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bodies would be obvious. The sun would appear to circle around the earth once each year. It would
cover 360o in 365 days, or move eastward at slightly less than 1 degree per day. The stars would move at
the same rate. (That's why different constellations are visible at different times of the year. Every
evening the same star appears to rise 4 minutes earlier.)
8.4.5. After half a year, when the earth reached the opposite extreme of its orbit, its dark side would be
turned in the opposite direction in space, facing a new field of stars. Hence, an observer at the equator
would see an entirely different sky at midnight in June than the one that appeared at midnight in
December. In fact, the stars seen at midnight in June are those that were above the horizon at midday in
December.
8.5. Seasons. The annual variation of the sun's declination and the consequent change of the seasons are
caused by the revolution of the earth (Figure 8.4). If the celestial equator coincided with the ecliptic, the
sun would always be overhead at the equator, and its declination would always be zero. However, the
earth's axis is inclined about 66 1/2o to the plane of the earth's orbit, and the plane of the equator is
inclined about 23 1/2o. Throughout the year, the axis points in the same direction. That is, the axis of the
earth in one part of the orbit is parallel to the axis of the earth in any other part of the orbit (Figure 8.5).
8.5.1. In June, the North Pole is inclined toward the sun so that the sun is at a maximum distance from
the plane of the equator. About June 22, at the solstice, the sun has its greatest northern declination.
8.5.2. The solstice brings the long days of summer, while in the Southern Hemisphere, the days are
shortest. This is the beginning of summer for the Northern Hemisphere and of winter for the Southern
Hemisphere. Six months later, the axis is still pointing in the same direction; but, since the earth is at the
opposite side of its orbit and the sun, the North Pole is inclined away from the Sun. At the winter
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solstice, about December 21, the sun has its greatest southern declination. Days are shortest in the
Northern Hemisphere, and winter is beginning.
8.5.3. Halfway between the two solstices, the axis of the earth is inclined neither toward nor away from
the sun, and the sun is on the plane of the equator. These positions correspond to the beginning spring
and fall.
Figure 8.4. Seasonal Changes of Earth's Position.
202 AFPAM11-216 1 MARCH 2001
Figure 8.5. Ecliptic With Solstices and Equinoxes.
Section 8C— Celestial Coordinates
8.6. Basics. Celestial bodies and the observer's zenith may be positioned on the celestial sphere, using a
coordinate system similar to that of the earth. Terrestrial lines of latitude correspond to celestial parallels
of declination. Lines of longitude establish the celestial meridians.
8.6.1. The observer's celestial meridian is a great circle containing the zenith, the nadir, and the celestial
poles (Figure 8.2). A line extended from the observer's zenith, through the center of the earth, intersects
the celestial sphere at the observer's nadir, the point on the celestial sphere directly beneath the
observer's position. The poles divide the celestial meridians into upper and lower branches. The upper
branch contains the observer's zenith. The lower branch contains the nadir.
8.6.2. A second great circle on the celestial sphere is the hour circle. The hour circle contains the
celestial body and the celestial poles. Unlike celestial meridians, which remain stationary, hour circles
rotate 15o per hour. Hour circles also contain upper and lower branches. The upper branch contains the
body. Again, the moon's hour circle moves at a different rate. The subpoint is the point on the earth's
surface directly beneath the celestial body.
8.6.3. You can locate any body on the celestial sphere relative to the celestial equator and the Greenwich
meridian using declination and Greenwich hour angle.
8.7. Declination (Dec). Declination is the angular distance a celestial body is north or south of the
celestial equator measured along the hour circle. It ranges from 0o to 90o and corresponds to latitude.
8.8. Greenwich Hour Angle (GHA). GHA is the angular distance measured westward from the
Greenwich celestial meridian to the upper branch of the hour circle. It has a range of 0o to 360o. The Air
Almanac lists the GHA and the Dec of the sun, moon, four planets, and Aries. The subpoint's latitude
matches its Dec, and its longitude correlates to its GHA, but not exactly. GHA is always measured
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westward from the Greenwich celestial meridian, and longitude is measured in the shortest direction
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