The "obliquity" of the Earth is the angle between the plane of the equator and the plane of the Earth's orbit, and it is quite important in determining climate belts around the Earth's sphere. .... ... Rubincam et al (National Aeronautics and Space Administration, US) review the periodic shrinkage of Earth's tropical zone. In the past year, the Earth completed one revolution around the Sun, the Moon went through its phases 13 times, and the tropic of Cancer moved another 14.7 meters south. The tropic of Cancer is the latitude at which the Sun is overhead at the June solstice, and that latitude is moving toward the equator. Similarly, the tropic of Capricorn, where the Sun stands at the December solstice, moved 14.7 meters northward. Almost 1100 square kilometers move from the tropics into the temperate zone each year. The reason for the shrinkage is the changing obliquity of the Earth, which is getting progressively smaller by approximately 47.5 arc-seconds per century. Various gravitational torques of the Sun, Moon, and other planets cause the Earth's obliquity to oscillate with a period of approximately 41,000 years.

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