Lunar Phases

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 facing the Sun is illuminated.

The lunar phase depends on the Moon\'s position in orbit around the Earth, and the Earth\'s position in orbit around the sun. 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 examples of syzygies. The time between two Full Moons (or between successive occurrences of any two phases - for examples, First Quarter Moon to the next First quarter Moon) is about 29.53 days (or 29 days, 12 hours, 44 minutes); 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 wholly-illuminated disk. As the Moon orbits 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 Earth and the Sun and thus the illuminated half cannot be seen at all.

Gibbous (red) and crescent (blue) shapes resulting from dividing a circle with an inscribed half-ellipse.

Animation of the Moon as it cycles through its phases. The apparent wobbling of the Moon is known as libration.

According to geometry, when a sphere is illuminated on one hemisphere and viewed from an angle, then the portion of the illuminated area visible from that angle will be observed to have the two-dimensional shape of a half-ellipse inscribed within a half-circle, where the major axis of the ellipse is the same as the diameter of the semicircular arc. If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous, and if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent. If the eccentricity of the ellipse is 1, then a half-circle will be seen; while if the eccentricity of the ellipse is 0, then either a fully-illuminated or fully-dark circular disk will be seen (i.e. a full moon or new moon). Of course, actual observations of the Moon\'s lit area will not completely correspond with these abstract geometrical shapes, due to varying surface reflectivities, the lesser brightness of illuminated areas which are viewed at a low angle, etc.

The different phases of the Moon have different names. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), 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.

It would seem that, once per month every time the Moon passes between Earth and the Sun (ie. at a New Moon), its shadow should fall on Earth, causing a solar eclipse. Likewise, during every Full Moon, we might expect the Earth\'s shadow to be visible on the Moon -- a lunar eclipse. This is, in fact, how eclipses happen, but they don\'t happen at every Full or New Moon. This is because 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, meaning that at the New and Full Moons the Moon usually lies to the north or south of a direct line through the Earth and Sun. An eclipse can only occur when the Moon is positioned at, or very nearly at, one of its nodes (or crossing points between the Moon\'s orbital path and the ecliptic). This happens twice a year (roughly), and so there are between 4 and 7 eclipses in a calendar year. Most of these are quite insignificant; major eclipses of the Moon or Sun are rather rare and newsworthy events.

Casual observers will not typically notice a waxing Crescent Moon until about 60 hours after it has passed conjunction with the Sun (i.e after New Moon); but some individuals have crafted a hobby out of attempting to view the Moon after a much shorter interval than this. Informal \"records\" and their confirmability vary; some have claimed to have seen the Moon as little as 12 hours after the moment of conjunction. Three factors increase the likelihood of spying a very \"young\" Moon: First, the angle the Moon makes with the ecliptic must be favourable for the applicable side of the Earth -- the optimum scenario for this would be a New Moon that falls in mid-March in the Northern Hemisphere or mid-September in the Southern Hemisphere; second, the Moon should be at or near