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The Earth and the Moon
Phases of the Moon - Relative Positions
of the Earth, Moon and Sun
References:
[Wikipedia: Lunar Phase, Arny 2002]
How can the Moon appear at its fullest when the Earth is between it
and the Sun, and why is the Moon not visible when it lies between
the Earth and Sun? To answer these questions consider first the
diagram below of the relative positions of the Earth, Moon and Sun
during the eight phases of the Moon.
Earth, Moon and Sun Positions During Lunar Phases |
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Globe image courstesy of
Wikipedia |
The diagram shown is typical of those used to describe the phases of
the Moon, but it is easy to become confused if the diagram is not
interpreted correctly. There are two representations of the Moon
shown for each of the eight lunar phases. The smaller half-gray
circles represent how the Moon is illuminated by the Sun
throughout its orbit. Each symbol is exactly the same indicating
that one-half or one hemisphere of the Moon remains illuminated
throughout its orbit. The larger white circles, on the other hand,
indicate how the Moon appears in the sky during each of the lunar
phases. To demonstrate how illumination and appearance are related
semi-transparent carets are positioned to indicate which part of the Moon's Earth-facing
side is illuminated during each phase. During the New Moon the far side of the Moon
is illuminated by the Sun, while the side that faces the Earth lies
in darkness so the Moon is invisible. As the Moon follows
its path from the New Moon position more and more of the Earth-facing
side becomes illuminated resulting in the “waxing” phases. When the
Moon is on the opposite side of the Earth from the Sun its Earth-facing
side is fully illuminated producing the Full Moon. As the Moon follows
its path from the Full Moon position less and less of the Earth-facing side
is illuminated resulting in the “waning” phases.
While the diagram above seems to explain why the Moon looks as it does
as it orbits the Earth, it doesn’t appear to take into account any
shadows that are cast either by the sunlit Earth or the sunlit Moon
during their respective orbits. In the next sections the discussion continues as the role
that shadowing plays in two of the most interesting celestial
phenomena— lunar and solar eclipses —is introduced.
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Basics of Celestial Motion. Copyright 2006
S. E. Scruggs
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