THE SATELLITES OF JUPITER.

333

tance is, of course, immensely greater tlian that of the 4th,or furthest, Satellite, which falls short of 1,200,000 miles.The diameter of a circular section of the shadow', perpendicularto its axis, is, therefore, even at this latter distance, little lessthan that of Jupiter itself.

The axis of the shadow' must evidently lie in the plane ofJupiter’s orbit, while the Satellites move nearly in the planeof Jupiter ’s ecpiator, which is only inclined at an angle ofabout three degrees to that of the orbit. It is consequentlyfound that the first three Satellites never escape the shadowwhen they are full moons ; and that the 1st and 2nd alwayspass deeply through it; while the 3rd may, at its greaterdistance, at times dip but comparatively little into it. Butthe 4th, for nearly half of its path, is so far removed above, orbelow, the plane of Jupiter ’s orbit, that it may, when full, pass

Fij?. LXXVI.—Comparative movements and relative positions of the three innermostmoons of Jupiter.*

altogether clear of the shadow'. When the Satellites travelcentrally through the shadow', their eclipses w'ill of course beof the greatest possible duration, and may last in the case ofthe 1st for 2 h 20“ ; 2nd, 2“ 56'“; 3rd, 3 h 43 m ; 4th, 4 h 56‘".

The first three can therefore never appear as full moons, butare always eclipsed when they otherwise would be so seen, aswould be the case with our own Moon if it moved very nearlyin the plane of the ecliptic, instead of in an orbit inclined to itat an angle of about five degrees.

But, in addition to the fact that the three inner Satellitesare always thus eclipsed when they would otherwise be seenas full moons, an exceedingly curious relation exists betweentheir motions, which involves the fact that they can never all

* Apliagram resembling the above, but more elaborate, may be seen inan article by Mr. Proctor in the Popular Science llecietc, vol. vi., p. 248.