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Animating World Attributes

As of Blender 2.5, Everything is animatable. Read more about keyframes Here.

The Sun lamp has a feature called “sky and atmosphere”, which simulates the physical colors of the sky using the sun position and several parameters. The interesting point for animation is that, depending on the orientation of the sun lamp, the sky is “mid night”, “mid day”, “sunset”/“sunrise”, etc. So you can animate a “blue sky” world background across whole day, just by animating the rotation of a sun lamp! See Here for more details.