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GHA of Aries.
1.18.2. GST, or GHA of Aries, specifies the position of the stars with relation to the earth. Thus, a given
star is in the same position relative to the earth at the same sidereal time each day.
1.19. Number of Days in a Year. The earth revolves around the sun in a year. The number of days in
the year equals the number of rotations of the earth during one revolution. The earth rotates eastward
about 366.24 times during its yearly eastward revolution. The total effect of one revolution and 366.24
rotations is that the sun appears to revolve around the earth 365.24 times per year. Therefore, there are
365.24 solar days per year. Since the sidereal day is measured with reference to a fixed point, the length
of the sidereal day is the period of the earth's rotation. Therefore, the number of sidereal days in the year
is equal to the number of rotations per year, 366.24.
34 AFPAM11-216 1 MARCH 2001
Figure 1.17. Air Almanac Conversion of Arc to Time.
AFPAM11-216 1 MARCH 2001 35
Figure 1.18. Greenwich Sidereal Time.
1.20. Navigator's Use of Time. Navigators use three different kinds of time— GMT, LMT, and ZT. All
three are based upon the motions of the fictitious mean sun. The mean sun revolves about the earth at the
average rate of the apparent sun, completing one revolution in 24 hours.
1.20.1. Time is based upon the motion of the sun relative to a given meridian. The time is 2400/0000 at
lower transit and 1200 at upper transit. In GMT, the reference meridian is that of Greenwich; in LMT,
the reference meridian is that of a given place; in ZT, the reference meridian is the standard meridian of
a given zone.
1.20.2. The difference between two times equals the difference of longitude of their reference meridians
expressed in time. GMT differs from ZT by the longitude of the zone's standard meridian; LMT differs
from ZT by the difference of longitude between the zone's standard meridian and the meridian of the
place. In interconverting ZT and GMT, the navigator uses the zone description. The zone difference is
the time difference between its standard meridian and GMT and it has a sign to indicate the correction to
ZT to obtain GMT. The sign is plus (+) for west longitude and minus (-) for east longitude.
Section 1E— Charts and Projections
1.21. Basic Information. There are several basic terms and ideas relative to charts and projections that
the reader should be familiar with before discussing the various projections used in the creation of
aeronautical charts.
1.21.1. A map or chart is a small scale representation on a plane of the surface of the earth or some
portion of it.
1.21.2. A chart projection is a systematic construction of lines on a plane surface to represent the
parallels of latitude and the meridians of longitude of the earth or a section of the earth.
36 AFPAM11-216 1 MARCH 2001
1.21.3. The chart projection forms the basic structure on which a chart is built and determines the
fundamental characteristics of the finished chart.
1.21.4. There are many difficulties that must be resolved when representing a portion of the surface of a
sphere upon a plane. Two of these are distortion and perspective.
1.21.5. Distortion cannot be entirely avoided, but it can be controlled and systematized to some extent in
the drawing of a chart. If a chart is drawn for a particular purpose, it can be drawn in such a way as to
minimize the type of distortion which is most detrimental to the purpose. Surfaces that can be spread out
in a plane without stretching or tearing, such as a cone or cylinder, are called developable surfaces, and
those like the sphere or spheroid that cannot be formed into a plane without distortion are called
nondevelopable (Figure 1.19).
Figure 1.19. Developable and Nondevelopable Surfaces.
1.21.6. The problem of creating a projection lies in developing a method for transferring the meridians
and parallels to the chart in a manner that will preserve certain desired characteristics as nearly as
possible. The methods of projection are either mathematical or perspective.
1.21.7. The perspective or geometric projection consists of projecting a coordinate system based on the
earth-sphere from a given point directly onto a developable surface. The properties and appearance of the
resultant map will depend upon two factors: the type of developable surface and the position of the point
of projection.
AFPAM11-216 1 MARCH 2001 37
1.21.8. The mathematical projection is derived analytically to provide certain properties or
characteristics which cannot be arrived at geometrically. Let us now consider some of the choices we
have for selecting projections which best accommodate these properties and characteristics.
　
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