Following waning crescent is New Moon, beginning a repetition of the
complete phase cycle of 29.5 days average duration. The time in days
counted from the time of New Moon is called the Moon's "age". Each
complete cycle of phases is called a "lunation".
Because the cycle of the phases is shorter than most calendar months,
the phase of the Moon at the very beginning of the month usually repeats
at the very end of the month. When there are two Full Moons in a month
(which occurs, on average, every 2.7 years), the second one is called a
"Blue Moon".
Although Full Moon occurs each month at a specific date and time, the
Moon's disk may appear to be full for several nights in a row if it is
clear. This is because the percentage of the Moon's disk that appears
illuminated changes very slowly around the time of Full Moon (also
around New Moon, but the Moon is not visible at all then). The Moon may
appear 100% illuminated only on the night closest to the time of exact
Full Moon, but on the night before and night after will appear 97-99%
illuminated; most people would not notice the difference. Even two days
from Full Moon the Moon's disk is 93-97% illuminated.
The relationship of the Moon's phase to its angular distance in the
sky from the Sun allows us to establish very exact definitions of
when the primary phases occur, independent of how they appear.
Technically, the phases New Moon, First Quarter, Full Moon, and Last
Quarter are defined to occur when the excess of the apparent
ecliptic (celestial) longitude of the Moon over that of the Sun is
0, 90, 180, and 270 degrees, respectively. These definitions are
used when the dates and times of the phases are computed for
almanacs, calendars, etc. Because the difference between the
ecliptic longitudes of the Moon and Sun is a monotonically and
rapidly increasing quantity, the dates and times of the phases of
the Moon computed this way are instantaneous and well defined.
The percent of the Moon's surface illuminated is a more refined,
quantitative description of the Moon's appearance than is the phase.
Considering the Moon as a circular disk, the ratio of the area
illuminated by direct sunlight to its total area is the fraction of
the Moon's surface illuminated; multiplied by 100, it is the percent
illuminated. At New Moon the percent illuminated is 0; at First and
Last Quarters it is 50%; and at Full Moon it is 100%. During the
crescent phases the percent illuminated is between 0 and 50% and
during gibbous phases it is between 50% and 100%.
For practical purposes, phases of the Moon and the percent of the
Moon illuminated are independent of the location on the Earth from
where the Moon is observed. That is, all the phases occur at the
same time regardless of the observer's position.
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New Moon - The
Moon's un-illuminated side is facing the Earth. The Moon is not
visible (except during a solar eclipse). |
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Waxing Crescent
- The Moon appears to be partly but less than one-half
illuminated by direct sunlight. The fraction of the Moon's disk that
is illuminated is increasing. |
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First Quarter -
One-half of the Moon appears to be illuminated by direct sunlight.
The fraction of the Moon's disk that is illuminated is increasing. |
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Waxing Gibbous
- The Moon appears to be more than one-half but not fully
illuminated by direct sunlight. The fraction of the Moon's disk that
is illuminated is increasing. |
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Full Moon - The
Moon's illuminated side is facing the Earth. The Moon appears to be
completely illuminated by direct sunlight. |
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Waning Gibbous
- The Moon appears to be more than one-half but not fully
illuminated by direct sunlight. The fraction of the Moon's disk that
is illuminated is decreasing. |
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Last Quarter -
One-half of the Moon appears to be illuminated by direct sunlight.
The fraction of the Moon's disk that is illuminated is decreasing. |
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Waning Crescent
- The Moon appears to be partly but less than one-half
illuminated by direct sunlight. The fraction of the Moon's disk that
is illuminated is decreasing. |