3. Some areas may be considered a high lighting environment only in specific circumstances. For example,
some surfaces, such as a forest with limited cultural lighting, normally have little reflectivity, requiring
dependence on significant moonlight to achieve a high lighting condition. However, when that same forest is
covered with snow, its reflectivity may support a high lighting condition based only on starlight. Similarly, a
desolate area, with little cultural lighting, such as a desert, may have such inherent natural reflectivity that it may
be considered a high lighting conditions area regardless of season, provided the cloud cover does not prevent
starlight from being reflected from the surface. Other surfaces, such as areas of open water, may never have
enough reflectivity or cultural lighting to ever be characterized as a high lighting area.
4. Through the accumulation of night flying experience in a particular area, the operator will develop the
ability to determine, prior to departure, which areas can be considered supporting high or low lighting conditions.
Without that operational experience, low lighting considerations should be applied by operators for both
pre-flight planning and operations until high lighting conditions are observed or determined to be regularly
available.
b. Astronomical Definitions and Background Information for Night Operations
1. Definitions
(a) Horizon. Wherever one is located on or 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).
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

Special Operations

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