21: Kepler

Explain xkcd: It's 'cause you're dumb.
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Original title: Monday's drawing
Science joke. You should probably move along.Original caption: Another one which, if you don't get, you're probably better off.
Title text: Science joke. You should probably move along.

Original caption: Another one which, if you don't get, you're probably better off.


This was the twentieth comic originally posted to LiveJournal. The previous one was 20: Ferret, and the next one was 44: Love.

Johannes Kepler was a German mathematician, astronomer, and astrologer, best known for his laws of planetary motion. By using Tycho Brahe's observations of our solar system (Brahe gave Kepler the job of observing and explaining the motion of the planet Mars), Kepler was able to deduce that planets in the system do not move in circular orbits around the Sun, but rather in elliptical ones. In doing so, he directly contradicted Brahe's own conviction that the Earth was the center of the universe. According to Kepler's Second Law, "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time," somewhat akin to sweeping a broom over the floor. In the comic, the janitor Kepler also sweeps the same area, although in this case, "area" is used in the sense of "surface" (of floor) rather than in the purely mathematical sense. It is also very monotonous, like a planet's set orbit, but Kepler doesn't mind this.

The comic could also be seen as a subtle reference to the Kepler space telescope that was searching for exoplanets (planets outside the Solar system) from March 2009 to August 2013, by looking at exactly the same spot in the night sky over and over again. Even though the telescope was not launched until 4 years after this comic was published, the details of Project Kepler had been disclosed by NASA press releases as early as 2001.

The title text assumes that the reader is scientifically illiterate and won't understand the joke, which is ironic, considering how xkcd came to be known for embracing STEM fields and nerdiness in general.


[Two Cueball-like guys stand in an aisle in a store.]
Cueball: Nice store. How do you keep the floors so clean?
Store manager: Oh, we hired this dude named Kepler, he's really good. Hard worker. Doesn't mind the monotony. Sweeps out the same area every night.

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Rikthoff (talk) The date of issue of this comic is off. Does anybody have the correct issue date?

It appears to be accurate; it's the date from Randall's xkcd LiveJournal entry. Blaisepascal (talk) 02:21, 16 August 2012 (UTC)

Sorry, I fixed the date (to livejournal date) on August 8, 2012 but did not see/update talk page.--B. P. (talk) 18:39, 17 August 2012 (UTC)

I thought it was a reference to the Kepler telescope being pointed in the same direction for a long time, but I don't really know enough to be sure about that 02:14, 17 April 2015 (UTC)

It could be, Kepler was planned to be launched around the time this comic was originally published, but it was delayed due to budget cuts. While I think Kepler's Second Law explanation seems more relevant to the joke in the comic, I have updated the explanation and trivia sections to include Project Kepler. 18:40, 16 February 2017 (UTC)

There is a community portal discussion of what to call Cueball and what to do in case with more than one Cueball. I have added this comic to the new Category:Multiple Cueballs. Since the store manager Cueball has the interesting part of this comic, I have listed him as Cueball. Just made a note that the other guy also looks like Cueball.--Kynde (talk) 18:13, 16 March 2015 (UTC)

Need title text explanation. 23:38, 21 October 2017 (UTC)