The moon runs through eight named phases every 29.5 days as the angle between the Sun, Earth, and Moon changes. This calculator turns any calendar date into the phase, the illuminated percentage, the moon's age, and β for your location β moonrise and moonset. Here is how it works and how to use each output.
How the calculator works
Every calculation starts by converting your date to a Julian Date, a continuous day count astronomers use. Subtracting the date of a known new moon (2000-01-06) and taking the remainder over the 29.53-day synodic month gives the moon age. A cosine of that age yields the illuminated fraction: 0% at new moon, 100% at full moon, 50% at the quarters.
Moonrise, moonset, and the EarthβMoon distance use an abridged version of the standard Meeus lunar-position series. The calculator samples the moon's altitude across the day and finds when it crosses the horizon, so rise/set times are typically accurate to a few minutes.
Inputs and what they mean
Date drives everything β phase, illumination, age, and the upcoming events list. Latitude and longitude only affect rise and set times; the phase looks the same from anywhere on Earth. Typing a city auto-fills its coordinates. Because the tool infers your timezone from longitude, rise/set times are clock-approximate and do not adjust for daylight saving time or local terrain.
Limits and edge cases
The phase and illumination formulas use the mean synodic month, so predicted new/full moon dates can differ from a precise ephemeris by up to roughly half a day far from the present. At high latitudes the moon can stay up or down for a full calendar day, in which case moonrise or moonset is shown as a dash. For navigation, eclipse timing, or scientific work, consult an authoritative source such as the U.S. Naval Observatory rather than this hobby tool.