116 ASTRONOMY. 
far from him. Marius observed them carefully, and in March, 1610, was 
convinced that those four small stars were moons of Jupiter. Galileo 
observed them for the first time on January 10th, 1610. . 
Indicating the satellites of Jupiter by L, IL, IL, IV., their mean distances 
from the centre of Jupiter are as follows :—232,000 (English) geographical 
miles for I.; 372,000 for II. ; 592,000 for III.; and 1,040,000 for IV. The 
eccentricities of their orbits ( pl. 10, fig. 6) are inconsiderable, as also their 
inclinations. The sidereal period of I. is 1 day, 18 hours, 28 minutes ; of IL, 
3 days, 13 hours, 14 minutes; of II., 7 days, 3 hours, 43 minutes; and of 
IV., 16 days, 16 hours, 32 minutes. These are uncommonly short, and con- 
sequently eclipses of Jupiter's moons occur with remarkable frequency. 
We often see one or another moon vanish suddenly and re-appear on the 
eastern side after the lapse of some hours. A tolerably attentive exami- 
nation soon shows that such an eclipse of the moon is produced by the 
shadow of the primary. This also shows incontestably that Jupiter and 
his four moons are opake bodies, deriving all their light from the sun. 
By the assistance of a good telescope, it will frequently be observed that 
these moons enter Jupiter’s disk on the eastern border, moving towards the 
western border, accompanied by circular dark spots, going in the same 
direction and with the same velocity. These spots are evidently nothing 
else than the shadows of the moons cast from them upon the surface of 
Jupiter. These phenomena are consequently eclipses of the sun to Jupiter, 
produced by his moons. The maximum duration of the eclipses amounts 
for satellite I, to 2 hours, 16 minutes; for IL, to 2 hours, 52 minutes; for 
llf., to 3 hours, 34 minutes; and for IV., to 4 hours, 45 minutes. In one 
vear of Jupiter, that is in almost 12 of our years, 4,400 eclipses of the moons, 
und as many of the sun, may be observed. The beginning and ending of 
the same eclipse of I. and IL. are never both seen, as before the opposition to 
Jupiter only the beginning, and after it, only the ending are observed. On 
the other hand, both beginning and ending in I]. and IV. may be perceived. 
With respect to eclipses of the sun, it is to be remarked that at the time of 
visible beginning, the shadows follow the satellites, and precede them at the 
time of ending. In conclusion, one moon of Jupiter may sometimes, though 
rarely, eclipse another. 
Observations of the so frequently occurring eclipses of Jupiter’s satellites 
offer an exceedingly ready means of determining geographical longitude. 
Unfortunately, the moment of such an eclipse, just as in the case of an 
eclipse of our moon, will be observed very differently at different places, 
owing to the unequal illumination and magnifying power of telescopes, and 
the different acuteness of sight of the several observers. It was the obser- 
vation of the eclipses of Jupiter’s satellites that led the Danish astronomer, 
Olaus Rémer (in the latter half of the seventeenth century), to the discovery 
of the velocity of light. 
When Jupiter is at his mean distance from the earth, the diameters of his 
moons appear to us respectively at angles of 1/.02, 0’’.91, 1/49, 1/'.27. 
Hence the apparent diameters of his moons to Jupiter will be 381/ 11”, 
116 
