The Observer’s Year
136 Equatorial Belt (SEB), with others at higher latitudes, such as the North and South Temperate Belts. Spots, wisps, and pestoons are also to be seen, and the detail can be amazingly complex. Moreover, Jupiter is a world in constant turmoil so that the surface is always changing. Because of the quick spin, the surface details appear to be carried across the disk from one side to the other; the shifts are obvious after only a few minutes’ observation. Jupiter does not rotate in the way that a solid body would do. There is a strong equatorial current, and the rotation period here (System I) is 9 h 50 m 30 s, for the rest of the planet, the period is 9 h 55 m 41 s (System II). To complicate matters still further, various discrete features have periods of their own, so that they drift in longitude (much less in latitude). The most famous feature on Jupiter is the Great Red Spot, which has been under observation for most of the time since the seventeenth century. It sometimes disappears for a while, but it always comes back and is associated with a ‘hollow’, in the South Equatorial Belt. Presumably, it has existed for so long because of its size – up to 30 000 miles long by 7000 miles broad – but we cannot be sure that it is permanent. Once thought to be a volcano it is now known to be a whirling storm – a phenomenon of Jovian ‘weather’; at times it is really very red. The cause of the colour is unclear, but may be due to phosphorus. The rotation period of the Spot differs from its surroundings, so that the longitude shifts, but the latitude remains almost constant at about 22°S.
May 7 The Satellites of Jupiter Jupiter has four large satellites; they were observed by Galileo in 1610, with his first telescope, and are always known as the Galileans. Details are as follows: Mean distance from Jupiter (miles) Io Europa Ganymede Callisto
262 417 666 1 170
000 000 000 000
d 1 3 7 16
Orbital period h m 18 13 3 16
28 14 43 32
Diameter (miles)
Mean opposition magnitude
2264 1945 3274 2981
5.0 5.1 4.6 5.6
All are plane-sized; Ganymede is actually larger than Mercury, though less massive. All four move almost in the plane of the Jovian equator, and with even a small telescope,