Moon Phase Calculator

The Moon is tidally locked to Earth: its rotation period equals its orbital period (both 27.32 days). This synchronous rotation means the same hemisphere always faces Earth. Tidal locking happened over billions of years as Earth's gravity slowed the Moon's initial faster rotation. We can see slightly more than 50% of the Moon's surface over time through libration — the Moon's slight wobble as seen from Earth.

A lunar eclipse occurs when the Earth passes between the Sun and the Moon, and the Moon moves into Earth's shadow (umbra). This only happens at full moon when the alignment is nearly perfect. Lunar eclipses do not occur every full moon because the Moon's orbit is tilted about 5 degrees relative to the ecliptic, so usually the Moon passes slightly above or below Earth's shadow.

A supermoon occurs when a full moon (or new moon) coincides with the Moon being near perigee (closest point in its elliptical orbit). The Moon's distance varies from about 356,000 km (perigee) to 406,000 km (apogee). At perigee full moon, the Moon appears about 14% larger and 30% brighter than at apogee full moon. The term supermoon is popular but not standard in scientific astronomy.

The month was originally defined by the synodic period of the Moon — the period from new crescent to new crescent. Ancient Babylonian, Egyptian, Islamic, Hebrew, and Chinese calendars all began months with the new moon. The Islamic calendar is still a pure lunar calendar, with months of 29 or 30 days. Most modern calendars are solar or lunisolar, but the word month derives from the same root as moon.

The popular definition of a blue moon is the second full moon in a single calendar month (which has 30 or 31 days, while the synodic month is 29.5 days, so occasionally two full moons occur in one month). The older astronomical definition is the third full moon in a season that contains four full moons. The phrase once in a blue moon reflects that this is a somewhat rare occurrence (about once every 2.7 years by the popular definition).

Spring tides (highest high tides and lowest low tides) occur at new moon and full moon, when the Sun, Earth, and Moon are aligned and gravitational forces add together. Neap tides (smaller range) occur at first quarter and last quarter, when the Sun and Moon are at right angles and their tidal forces partially cancel. The tidal effect of the Moon is about 2.2 times stronger than that of the Sun.

The sidereal month (27.32 days) is the Moon's orbital period measured with respect to the stars — the time to return to the same position against the background constellations. The synodic month (29.53 days) is measured with respect to the Sun — the time from one new moon to the next. The synodic month is longer because Earth has moved along its orbit, requiring the Moon to complete slightly more than one full orbit to realign with the Sun.

Biodynamic agriculture claims that planting at certain moon phases improves yields. Scientific studies have generally not found statistically significant effects of moon phase on plant growth when other variables are controlled. The Moon's light at full moon is about 1/400,000th the intensity of sunlight — too dim to significantly drive photosynthesis. Any tidal effects on soil water are similarly negligible for agricultural purposes.

Libration refers to the apparent wobbling of the Moon as seen from Earth, caused by: (1) Libration in longitude — the Moon moves at varying speed in its elliptical orbit while rotating at constant speed, revealing slightly different longitudes. (2) Libration in latitude — the Moon's orbit is tilted 5.1 degrees, alternately revealing more of its north or south polar regions. (3) Diurnal libration — Earth's rotation provides slightly different viewing angles at different times of day. Combined, these allow us to see about 59% of the Moon's surface over time.