THE SATELLITES OF JUPITER.
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which distance causes to appear so small, if we notice that thenearest of these four Satellites , which appear to be so close toJupiter , is really more than 20,000 miles further from itscentre than the Moon is (upon an average) from that of theEarth ; while the 4th Satellite is at a distance nearly equalto five times that of our Moon ; and yet, such is the vastradius of the planet itself, the respective distances of thefour are only about 5 T ”y, 9|, 15, and 26£ times the radius ofJupiter. Our own Moon ’s distance being about 60 times theEarth ’s radius, it follows, that the furthest Satellite ofJupiter is more than twice as near, in comparison with theradius of its primary.
The four above-mentioned distances being in the ratios ofthe numbers 59, 96, 150, 265, attempts have been made todiscover some relationship between them similar to that whichBode’s series indicates in the case of the planets. It may notbe out of place if we here mention two of the most successfulof these attempts, although we consider them to be ratherfanciful than useful.
If we take the series 5, 5 + 3, 5 + 3 x 2£, 5 + 3 x 2| x 2|,we obtain the numbers 5, 8, 12£, 23£. These, if multipliedrespectively by 12, give 60, 96, 150, 279.
Or we may take
20
20
20
20
Add
20
40
60
80
And again add
20
40
80
160
The sums of which, viz.—
60
100
160
260
as well as the numbers given by the previous series, do notdiffer much from the true ratios of the distances.
A much more striking relation may, however, be noticed inthe periodic times of the Satellites . If we take these to be *l d 18 h 28 m ; 3 d 13 h 14"'; 7 d 3 h 43 m ; 16 d 16 h 32 m ; or in daysand decimals of a day 1 *77, 3 - 55, 7‘ 15, 16 - 69 days ; we see thatthe 2nd is almost exactly double the 1st, and that the 3rd is
* In this case we use the actual periods in which they revolve aroundJupiter , which are termed their sidereal periods, and are, as explained inthe note upon p. 323 , somewhat shorter than the corresponding synodicperiods.