Figure 15. Geometry of the sun-earth system. The earth’s
orbit, the large ellipse with major axis AP and the sun at one focus, defines
the plane of the ecliptic. The plane of the earth's axial tilt (shaded)
is shown passing through the sun corresponding to the time of the southern
summer solstice. The earth moves around its orbit in the direction of the
solid arrow (period one year) while spinning about its axis in the direction
shown by the thin curved arrows (period one day). The earth’s spin axis
describes a slow retrograde motion, called precession, shown by the thicker
curved arrows (period about 22,000 years), and varies in degree of tilt
from 21.5° to 24.5° (period 41,000 years).
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