322
permost line of Fig . LXXIY. The following night he againsaw three, but they were all on one side of Jupiter , and nearerto one another than on the previous night. This is shown inthe second line of Fig . LXXIV. The next night he saw onlytwo, both on the opposite side of the planet. The nights of the13th and 15th were the first upon which he saw all the four ,when their positions were such as are indicated in the thirdand fourth lines of Fig . LXXIV., in which the original draw-ings published by Galileo are reproduced.*
Galileo continued to observe the Satellites , and to recordtheir places, on every clear night with the utmost care; occa-sionally marking down, in addition, the proximity of smallstars, which, however, were proved not to be Satellites, since
Date.
East. West.
January 7th
b • o
January 8th
o ° • *
January 13th
• o • • *
1 January loth
o • ° ° •
Fig. LXXIV.—Configurations of the Satellites of Jupiter, drawn by Galileo , in the year 1610.
the Satellites accompanied the planet as it advanced in itsorbit, while the stars were left behind.
From the first, he noticed that all the four Moons werenot equally brilliant. He was also soon able to conclude thatthey rotated round Jupiter , alternately passing between it andthe Earth , and beyond it to its further side. And he not onlycame to this conclusion through seeing them sometimes to theeast and sometimes to the west of the planet, but he alsoascertained that the size of the orbits in which they revolvedwas much greater for the outermost than for the innermost.This very important fact wasyliscovered as follows :—
Galileo observed that he often found two or three of the
* A much more complete reproduction of sixty-four of Galileo ’s draw-ings may he seen in the translation of the “ Sidereal Messenger ” (byMr. Carlos) previously referred to.