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Although the typically cloudy skies
of Erie, Pennsylvania, will make it a challenge, I have decided
to try to capture images of the moon on as many of its 29.5 day
cycle as possible. It would be really cool to be able to get 28-,
29-, and 1-day-old moon shots by being able to occult the sun's
disk in the aperture, but I don't have the equipment necessary.
People have been successful at getting 27- and 2-day-old moon shots
pretty easily. My first shot was the evening of April 2, 2012.
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Here are some excerpts about phases of the moon from the
U.S. Naval Observatory's
website: *** Begin USNO *** From any location on the Earth,
the Moon appears to be a circular disk which, at any specific time,
is illuminated to some degree by direct sunlight. Like the Earth,
the Moon is a sphere which is always half illuminated by the Sun,
but as the Moon orbits the Earth we get to see more or less of the
illuminated half. During each lunar orbit (a lunar month), we see
the Moon's appearance change from not visibly illuminated through
partially illuminated to fully illuminated, then back through partially
illuminated to not illuminated again. Although this cycle is a continuous
process, there are eight distinct, traditionally recognized stages,
called phases. The phases designate both the degree to which the
Moon is illuminated and the geometric appearance of the illuminated
part. These phases of the Moon, in the sequence of their occurrence
(starting from New Moon), are listed below.
- New Moon - The Moon's unilluminated side is facing the Earth.
The Moon is not visible (except during a solar eclipse).
- 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.
- 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.
- 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.
- Full Moon - The Moon's illuminated side is facing the Earth.
The Moon appears to be completely illuminated by direct sunlight.
- 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.
- 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.
- 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.
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". See the article "Once in a Blue Moon" for
the story of how the usage of this term has evolved
(Ref: Philip Hiscock, Sky & Telescope, March 1999, pp. 52-55.).
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. New Moon, First Quarter, Full
Moon, and Last Quarter phases are considered to be primary phases
and their dates and times are published in almanacs and on calendars.
The two crescent and two gibbous phases are intermediate phases,
each of which lasts for about a week between the primary phases,
during which time the exact fraction of the Moon's disk that is
illuminated gradually changes.
The phases of the Moon are related to (actually, caused by) the
relative positions of the Moon and Sun in the sky. For example,
New Moon occurs when the Sun and Moon are quite close together in
the sky. Full Moon occurs when the Sun and Moon are at nearly opposite
positions in the sky - which is why a Full Moon rises about the
time of sunset, and sets about the time of sunrise, for most places
on Earth. First and Last Quarters occur when the Sun and Moon are
about 90 degrees apart in the sky. In fact, the two "half Moon"
phases are called First Quarter and Last Quarter because they occur
when the Moon is, respectively, one- and three-quarters of the way
around the sky (i.e., along its orbit) from New Moon. 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. *** End USNO ***
Posted April 5, 2012
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