Monday, April 30, 2007

Moon Phases

Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing geometry of the Earth, Moon, and Sun. One half of the lunar surface is always illuminated by the Sun, and is hence bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon) to 0% (new moon). The boundary between the illuminated and unilluminated hemispheres is called the terminator.

The phases of the Moon have been given the following names, which are listed in sequential order:

Phase Name [PN] Northern hemisphere [NH] Southern hemisphere [SH]
New [PN] Not visible [NH] Not visible [SH]
Waxing Crescent [PN] Right 1-49% visible [NH] Left 1-49% visible [SH]
First Quarter [PN] Right 50% visible [NH] Left 50% visible [SH]
Waxing Gibbous [PN] Right 51-99% visible [NH] Left 51-99% visible [SH]
Full [PN] Fully visible [NH] Fully visible [SH]
Waning Gibbous [PN] Left 51-99% visible [NH] Right 51-99% visible [SH]
Third Quarter [PN] Left 50% visible [NH] Right 50% visible [SH]
Waning Crescent [PN] Left 1-49% visible [NH] Right 1-49% visible [SH]
New [PN] Not visible [NH] Not visible [SH]

When the Sun and Moon are aligned on the same side of the Earth, the Moon is "new", and is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress from 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 new moon phases. The terms old moon and new moon are interchangeable, although new moon is more common. Half moon is often used to mean the first- and third-quarter moons.

In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving towards a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (moving towards a new moon). Assuming that one is in the northern hemisphere, the right portion of the Moon is the part that is always growing. The acronym mnemonic "DOC" represents this: "D" is the waxing moon; "O" the full moon; and "C" the waning moon. (One phrase that can be used to remember this is "Dog comes; Cat goes".) In the Southern hemisphere, this order is reversed, and the mnemonic is "COD".

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