Provenance
Methodology and Sources
Where every number on this site comes from, and what it is accurate to.
How these figures are computed
Phase, illumination, libration, geocentric distance, moonrise and moonset are calculated from astronomical ephemerides — not looked up in a table and not fetched from an API. The same code runs everywhere on the site, so any two pages describing the same instant will agree.
Most pages are generated ahead of time, which is why they load instantly. That is the right trade for a page about a particular date, since a date does not change. It is the wrong trade for anything claiming to describe the moon now: illumination can move by more than eleven percentage points in a day around the quarters. So the home page recomputes in your browser when it loads and every minute after, and the calculator does the same on every input. What you read there is current to the minute, not to the last build.
Planetary positions come from the VSOP87 theory. The moon's come from a series derived from E. W. Brown's lunar theory by way of the Nautical Almanac Office's Improved Lunar Ephemeris of 1954. The library holds itself to one arcminute against NOVAS running on JPL's DE405 ephemeris, and is cross-checked against JPL Horizons. One arcminute is far finer than anything the eye can resolve: the moon's whole disc is about thirty arcminutes across.
All times are given in Coordinated Universal Time (UTC) unless a page says otherwise. Because phase events are instants rather than days, a full moon can fall on different calendar dates in different time zones — a moon listed here as the 1st at 00:30 UTC is still the previous evening in the Americas.
The photographic surface texture used by the calculator's WebGL disc is adapted from NASA's CGI Moon Kit. NASA content is generally not copyrighted; we use the texture with credit, without NASA logos, and without implying NASA endorsement. The phase, terminator and libration are still computed here rather than baked into the image.
Method and reference data
- Astronomy EngineOpen-source ephemeris library
- JPL Horizons On-Line Ephemeris SystemNASA
- Astronomical AlgorithmsWillmann-Bell, 2nd edition, 1998
- Five Millennium Canon of Lunar Eclipses: −1999 to +3000NASA/TP-2009-214172, 2009
- Five Millennium Canon of Solar Eclipses: −1999 to +3000NASA/TP-2006-214141, 2006
- CGI Moon KitLRO/LROC WAC colour map and LOLA displacement data
- Astronomy on the Personal ComputerSpringer — source of the lunar series used here, after the Nautical Almanac Office’s Improved Lunar Ephemeris (1954) and E. W. Brown’s lunar theory
Historical and folklore claims
Astronomy is computed; history is cited. The traditional moon names and the stories attached to them come from the literature rather than from calculation, and the sources for each are listed at the foot of the page that makes the claim.
That distinction matters most for the full moon names. The familiar list was popularised by the Maine Farmers' Almanac in the 1930s and reprinted ever since, but its attributions are frequently unsourced, and the names are a mixture of Algonquian, Colonial American and English origins rather than a single coherent tradition. Individual nations kept their own lunar calendars, which often did not align with Gregorian months at all. Where a name has a documented origin the relevant page says so; where it is uncertain, it says that too.