Lunar cycle
The lunar cycle is the repeating change in the Moon's visible shape as it moves around Earth. A complete cycle, from one New Moon to the next, takes about 29.5 days and is called a synodic month.
The four principal phases are New Moon, First Quarter, Full Moon, and Last Quarter. Astronomically, these occur when the Moon's ecliptic longitude differs from the Sun's by about 0, 90, 180, and 270 degrees. Published phase times are usually geocentric, calculated for the center of Earth, so local viewing conditions can vary slightly.
At New Moon, the Moon is near the Sun in the sky and its sunlit side faces mostly away from Earth. As the Moon moves eastward along its orbit, more of the bright side becomes visible: this is the waxing half of the cycle. The light grows through Waxing Crescent, First Quarter, and Waxing Gibbous until Full Moon, when the Moon is opposite the Sun and rises near sunset.
After Full Moon, the illuminated part shrinks. This waning half passes through Waning Gibbous, Last Quarter, and Waning Crescent before returning to New Moon. The exact phase is not caused by Earth's shadow; it is the viewing angle between the Sun, Earth, and Moon. Earth's shadow creates a lunar eclipse only when the alignment is especially exact.
The intermediate phases are the crescent and gibbous phases between the four principal points. On average, each intermediate phase lasts about one quarter of a synodic month, roughly 7.4 days, but the Moon's elliptical orbit means the timing is not perfectly even.
A sidereal month, the Moon's orbit relative to the stars, is about 27.3 days. The phase cycle is longer because Earth is also moving around the Sun; the Moon must travel a little farther before the Sun-Earth-Moon geometry repeats.
The Moon always keeps nearly the same face turned toward Earth because its rotation period is locked to its orbital period. Even so, a gentle rocking motion called libration lets observers see a little more than half of the lunar surface over time.
The Moon's phase appearance also depends on the observer's latitude. A crescent that looks upright in one region can appear tilted elsewhere, and the view is effectively rotated between the Northern and Southern Hemispheres.
The thin crescent phases can show earthshine: faint light on the dark part of the Moon, caused by sunlight reflecting from Earth back onto the lunar surface. Around Full Moon the Moon is brightest, but surface shadows are short; near the quarter phases, long shadows make craters and mountain ranges easier to see.
Moonrise and moonset shift later from day to day because the Moon moves along its orbit while Earth rotates. The interval is not fixed, but it is often close to 50 minutes. Distance also changes during the month, so some Full Moons appear slightly larger near perigee and slightly smaller near apogee.
Eclipses do not happen every month because the Moon's orbital plane is tilted by about five degrees relative to Earth's orbital plane, the ecliptic. A solar eclipse requires a New Moon near a lunar node, while a lunar eclipse requires a Full Moon near one of those nodes.
Lunar phases have also been used for timekeeping. Pure lunar calendars follow lunations directly, while lunisolar calendars add adjustments because twelve lunar months are shorter than a solar year by about ten or eleven days.
