Iapetus (moon)

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Iapetus (pronounced /aɪˈæpɨtəs/,[6] or as Greek Ιαπετός), occasionally Japetus (pronounced /ˈdʒæpɨtəs/),[7] is the third-largest moon of Saturn, and eleventh in the solar system,[8] discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.



Iapetus was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in October 1671. He had discovered the moon on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. The pattern continued the following year as he was able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.[9][10]

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour.


Iapetus is named after the Titan Iapetus from Greek mythology. In fact, all Saturnian moons are named after Titans. The name was suggested by John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[7] in which he advocated naming the moons of Saturn after the Titans, sisters and brothers of the Titan Cronus (whom the Romans equated with their god Saturn).

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