Aeronautical Information Manual (AIM), page 586

Index   585 -- Page 586 -- 587

(2) Significant reflected cultural lighting
(such as the illumination caused by the reflection of
a major metropolitan area's lighting reflecting off a
cloud ceiling), or

(3) Limited cultural lighting combined
with a high level of natural reflectivity of celestial
illumination, such as that provided by a surface
covered by snow or a desert surface.

2. Low lighting conditions are those that do not
meet the high lighting conditions requirements.
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 character-
ized 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 experi-
ence, 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 consid-
ered 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 com-
puted 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).

10-2-8 Special Operations

Page 586 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   585 -- Page 586 -- 587