Alpha Centauri Wiki
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Nessus, the outer moon of Chiron, has about 1/2 the diameter and 9% the mass of Earth's moon. Its orbit is also nearly circular, with a radius of about 124,000 miles. Its synodic month is 10.6 local days, and there are 50 such months in a local year. Nessus is relatively bright, with several large craters associated with "rays" of reflective dust.

The orbits of Nessus and Pholus fall in the same plane, and the orbital radius of Nessus is almost exactly twice that of Pholus. As a result, the two moons appear to be roughly the same size from the surface of Chiron, each a little smaller than the Moon once appeared in Earth's sky. Each moon partially eclipses Alpha Centauri A from time to time (they are too small to cover the sun's disk entirely). Far more frequent are lunar eclipses, in which one or the other moon moves through Chiron's shadow. Even more frequent are the occasions on which Pholus passes in front of Nessus from the point of view of observers on Chiron.

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