Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2005 November 2
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Epimetheus and Janus: Interchangeable Moons of Saturn
Credit : Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: These two moons change places. Epimetheus and Janus, two small moons of Saturn, actually switch positions as they orbit their home planet. The orbital radii of the moons are strangely separated by less than the radii of the moons themselves: about 50 kilometers. One moon orbits Saturn well ahead of the other, at first. As the two moons gravitationally attract, they approach each other and, every few years, actually pass and trade orbits. This strange dance creates speculation that Epimetheus and Janus were once joined and later split from each other. Pictured above, the two moons were photographed rounding their orbits just outside of Saturn's F ring. The above image was taken in early September by the robot Cassini spacecraft, also orbiting Saturn.

Tomorrow's picture: Pluto's Moons

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