How moon days are calculated
Where the dates come from, and why they can differ by a day from city to city.
The short version
Section titled “The short version”A moon day is the day on which the full moon or new moon lunar day is running at sunrise in the city you follow. Tristhana calculates this for each city, so the dates fit the place.
Lunar days
Section titled “Lunar days”The lunar month has 30 lunar days, called tithi. They follow the angle between the Moon and the Sun: every 12 degrees is one lunar day.
- The 15th lunar day is the full moon, Purnima.
- The 30th is the new moon, Amavasya.
A lunar day is a little shorter than a calendar day on average, and it can begin at any hour.
Why sunrise
Section titled “Why sunrise”By tradition a calendar day takes the lunar day that is running at sunrise. So Tristhana finds the moment of sunrise in the city and looks at which lunar day it falls in. If it is the 15th or the 30th, that date is a moon day. When the same lunar day is running at two sunrises in a row, the moon day is the first of the two dates.
Why two cities can differ
Section titled “Why two cities can differ”Sunrise comes at different moments in different places. If a lunar day begins between sunrise in one city and sunrise in another, the two cities get the moon day on different dates. Both are right for their place.
When a lunar day fits between two sunrises
Section titled “When a lunar day fits between two sunrises”Sometimes the full or new moon lunar day begins after one sunrise and ends before the next, so it isn’t running at any sunrise. Then the moon day is the date on which it begins.
What Tristhana uses
Section titled “What Tristhana uses”- The positions of the Sun and the Moon and the sunrise times come from the Swiss Ephemeris, a standard astronomical library.
- Calendars are calculated ahead for a list of cities. If yours isn’t on the list, choose the nearest one or Mysuru.
- The dates were compared with the moon days Sharath Yoga Centre published for Mysuru for 2025: 23 of 24 are the same. For one new moon, one of these short lunar days, the calculation gives 27 February and the centre’s list has 28 February. Both ways of placing such a day are in use. If you practice in Mysuru, follow the centre’s list. Tristhana has no connection with the centre.
If your shala says something else
Section titled “If your shala says something else”Follow your teacher. Shalas sometimes observe a moon day a day earlier or later, and a calculation can’t know that. /moon lets you choose another city, and a shala can hold or cancel classes on any date.