Janus

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(jā'nəs)
n.

1. Roman Mythology. The god of gates and doorways, depicted with two faces looking in opposite directions.
2. A satellite of Saturn.

[Latin Iānus, iānus, archway, doorway.]

The sixth moon of Saturn, also known as Saturn X; it is nearly co-orbital (shares its orbit) with Epimetheus and is located between Saturn's F- and G-rings at a mean distance of 151,472 km from the planet's center. Because Janus and Epimetheus differ in their orbital radii by only 50 km, their orbital velocities are very nearly equal and the lower, faster one gradually catches up and overtakes the other. As the moons approach each other they exchange a small amount of momentum that boosts the lower one into a higher orbit while the higher one drops to a lower orbit. This exchange of places happens about once every four years. Audouin Dollfus is credited with the discovery of Janus in 1966 but it is not certain whether the object he saw was Janus or Epimetheus and his observations led to a spurious orbit. (Richard Walker discovered it independently but his telegram arrived a few hours after Dollfus's.) Stephen Larson and John Fountain determined in 1978 that there are two moons at about 151,000 km from Saturn—a fact confirmed in 1980 by Voyager 1. Janus measures 196 × 192 × 150 km and is extensively cratered, with several craters larger than 30 km, but has few linear features.

For more information on Janus, visit Britannica.com.

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For a list of words related to Janus, see:

• Gods and Divinities - Janus: god of beginnings and doorways (Rome)

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See words rhyming with "Janus."

  See crossword solutions for the clue Janus.

Some good "Janus" pages on the web: