The discovery of these two satellites is, perhaps, the most interesting telescopic visual discovery made with the large telescopes of the last half century; photography having been the means of discovering all the other new satellites except Jupiter's fifth (in order of discovery).
[Illustration: JUPITER. From a drawing by E. M. Antoniadi, showing transit of a satellite's shadow, the belts, and the "great red spot" (_Monthly Notices_, R. A. S., vol. lix., pl. x.).]
_Jupiter._--Galileo's discovery of Jupiter's satellites was followed by the discovery of his belts. Zucchi and Torricelli seem to have seen them. Fontana, in 1633, reported three belts. In 1648 Grimaldi saw but two, and noticed that they lay parallel to the ecliptic. Dusky spots were also noticed as transient. Hooke[5] measured the motion of one in 1664. In 1665 Cassini, with a fine telescope, 35-feet focal length, observed many spots moving from east to west, whence he concluded that Jupiter rotates on an axis like the earth. He watched an unusually permanent spot during twenty-nine rotations, and fixed the period at 9h. 56m. Later he inferred that spots near the equator rotate quicker than those in higher latitudes (the same as Carrington found for the sun); and W. Herschel confirmed this in 1778-9.
Jupiter's rapid rotation ought, according to Newton's theory, to be accompanied by a great flattening at the poles. Cassini had noted an oval form in 1691. This was confirmed by La Hire, Römer, and Picard. Pound measured the ellipticity = 1/(13.25).
W. Herschel supposed the spots to be masses of cloud in the atmosphere--an opinion still accepted. Many of them were very permanent. Cassini's great spot vanished and reappeared nine times between 1665 and 1713. It was close to the northern margin of the southern belt. Herschel supposed the belts to be the body of the planet, and the lighter parts to be clouds confined to certain latitudes.
In 1665 Cassini observed transits of the four satellites, and also saw their shadows on the planet, and worked out a lunar theory for Jupiter. Mathematical astronomers have taken great interest in the perturbations of the satellites, because their relative periods introduce peculiar effects. Airy, in his delightful book, _Gravitation_, has reduced these investigations to simple geometrical explanations.