The sun illuminates one side of the moon and leaves the other side in shadow.
Anyone can see that the side of the moon facing towards the Sun is bright,
and the side facing away from the sun is dark.
As the moon orbits the Earth we
see these lighted and dark parts of the surface in changing proportions.
The changing
configurations of illumination on the moon's surface are known as lunar phases.
Throughout human history people have used the phases of the moon as a calendar.
There are approximately seven days separating the major phases of the moon, which serves as weeks,
and each entire cycle is approximately one month. The word "month" is derived from the word "moon."
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
Lunar Surface, 2009. Photo by Brad Snowder.
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New phase occurs when the Sun and
Moon are on the same side of the Earth and we see only the dark side. On that day the Sun
and Moon rise and set approximately together. The new moon is visible all day long.
Waxing Crescent phase occurs when the western edge of the Moon is lit but most of the surface
visible from Earth is dark. The amount of illumination visible is growing from day to day
during this phase which is what is meant by "waxing."
First Quarter phase occurs when the western half of the Moon is illuminated so that it looks
like the letter "D". On that day
the Moon rises at noon and sets at midnight.
Waxing Gibbous phase occurs when the Moon is mostly lit and the illuminated portion
is egg-shaped (gibbous) with the eastern edge shaded. The
amount of illuminated area visible is increasing from one day to the next which is what is meant
by "waxing".
Full phase occurs when the Moon is on the
opposite side of the Earth from the Sun and we see only the illuminated side. On that day
it rises as the Sun is setting and sets and the Sun is rising. It is visible all night long.
Waning Gibbous phase occurs when the Moon is mostly lit and the illuminated portion
is egg-shaped (gibbous) with the western edge shaded. The
amount of illuminated area visible is decreasing from one day to the next which is what is meant
by "waning".
Third Quarter phase occurs when the eastern half of the Moon is illuminated. On that day it rises at midnight and sets at noon.
Waning Crescent phase occurs when the eastern edge of the Moon is lit but most of the visible
surface is dark. The amount of illumination is decreasing from day to day which is what is meant by
"waning." During this time the illuminated portion of the Moon looks like the letter "C".