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understand transit. Notice in Figure 1.12 that the poles divide the observer's meridian into halves. The
observer's position is in the upper branch. The lower branch is the opposite half. Every day, because of
the earth's rotation, every celestial body transits the upper and lower branches of the observer's meridian.
The first kind of time presented here is solar time.
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Figure 1.12. Transit Is Caused by the Earth's Rotation.
1.10. Apparent Solar Time. The sun as you see it in the sky is called the true sun or the apparent sun.
Apparent solar time is based upon the movement of the sun as it crosses the sky. A sundial accurately
indicates apparent solar time. Apparent solar time isn't useful because the apparent length of day varies
throughout the year. A timepiece would have to operate at different speeds to indicate correct apparent
time. However, apparent time accurately indicates upper and lower transit. Upper transit occurs at noon;
apparent time and lower transit at midnight apparent time. Difficulties in using apparent time led to the
introduction of mean time.
1.11. Mean Solar Time. A mean day is an artificial unit of constant length, based on the average of all
apparent solar days over a period of years. Time for a mean day is measured with reference to a fictitious
body, the mean sun, so designed that its hour circle moves westward at a constant rate along the celestial
equator. Time computed using the mean sun is called mean solar time. The coordinates of celestial
bodies in the Air Almanac are tabulated in mean solar time, making it the time of primary interest to
navigators. The difference in length between the apparent day (based upon the true sun) and the mean
day (based upon the mean sun) is never as much as a minute. The differences are cumulative, however,
so that the imaginary mean sun precedes or follows the apparent sun by approximately 15 minutes at
certain times during the year.
1.12. Greenwich Mean Time (GMT). GMT is used for most celestial computations. GMT is mean
solar time measured from the lower branch of the Greenwich meridian westward through 360o to the
upper branch of the hour circle passing through the mean sun (Figure 1.13). The mean sun transits the
Greenwich meridian's lower branch at GMT 2400 (0000) each day and the upper branch at GMT 1200.
The meridian at Greenwich is the logical selection for this reference, as it is the origin for the
measurement of Greenwich hour angle (GHA) and the reckoning of longitude. Consequently, celestial
coordinates and other information are tabulated in almanacs with reference to GMT. GMT is also called
Zulu or Z time.
30 AFPAM11-216 1 MARCH 2001
Figure 1.13. Measuring Greenwich Mean Time.
1.13. Local Mean Time (LMT). Just as GMT is mean solar time measured with reference to the
Greenwich meridian, LMT is mean solar time measured with reference to the observer's meridian. LMT
is measured from the lower branch of the observers meridian, westward through 360o, to the upper
branch of the hour circle passing through the mean sun (Figure 1.13). The mean sun transits the lower
branch of the observer's meridian at LMT 0000 (2400) and the upper branch at LMT 1200. For an
observer at the Greenwich meridian, GMT is LMT. Navigators use LMT to compute local sunrise,
sunset, twilight, moonrise, and moonset at various latitudes along a given meridian.
Section 1D— Relationship of Time and Longitude
1.14. Introduction. The mean sun travels at a constant rate, covering 360o of arc in 24 hours. The mean
sun transits the same meridian twice in 24 hours. The following relationships exists between time and
arc:
Time Arc
24 hours 360o
1 hour 15o
4 minutes 1o
1 minute 15'
1.14.1. Local time is the time at one particular meridian. Since the sun cannot transit two meridians
simultaneously, no two meridians have exactly the same local time. The difference in time between two
meridians is the time of the sun's passage from one meridian to the other. This time is proportional to the
angular distance between the two meridians. One hour is equivalent to 15o.
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1.14.2. If two meridians are 30o apart, their time differs by 2 hours. The easternmost meridian has a later
local time because the sun has crossed its lower branch first; thus, the day is older there. These
statements hold true whether referring to the apparent sun or the mean sun. Figure 1.14 demonstrates
that the sun crossed the lower branch of the meridian of observer # 1 at 60o east longitude 4 hours before
it crossed the lower branch of the Greenwich meridian (60 divided by 15) and 6 hours before it crossed
the lower branch of the meridian of observer #2 at 30o west longitude (90 divided by 15). Therefore, the
　
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