ĀRKThe almanac
Where the Moon is tonight
The Moon takes about twenty-seven and a third days to complete its orbit against the stars. Dividing that circle into twenty-seven equal segments gives the traditional lunar mansions, known in Sanskrit as nakṣatrasNakṣatra · lunar mansionOne of twenty-seven equal segments of the Moon’s path, each about a day’s travel., representing the Moon’s station for roughly one day each.
A lunar mansion (nakṣatra) represents one twenty-seventh of the Moon’s 360° circuit, spanning exactly 13.33° (or 13°20′). The illuminated marker indicates where the Sūrya-siddhānta places the Moon right now. This mean position aligns with the correct astronomical mansion roughly two out of every three days, so on any given night it may disagree with the true position shown on the ring above.
The working
- 01
What this is computing
Days elapsed since the text’s epoch (the ahargaṇaAhargaṇa · the day countThe number of days elapsed since the system’s zero point.), multiplied by the Moon’s revolutions per cosmic cycle (mahāyugaMahāyuga · the great ageA cycle of 4,320,000 years, the span the text states its orbit counts over.), divided by the total civil days. The remaining fraction indicates where along the 360-degree circle the Moon has travelled. This calculation follows chapter 1 of the Sūrya-siddhānta directly in your browser without any external requests, so it works completely offline.
- 02
How accurate this is
This calculates the mean (average) position from chapter 1. Chapter 2 introduces further corrections for variations in the Moon’s speed, which are not applied here.
Compared against modern NASA lunar ephemeris data over a two-year span, this mean calculation identifies the correct lunar mansion on about two days out of three, averaging within 4.6° of true celestial longitude. Because each mansion spans 13.33°, any discrepancies occur primarily when the Moon is crossing a boundary between two mansions.
- 03
Why we publish the miss
We publish this measured accuracy openly because the transparency matters. The basic formula requires only four historical integers and two divisions to place the Moon within a few degrees of its modern position.