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It’s becoming harder and harder to categorize moons as moons.

To learn more, start your googling with these keywords:
Moon: a natural satellite of a satellite of a star.
Satellite: A celestial body orbiting a larger celestial body.
Orbit: The path followed by one object revolving around another object under the influence of gravity.
Barycenter: The center of mass that two or more bodies orbit around.
Binary System: A system in which two similarly sized object orbit the same barycenter.
Hydrostatic equilibrium: Roundness that occurs when gravity is balanced by a pressure gradient force.

If you liked this week’s video, you might also like:
That’s no moon … it’s a space station!
And here’s a tidy illustrated difference between geocentrism and heliocentrism:
And MinuteLabs interactive chaotic planet orbit simulation:

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Credits (and Twitter handles):
Script Writer: David Goldenberg (@dgoldenberg)
Script Editor: Henry Reich (@MinutePhysics)
Video Illustrator: Ever Salazar (@eversalazar)
Video Director: Henry Reich (@MinutePhysics)
Video Narrator: Kate Yoshida (@KateYoshida)
With Contributions From: Alex Reich, Emily Elert, Peter Reich
Music by: Nathaniel Schroeder:



Norton, J. (2008) Could a moon have moons? Popular Science. Retrieved from:

H. A. Weaver, S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young, L. A. Young (2006). The Discovery of Two New Satellites of Pluto. Nature. 439: 943-945. Retrieved from:

Tiscareno, M., Burns, J., Sremcevi, M., Beurle, K., Hedman, M. Copper, N., Milano, A., Evans, M., Porco, C., Spitale, J., and WEiss, J. (2010) Physical Characteristics And Non-Keplerian Orbital Motion Of “Propeller” Moons Embedded In Saturn’s Rings. The Astrophysical Journal Letters, 718: 92-96. Retrieved from:

Spahn, F., and Schmidt, J. (2006) Saturn’s Bared Mini-Moons. Nature, 440: 30-31. Retrieved from:


Our Definition For “Moon” Is Broken (Collab. w/ MinutePhysics)

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