[18] Modern astronomers agree in assigning an atmosphere to Jupiter, but consider it not extensive enough to affect the brightness of the satellites.—(WEBB, _Celestial Objects for Common Telescopes_.) Their absolute magnitudes are different, and their surfaces have been observed to be obscured by spots, which may account for the variations of their brightness. These spots, like the lunar spots, are probably due to variations of reflective power at different parts of their surfaces, for as they always turn the same face to Jupiter, they present different portions of their surfaces to us periodically, and it has been ascertained by observation that “these fluctuations in their brightness are periodical, depending on their position with respect to the Sun.”—(HERSCHEL, _Outlines of Astronomy_; ARAGO, _Astronomie Populaire_, 1854.) _Original Configurations of Jupiter’s Satellites observed by Galileo in the months of January, February, and March 1610, and published with the 1st edition of his book_ Sidereus Nuncius, _Venice, 1610._ —————————+———————————+———————————————+——————————————— FIG. | DATE. | EAST. | WEST. —————————+———————————+———————————————+——————————————— 1 | Jan. 7 | | | • • ◯ • —————————+———————————+——————————————————————————————— 2 | 8 | | | ◯ • • • —————————+———————————+——————————————————————————————— 3 | 10 | | | • • ◯ —————————+———————————+——————————————————————————————— 4 | 11 | | | • • ◯ —————————+———————————+——————————————————————————————— 5 | 12 | | | • •◯ • —————————+———————————+—————————-————————————————————— 6 | 13 | | | • ◯ • ⠁ • —————————+———————————+——————————————————————————————— 7 | 15 | • | | ◯ • • • —————————+———————————+——————————————————————————————— 8 | 15 | | | ◯ • • • —————————+———————————+——————————————————————————————— 9 | 16 | | | • ◯ • • —————————+———————————+——————————————————————————————— 10 | 17 | | | • ◯ • —————————+———————————+——————————————————————————————— 11 | Jan. 17 | • • | | • ◯ —————————+———————————+——————————————————————————————— 12 | 18 | | | • ◯ • —————————+———————————+——————————————————————————————— 13 | 19 |