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AIM

10/12/17

10−2−8

Special Operations

(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).