi),
• a tithi comes twice in the same civil date (vṛddhi tithi – “double” tithi). ( jainqq.org)

2. How calendars practically use tithi (udaya tithi rule)

Modern Jain panchāngs (both Digambar and Shwetambar) generally follow the Hindu lunisolar calculation but apply Jain rules for festivals:

1. Tithi is calculated by astronomy (Moon–Sun angle as above).
2. For daily use, most follow uday tithi rule:

- The tithi present at local sunrise names that whole civil day.

1. Because sunrise time and location differ, the same civil date can have:

- one tithi in India, - another tithi in America, etc. ( jainknowledge.com)

3. Jain month and pakṣa

• One lunar month has 30 tithis.
• It is divided into two halves:

- Śukla / Sud pakṣa (bright half, Moon increasing): tithis 1–15, - Kṛṣṇa / Vad pakṣa (dark half, Moon decreasing): tithis 1–15.

• 15th of bright half = Pūrṇimā (Poonam, full moon).
• 15th of dark half = Amāvasyā (Amas, new moon). ( jainqq.org)

4. Special Jain points

From Jain Āgams like Sūrya Prajñapti and Candra Prajñapti:

• Tithi is clearly based on Moon’s motion, slightly less than a full day.
• Old Jain āgamic calendar used a 5‑year cycle (yuga) with fixed rules:

- extra month after fixed intervals, - one tithi regularly “dropped” after a set number of days. ( jainqq.org)

Today, published Jain calendars (of most sects) do not strictly follow this ancient 5‑year āgamic structure; they mostly adapt the common Indian lunisolar calculations, but:

• keep Jain names and parva‑tithis (festival tithis),
• apply Jain rules about which tithi to use when there is kṣaya or vṛddhi for important parvas. ( jainqq.org)

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If you want a very simple, beginner‑level explanation of “what is tithi” and “why there are different tithis on the same date,” you can also read a short Q&A here