The Moon has phases because we see different portions of its sunlit hemisphere as the Moon travels around Earth. The Moon itself does not produce significant visible light of its own; it reflects sunlight. At every moment, approximately half of the Moon is illuminated by the Sun, just as half of Earth is illuminated at any given time.
From Earth, however, we do not always see the entire illuminated half because the Moon changes position relative to Earth and the Sun during its orbit. When the Moon is positioned approximately between Earth and the Sun, its illuminated side faces mostly away from Earth and we see a new Moon. As it moves onward, an increasing fraction of its sunlit side becomes visible, producing waxing phases. After full Moon, the visible illuminated fraction decreases, producing waning phases. The phases are therefore a consequence of geometry rather than Earths shadow.
The principal lunar phases are new Moon, first quarter, full Moon and third quarter, with intermediate phases called waxing crescent, waxing gibbous, waning gibbous and waning crescent. During new Moon, the Moon is approximately aligned between Earth and the Sun, although its orbit is usually tilted enough that the Moon passes above or below the Suns apparent position. At first quarter, approximately half of the lunar disc visible from Earth is illuminated.
At full Moon, Earth lies approximately between the Sun and Moon, so the hemisphere facing Earth is almost completely illuminated. At third quarter, the visible illuminated portion is again approximately half, but the opposite side is illuminated compared with first quarter. The terms first and third quarter refer to the Moons position through its orbital cycle rather than to the amount of illumination alone.
The Moon completes one orbit relative to the distant stars in approximately 27.3 days, called the sidereal month. However, the cycle from one new Moon to the next takes approximately 29.5 days and is called the synodic month. The difference occurs because Earth is simultaneously moving around the Sun. After the Moon has completed one orbit relative to the stars, the Sun Earth geometry has changed, so the Moon must travel somewhat farther to return to the same phase. This is the same general geometric principle that produces the difference between a sidereal and solar day. The approximately 29.5 day synodic cycle is the basis of the familiar sequence of lunar phases and has influenced calendars throughout human history.
Lunar phases do not occur because Earths shadow covers the Moon during ordinary monthly cycles. Earths shadow causes a lunar eclipse only when the Sun, Earth and Moon become suitably aligned and the Moon passes through Earths shadow. During most full Moons, the Moon passes above or below the shadow because its orbital plane is tilted by about 5 degrees relative to the ecliptic plane.
Similarly, a new Moon usually does not produce a solar eclipse because the Moons shadow misses Earth. This distinction is one of the most important misconceptions about lunar phases. The phase of the Moon describes how much of its illuminated hemisphere is visible from Earth, whereas an eclipse occurs when one celestial body physically passes through another bodys shadow or blocks its light under particular alignment conditions.
The appearance of the Moon changes predictably throughout the lunar cycle. A waxing Moon has an increasing illuminated fraction, while a waning Moon has a decreasing illuminated fraction. In the Northern Hemisphere, the illuminated portion of a crescent or gibbous Moon appears on characteristic sides of the lunar disc, although the apparent orientation changes with latitude, viewing time and hemisphere. In the Southern Hemisphere the familiar orientation is reversed relative to many Northern Hemisphere diagrams.
Near the equator the Moon can appear in orientations that differ from standard textbook illustrations. These differences do not represent different physical phases. They result from the observers position on the rotating spherical Earth and the changing orientation of the lunar path in the sky.
Lunar phases have historically been important for timekeeping and calendars because the cycle is readily observable without instruments. Many traditional calendars are lunar or lunisolar, using lunar months while incorporating adjustments to keep months aligned with the solar year. The word month is historically connected to the Moon in several languages and cultural traditions. The Islamic calendar is a lunar calendar, while several other calendar systems are lunisolar and periodically insert additional months or other adjustments. Modern civil calendars are primarily solar but retain historical relationships to lunar timekeeping. The phases have also been important in navigation, agriculture, cultural practices and observational astronomy, although the Moon does not provide a perfectly uniform month because its orbital speed and geometry vary.
The brightness and apparent size of the Moon also change slightly through the cycle because the Moons orbit is elliptical. When the full Moon occurs near perigee, its closest approach to Earth, it can appear somewhat larger than when full Moon occurs near apogee, its farthest point. This geometric variation is separate from the phase mechanism itself. The Moon also reflects different amounts of sunlight depending on surface terrain and viewing geometry, producing subtle brightness variations.
Earthshine can make the unilluminated portion of a young crescent faintly visible because sunlight reflected from Earth illuminates the Moons night side. These effects add complexity to the appearance of the Moon but do not alter the fundamental explanation: the Moon always has a sunlit half, and its orbital position determines how much of that illuminated half is visible from Earth.
The phases are ultimately a consequence of the three dimensional geometry of the Sun, Earth and Moon. The Sun provides the illumination, the Moon orbits Earth, Earth rotates beneath the changing lunar position, and the observer sees different fractions of the illuminated lunar hemisphere. Because the Moon is tidally locked, the same broad lunar hemisphere faces Earth, but this does not prevent phases because the phase depends on the direction of sunlight relative to the Earth facing hemisphere.
The sequence from new Moon through waxing phases to full Moon and then through waning phases repeats approximately every 29.5 days. Understanding the distinction between illumination, orbital position, eclipses and observer geometry provides a complete explanation of why the Moon appears to change shape even though its physical spherical shape remains essentially constant.
