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The phases of the
moon are caused by the relative positions of the earth, sun, and
moon. The moon goes around the earth, on average, in 27 days 7
hours 43 minutes.
The sun always illuminates the half of the moon facing the sun
(except during lunar eclipses, when the moon passes thru the
earth's shadow). When the sun and moon are on opposite sides of
the earth, the moon appears "full" to us, a bright, round disk.
When the moon is between the earth and the sun, it appears dark,
a "new" moon. In between, the moon's illuminated surface appears
to grow (wax) to full, then decreases (wanes) to the next new
moon.
The edge of the shadow (the terminator) is always curved, being
an oblique view of a circle, giving the moon its familiar
crescent shape. Because the "horns" of the moon at the ends of
the crescent are always facing away from the setting or rising
sun, they always point upward in the sky. It is fun to watch for
paintings and pictures which show an "impossible moon" with the
horns pointed downwards.
See the moon phases
on a calendar
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