A phase is a moment; a tithi is an interval
The U.S. Naval Observatory’s explanation of lunar phases defines a primary phase by the angular relationship of the Sun and Moon. Their published phase tables give one Universal Time for each new or full Moon. A fixed UTC offset moves that same instant to a local clock date; it does not alter the event itself. The full Moon in one country can therefore be dated one day later than in another without any disagreement about the underlying UTC moment.
The Positional Astronomy Centre’s Rashtriya Panchang divides the Moon’s gain in ecliptic longitude on the Sun into thirty 12° tithis. The waxing fifteenth tithi is Shukla Purnima, covering the interval immediately before the 180° longitude separation. The waning fifteenth is Krishna Amavasya, immediately before the next 360° or 0° separation. Each tithi begins at one angular crossing and ends at the next. Its length changes with the Moon’s speed, so an interval can cross local midnight.
What the calculation does
This page uses the locally bundled Astronomy Engine 2.1.19 to search for 0° and 180° geocentric Moon–Sun phase moments. Separately, it samples the apparent geocentric Sun and Moon ecliptic longitudes every two hours and bisects the 12° tithi boundaries to within fifteen seconds. It matches each phase to the Purnima or Amavasya interval that ends at approximately the phase moment. These two functions can differ by seconds because their internal methods differ; the displayed table rounds to a minute.
A tithi depends on the difference in Sun and Moon longitude. Subtracting a common Lahiri or other ayanamsa from each longitude cancels, so this particular calculation has no sidereal-origin option. That does not make every regional panchanga identical: ephemeris, apparent-position method, sunrise model, legal time, and observance rule still matter.
Why an observance date can differ
The phase date in the result is simply the civil date containing the astronomical instant under your chosen fixed offset. A traditional calendar may instead assign a tithi to the day on which it is present at local sunrise. A named vrat or festival can apply an additional rule tied to moonrise, twilight, or another part of the day. Those rules are specific to the observance and tradition, so one all-purpose “Purnima festival date” would be misleading.
For example, Drik Panchang’s New Delhi Purnima list separately prints Purnima Vrat and Purnima in some months, along with tithi beginning and ending times. Its local almanac date should not be copied from a phase table. Our calendar does not assign a vrat, fasting, worship, or festival day. To see which tithi is present at sunrise for your place, use the monthly tithi calendar and enter latitude and longitude.
Checking a worked month
For September 2026, the USNO phase API lists new Moon at 03:27 UTC on September 11 and full Moon at 16:49 UTC on September 26. At UTC+05:30 these are 08:57 on September 11 and 22:19 on September 26. The calculated phase instants on this page should agree to the displayed minute or close to it. The corresponding tithi starts before those instants. In another fixed offset, either phase may fall on a different local civil date.
Clock assumptions and common questions
Why does the table give both UTC and local time?
UTC makes an event comparable across publications. The local time applies your entered offset to that same moment. A fixed offset cannot follow daylight-saving changes; choose the offset actually in force for the date or use UTC with an external time-zone conversion.
Is a full Moon visible at the exact phase?
The computed moment is a geometric longitude relationship, not a prediction of visibility, weather, or moonrise at your location. The Moon can appear nearly full over more than one evening.
Does Amavasya mean the local date of the new Moon?
In this table, “local phase date” does mean the civil date of astronomical conjunction. A regional Amavasya observance may be assigned under a different day rule. Check a location-specific panchanga for that purpose.