Image:Wfm sketch seasons.png

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This is a very sketchy diagram trying to illustrate how the axial tilt of the earth (with respect to the ecliptic) causes the seasons and the concomitant changes in day length.

This is a view from above the sun's "north pole", looking down at the orbit of the earth. The earth's axis of rotation is shown (its tilt is exaggerated). The earth is shown at the vernal equinox, the summer solstice, the autumnal equinox, and at the winter solstice. Note that at the equinoxes the axis (when viewed in just this dimension) is parallel (tangential) to the earths orbit. Note that at the solstices it is perpendicular.

Note that the axis remains at a fixed orientation (with respect to some "fixed" point - ahem) for the course of its rotation. And remember that the earth is rotating around its axis, one cycle every 24 hours.

So at the equinoxes, the shadowed half of the earth is evenly "shared" by each point on the earth's surface, whereas at the solstices one hemisphere or other is "greedy" and spends more time in the shadow than in the light.

This diagram was knocked up in ten minutes by the artistically-challenged User:Finlay McWalter. I can upload the Staroffice-Draw source if someone wants to actually tweak it enough to be used in an article (otherwise it should probably be deleted once the discussion is over).

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