| 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. |
