Number 21
Date Jan. 1st 2006
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Kepler is the 21st xkcd comic.



Animated gif of Kepler's second law. Click to view the animation.

This comic is a reference to Kepler's laws of planetary motion, which were discovered by Johannes Kepler in the early 1600s. It specifically refers to the second law:

A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

The joke is that the store's janitor also sweeps equal areas in equal amounts of time.

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"Science joke. You should probably just move along."

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