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Measurement of the speed of light from the time it takes Io to orbit Jupiter, using eclipses of Io by Jupiter's shadow to precisely measure its orbit. Io is the innermost of the four moons of Jupiter discovered by Galileo in January 1610. Rømer and Cassini refer to it as the "first satellite of Jupiter".
The speed of light is the upper limit for the speeds of objects with positive rest mass, ... Rømer and Huygens, moons of Jupiter: 220 000 000 [96] [124] −27% 1729:
Cassini had observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed. In 1672 Rømer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant.
Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons. [27] Ganymede is the only Galilean moon of Jupiter named after a male figure—like Io, Europa, and Callisto, he was a lover of Zeus. In English, the Galilean satellites Io, Europa and Callisto have the Latin spellings of ...
Ole Rømer deduced that light does not travel instantaneously (a conclusion that Cassini had earlier rejected), [52] and this timing discrepancy was used to estimate the speed of light. [159] [160] In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch (910 mm) refractor at Lick Observatory in California.
The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's Sidereus Nuncius in March 1610. [1] While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name Medicea Sidera (Medicean Stars) after his new patrons, the de'Medici family of his native ...
A picture of Jupiter and its moon Io taken by the Hubble Space Telescope. The black spot is Io's shadow. Jupiter and the Great Red Spot (visible on the lower right), with Ganymede (immediately on the upper right) casting its shadow on Jupiter. The other moons are Europa (further right), Io (first on left), and Callisto (furthest on left).
A montage of Jupiter and its four largest moons (distance and sizes not to scale) There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. [1] [note 1] This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. [4]