Moon Phases
As the Moon moves around the Earth in its orbit, its position in the sky relative to the Sun changes constantly. The portion of the Moon's surface that we see illuminated by sunlight changes over the course of the Moon's 29 1/2-day phasing cycle.
The diagram to the right shows a view of the
Earth-Moon system as seen looking "down"
upon Earth's north pole. The Sun's light is
shining in from the right side. The inner ring
shows the Moon in four "cardinal" orbital
positions, the illuminated half always facing the
Sun. The outer images show how the Moon's phase
will appear from the Earth.
Click on the diagram for a Moon phase
simulation.
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| At any given moment, the Moon, spherical in shape, is half illuminated by the Sun, and half in darkness. |
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New
Moon: When the Moon is nearly between us and the Sun, its night side faces the Earth and its disk appears completely dark. This is called the New Moon. |
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First
Quarter Moon: When the Moon has moved though one quarter of its orbit around the Earth, half of the side facing Earth is lit, the other dark. This is called the First Quarter phase because the Moon has moved along one quarter of its orbit (with New Moon as the starting point). |
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Full
Moon: Upon completing one half of its orbit, the Full Moon phase is seen from Earth and the entire disk of the Moon facing us is illuminated. Before and after the Full Moon, when more than half (but not all) of the lunar surface is illuminated, the Moon phases are called "gibbous". |
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Third
Quarter Moon: Third Quarter phase occurs when the Moon has traversed three quarters of its orbit, and again we see half of the lunar surface illuminated, though it is the opposite half with respect to First Quarter phase. |
The terms "waxing" and "waning" are used to describe whether the Moon's phase is growing or declining. A crescent Moon that is growing, following the New Moon, is called a "waxing crescent", whereas the gibbous Moon following the Full Moon is a "waning gibbous". |
