In astronomy, a phase of the Moon is any of the aspects or appearances presented by the Moon as seen from Earth, determined by the portion of the Moon that is visibly illuminated by the Sun. The lunar phases vary cyclically as the Moon orbits the Earth, according to the relative positions of the Earth, the Moon, and the Sun. Since the Moon appears bright only due to the Sun's reflected light, only the half of the Moon closest to the Sun is illuminated.
The lunar phase depends on the Moon's position in orbit around the Earth. This diagram looks down on Earth from the north. Earth's rotation and the Moon's orbit are both counter-clockwise here. From this diagram, we can see, for example, that the full moon will always rise at sunset, and that the waning crescent moon is high overhead around 9:00 AM local time.
Lunar phases are the result of our seeing the illuminated half of the Moon at different angles. The Moon exhibits different phases as the relative positions of the Sun, Earth and Moon change, appearing as the full moon when the Sun and Moon are on opposite sides of the Earth, and becoming invisible as the new moon (also named dark moon) when they are on the same side: these two phases are called syzygies. The time between two full moons is about 29.5 days; it is longer than the time it takes the Moon to orbit the Earth since the Earth-Moon system is orbiting the Sun. The phases are not created by the shadow of the Earth on the moon (that would be a Lunar eclipse); instead, they are a result of our seeing only part of the illuminated half of the Moon.
- Dark Moon - Not visible
- New Moon - Not visible, or traditionally: first visible crescent of the Moon
- Waxing crescent Moon - Right 1-49% visible
- First quarter Moon - Right 50% visible
- Waxing gibbous Moon - Right 51-99% visible
- Full Moon - Fully visible
- Waning gibbous Moon - Left 51-99% visible
- Third quarter Moon - Left 50% visible
- Waning crescent Moon - Left 1-49% visible
- New Moon - Not visible
In the southern hemisphere, the above is reversed. For example:
- Waxing crescent Moon - Left 1-49% visible
- Waning crescent Moon - Right 1-49% visible
When the Sun and Moon are on opposite sides of the Earth, the Moon appears full: the moon appears as a whole disc. As the Moon orbits the earth, the moon wanes, as the amount of illuminated lunar surface reduces, until the moon effectively disappears at the New Moon, when the Moon is between the Earth and the Sun and thus the illuminated half cannot be seen at all.
The different phases of the moon have different names. As the moon waxes (the amount of illuminated surface is growing), the moon moves through the New Moon, Crescent Moon, First-Quarter Moon, Gibbous Moon and Full Moon phases, before returning through the Gibbous Moon, Third-quarter Moon, Crescent Moon and Old Moon phases. Old Moon and New Moon are interchangeable, although New Moon is used in preference, and Half Moon is often used to mean the First- and Third-Quarter Moons.
Note that the plane of the Moon's orbit around the Earth is tilted by about 5 degrees with respect to the Earth's orbit around the Sun. Therefore, eclipses of the Moon during the full Moon and of the Earth by a new Moon are rare and usually newsworthy.
In the northern hemisphere, if the right side of the Moon is dark, the light part is shrinking: the Moon is waning (moving towards a new Moon). If the left side is dark, the Moon is waxing (moving towards a full Moon). The acronym mnemonic "DOC" represents this ("D" is the waxing Moon; "O" the full moon; and "C" the waning moon). In the Southern hemisphere, this is reversed, and the mnemonic is "COD". A French mnemonic is that the waxing moon at its first "premier" quarter phase looks like a 'p', and the waning moon at its last "dernier" quarter looks like a 'd'. The southern hemisphere equivalent for 'p' and 'd' is that the moon is 'past it', or 'doing it'. One more (Northern hemisphere) mnemonic, which works for most Romance languages, says that the Moon is a liar: it spells "C", as in crescere (Italian for "to grow") when it wanes, and "D" as in decrescere ("decrease") when it waxes. For Polish it is easy to remember that C stands for "cofa się" ("is going back") and D - for "dopełnia się" ("is filling up"). In German, one mnemonic uses the cursive forms of the capital letters A for "abnehmend" (waning) and Z for "zunehmend" (waxing). T
A first quarter Moon follows a daily path in the sky corresponding to that of the Sun after three months. Hence it comes at the highest altitude in Spring. Similarly, full moon comes highest in Winter, last quarter Moon in Autumn, and (almost) new Moon in Summer.
For an animation of how the Moon appears from Earth over the course of an orbit, see libration.