How We Calculate Moon Data

Every number on MoonPhaseToday is calculated, not copied: one lunar engine produces the phases, times, positions and calendars on every page, in the widget and in the API.

The engine

We use Astronomy Engine by Don Cross, an open-source (MIT) library whose author reports positions accurate to about ±1 arcminute, tested against NASA JPL Horizons and the US Naval Observatory’s NOVAS. Phase times are found to within about a second of the model; we display them to the minute.

Definitions we use

• Phase angle: the Moon’s ecliptic longitude minus the Sun’s (0° new moon, 90° first quarter, 180° full moon, 270° last quarter).
• Primary phases are instants. A calendar day is named after a primary phase when that instant falls inside the day; a live “now” view uses the primary name within ±12 hours of the instant. Between them we use waxing/waning crescent and gibbous.
• Illumination is the illuminated fraction of the Moon’s visible disc.
• Moon age is the time since the previous new moon.
• Supermoon: a full moon within 90% of perigee for that orbit (Richard Nolle’s definition); micromoon: within 90% of apogee. We always show the distance.
• Blue Moon: monthly (second full moon in a UTC calendar month) or seasonal (third of four full moons between an equinox and a solstice).
• Full moon names follow the North American almanac tradition by UTC month. The Harvest Moon is the full moon nearest the September equinox.
• Moonrise and moonset are when the Moon’s upper edge crosses a flat sea-level horizon, including standard atmospheric refraction.
• Moon sign: the tropical zodiac sign of the Moon’s apparent geocentric ecliptic longitude of date.
• Your location on live pages is approximated from your internet connection by Cloudflare (city level) unless you choose “Use my precise location”; it is not stored.

Time zones

Calendars and date pages use Coordinated Universal Time (UTC) so that every reader sees the same page. Live panels, city pages and the “your time” labels convert to local time, including daylight saving time.

Drawing the Moon

The Moon image is drawn from the computed illuminated fraction, the position angle of the bright limb, and the tilt of the Moon’s axis. On the homepage it is rotated to your zenith, so the drawing matches what you see when you look up. The surface texture is an original illustration.

Corrections

Found a value that disagrees with another reputable source? Tell us the date, place and both values; we publish corrections on the affected page.