ASTR 100 Lecture Notes - Lecture 2: Top, Precession
26 Sep 2016
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*earth"s axis precesses like the axis of a spinning top. Seasons are caused by the tilt of earth"s axis which causes sunlight to hit different parts of the earth more directly during the summer and less directly during the winter. We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon. The summer and winter solstices are when the northern hemisphere gets its most and least direct sunlight respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. The tilt of earth"s axis remains about 23. 5 degrees (so the season pattern is not affected), but earth has a 26000 cycle that moves about in a circle. Moon rotates on its own axis at the same rate it takes to rotate around the earth.
