Moon Phase Apps: How to Track the Lunar Calendar and Understand Each Phase

The moon goes through a predictable cycle every 29.5 days, moving through eight distinct phases from New Moon to Full Moon and back again. Tracking these phases is useful for a surprising variety of reasons — planning nighttime photography, scheduling fishing trips, gardening by lunar cycles, or satisfying your curiosity about the bright or dark sky overhead.

Several excellent free apps make it easy to see the current moon phase, the phase for any past or future date, and the exact time of moonrise and moonset in your location. The best options for most people are the following: Moon — Current Moon Phase (available on iPhone and Android), My Moon Phase (Android), and the built-in lunar calendar in Stellarium (covered separately). The USNO Moon Phase Calendar at aa.usno.navy.mil is a free government website that requires no app at all.

Understanding the phases themselves is straightforward. A New Moon occurs when the moon is between the Earth and Sun — the side facing us is in shadow, so we see little or nothing. A Full Moon occurs when Earth is between the sun and moon — the side facing us is fully lit. The phases in between (crescent, quarter, gibbous) describe how much of the lit surface we can see from Earth.

Full Moons are often bright enough to cast shadows and are popular for outdoor nighttime activities. New Moon nights are the darkest and best for stargazing, since there is no competing moonlight. Tracking the moon helps you plan accordingly.