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near the Earth's surface, the Earth is perceived as
essentially flat and, therefore, as a plane. If there are
no visual obstructions, the apparent intersection of
the sky with the Earth's (plane) surface is the horizon,
which appears as a circle centered at the observer. For
rise/set computations, the observer's eye is considered
to be on the surface of the Earth, so that the
horizon is geometrically exactly 90 degrees from the
local vertical direction.
(b) Rise, Set. During the course of a day the
Earth rotates once on its axis causing the phenomena
of rising and setting. All celestial bodies, the Sun,
Moon, stars and planets, seem to appear in the sky at
the horizon to the East of any particular place, then to
cross the sky and again disappear at the horizon to the
West. Because the Sun and Moon appear as circular
disks and not as points of light, a definition of rise or
set must be very specific, because not all of either
body is seen to rise or set at once.
(c) Sunrise and sunset refer to the times when
the upper edge of the disk of the Sun is on the horizon,
considered unobstructed relative to the location of
interest. Atmospheric conditions are assumed to be
average, and the location is in a level region on the
Earth's surface.
(d) Moonrise and moonset times are computed
for exactly the same circumstances as for
sunrise and sunset. However, moonrise and moonset
may occur at any time during a 24 hour period and,
consequently, it is often possible for the Moon to be
seen during daylight, and to have moonless nights. It
is also possible that a moonrise or moonset does not
occur relative to a specific place on a given date.
(e) Transit. The transit time of a celestial
body refers to the instant that its center crosses an
imaginary line in the sky - the observer's meridian -
running from north to south.
(f) Twilight. Before sunrise and again after
sunset there are intervals of time, known as
“twilight,” during which there is natural light
provided by the upper atmosphere, which does
receive direct sunlight and reflects part of it toward
the Earth's surface.
(g) Civil twilight is defined to begin in the
morning, and to end in the evening when the center of
the Sun is geometrically 6 degrees below the horizon.
This is the limit at which twilight illumination is
sufficient, under good weather conditions, for
terrestrial objects to be clearly distinguished.
2. Title 14 of the Code of Federal Regulations
applies these concepts and definitions in addressing
the definition of night (Section 1.1), the requirement
for aircraft lighting (Section 91.209) and pilot
recency of night experience (Section 61.67).
2/14/08 AIM
Special Operations 10-2-9
c. Information on Moon Phases and Changes in
the Percentage of the Moon Illuminated
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. 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. 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 in FIG 10-2-3.
FIG 10-2-3
Phases of the Moon
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.
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