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horizon is geometrically exactly 90 degrees from the
local vertical direction.
AIP ENR 6.2−9
United States of America 15 MAR 07
Federal Aviation Administration Nineteenth Edition
2.2.1.2Rise, 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.
2.2.1.3Sunrise 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.
2.2.1.4Moonrise 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.
2.2.1.5Transit.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.
2.2.1.6Twilight.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.
2.2.1.7Civil 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.
2.2.2Title 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).
2.2.3Information 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
NewMoon), are listed in FIG ENR 6.2−3.
2.2.3.1The 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, 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%.
2.2.3.2For 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.
2.2.3.3For more detailed information, refer to the
United States Naval Observatory site referenced
below.
ENR 6.2−10 AIP
15 MAR 07 United States of America
Nineteenth Edition Federal Aviation Administration
FIG ENR 6.2−3
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.
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