256 
POPULAH SCIENCE KEVIEW. 
orbits of the two interior satellites are described as circular, and 
that of the third as of small eccentricity, yet these orbits have 
an ellipticity due to the mutual attractions of the satellites. 
This ellipticity is wholly different from the ellipticity of the 
planetary orbits. The former is centric, the latter eccentric, the 
sun being in the focus of each planetary eclipse, while Jupiter 
is at the centre of the ellipse traversed by the inner satellites. 
The following facts combined with the information afforded 
by fig. 2, will suffice to enable the telescopist satisfactorily to 
observe the phenomenon of August 21. The planet rises at 
half-past seven, almost at the same moment that the sun sets. 
At 7 h 44 m Greenwich mean time, the shadow of the third satellite 
passes on to the disc, and the satellite itself passes on to the disc 
at 8 h 14 m . The first phenomenon will not be observable, as the 
sun will not be low enough beneath the horizon nor Jupiter high 
enough above the horizon. Neither will the second phenomenon 
nor the entry of the fourth satellite’s shadow on the disc, which 
occurs at 8 h 17 m ,be easily seen. The remaining eleven phenomena 
will be readily seen, however. At 9 h 10 m the second satellite will 
disappear in the shadow of the planet. At 9 h 28 m the fourth 
satellite will enter on Jupiter’s disc. At 9 h 57 m the shadow of the 
first satellite will make its appearance, followed in seven minutes 
by the entry of the satellite itself on Jupiter’s disc. At this 
time (10 h 4 m ) Jupiter will be without satellites in telescopes of 
moderate power, but large telescopes will exhibit three satellites 
on his disc, together with their three shadows. At ll h 23 m , the 
shadow of the third satellite passes off the disc, at ll h 49 m the 
satellite itself. At 12 h 13 m the second satellite reappears from 
behind Jupiter, at 12 h 1 6 m the shadow of the first satellite passes 
off the disc, the satellite itself seven minutes later. Lastly, at 
12 h 59 m the shadow of the fourth satellite, and at 13 h 54 m the 
fourth satellite itself, pass off Jupiter’s disc. 
In fig. 2, the paths traversed by the satellites and their 
shadows, are indicated. The figure represents the appearance 
presented in an inverting telescope. It is only necessary to 
invert the figure to see the actual configuration. It will of course 
be understood that the apparent slope of the paths will vary 
with the hour of observation. I have made the planet’s equator 
horizontal, instead of estimating the slope for any assigned 
hour; because the planet’s oblateness being very observable, 
affords a natural feature of reference. 
It will be observed that the shadows of the four satellites are 
very different in appearance. I have drawn them, not as they have 
been seen in the telescope, but as it is certain that they would 
appear in telescopes of adequate power. The figure and extent 
of the penumbrse have been determined from the simplest optical 
principles applied to the known distances and magnitudes of the 
