Nereid (moon)

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Nereid (pronounced /ˈnɪəri.ɪd/; also /ˈnɛri.ɪd/, as in Greek Νηρηΐδα),[note 3] also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949.


Discovery and naming

Nereid was discovered on May 1, 1949, by Gerard P. Kuiper, on photographic plates taken with the 82-inch telescope at the McDonald Observatory. He proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune.[1] It was the second and last moon of Neptune to be discovered before the arrival of Voyager 2 (not counting a single observation of an occultation by Larissa in 1981).[5]

Orbit and rotation

Nereid orbits Neptune in the prograde direction at an average distance of 5,513,400 km (3,425,900 mi), but its high eccentricity of 0.7507 takes it as close as 1,372,000 km (853,000 mi) and as far as 9,655,000 km (5,999,000 mi) from the planet.[2]

The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.[6]

In 1991 a rotation period of Nereid of about 13.6 hours was determined by analysis the moon's light curve.[7] Later in 2003 another rotation period of about 11.52 ± 0.14 hours was measured.[4] However this determination was later disputed. Other researchers have failed so far to detect any periodic modulation in Nereid's light curve.[8]

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