February 10, 2026 · ToolDai team · calendar · astronomy · nepal
How our Nepali calendar actually works — tithi, nakshatra and sunrise
A peek at the astronomy library that powers ToolDai's panchanga, and why the dates sometimes shift by a day from a published patro.
A traditional Nepali patro — Hamro Patro, Nepali Patro, printed almanacs — publishes authoritative tithi (lunar day) and nakshatra (lunar mansion) values each day. The daily values come from an ephemeris: a precise table of where the Sun and Moon actually are in the sky at a reference moment, usually local sunrise.
Our calendar uses a simpler model. We compute:
- Tithi from the Moon's mean motion relative to the Sun, from a known new-moon reference date.
- Nakshatra from the Moon's mean ecliptic longitude at UTC noon.
- Sunrise and sunset at Kathmandu using the standard NOAA formula, without an atmospheric refraction correction.
The result is accurate to a few hours for tithi and a few minutes for sunrise/sunset. For a quick "what tithi is today?" look-up or a one-year vrat calendar, that's plenty.
Where it falls short: near a tithi transition — the moment the moon crosses from one lunar day into the next — our value can disagree with a published patro by a single tithi. A patro counts the tithi at sunrise; we count at UTC midnight. For precise muhurta (astrologically-chosen moments) or ritual timing, always use the published patro.
Our Ekadashi/Pradosh calendar flags the day whose midnight sits inside tithi 11, 13, 26 or 28. In almost every case it matches the published fasting day; near a transition it may slip by one calendar date, so we add a note on each tool's page reminding users.