SATELLITES. 
a shadow upon the face of that planet, 
which we see as an obscure round spot. 
And lastly, when the satellite comes into a 
line between Jupiter and us, it is said to 
transit the disc of the planet, upon which it 
appears as a round black spot. The periods 
or revolutions of Jupiter’s satellites, are 
found out from their conjunctions with that 
planet, after the same manner as those ot 
the primary planets are discovered from 
their opposition to the Sun. And their 
distances from the body of Jupiter, are 
measured by a micrometer, and estimated 
in semi-diameters of that planet, and thence 
in miles. The periodical times and distances 
of these satellites, and the angles under 
which their orbits are seen froni the Earth, 
at its mean distance from Jupiter, are as 
below. 
Periodic Times. 
Distances in 
Anciles of 
Orbit. 
Semidia- 
meter 
Miles. 
I'il8>‘2?'34" 
5| 
266,000 
3' 55' 
3 13 13 42 
9 ^ 
423,000 
6 Id* 
7 3 42 36 
676,000 
9 58 
16 16 32 9 
25* 
1,189,000 
17 30 
1 
The eclipses of the satellites, e.specially 
of those of Jupiter, are of very great use 
in astronomy. First, in determining pretty 
exactly the distance of Jupiter from the 
Earth. A second advantage still more con- 
siderable, which is drawn from these 
eclipses, is the proof which they give of the 
progressive motion of light. It is demon- 
strated by these eclipses, that light does not 
come to ns in an instant, although its rno- 
tion is extremely rapid. For if the motion 
of light were infinite, or came to us in an in- 
stant, it is evident that we should see the 
commencement of an eclipse of a satellite 
at the same moment, at whatever distance 
we might be from it ; but, on the contrary, 
if light move progressively, then it is as evi- 
dent, that the further we are from a planet, 
the later we shall be in seeing the moment 
of its eclipse, because the light will take up 
a longer time in arriving at us ; and so it is 
found in fact to happen, the eclipses of these 
satellites appearing always later and later 
than the true computed times, as the Earth 
removes further and further from the pla- 
net. When Jupiter and file Earth are at 
their nearest distance, being in conjunction 
bothion the same side of the Sun, then the 
eclipses are ^e?Ti to happen the soonest j 
and when tlie Sun is directly between Jupi- 
ter and the Earth, they are at their greatest 
distance asimder, the distance being more 
than before by the whole diameter of the 
Earth’s annual orbit, or by double the 
Earth’s distance from the Sun, then the 
eclipses are seen to happen the latest of any, 
and later than before by about a quarter 
of an hour. Hence therefore it follows, 
that light takes up a quarter of an hour 
in travelling across the otbit of the Earth, 
or nearly eight minutes in passing from the 
Sun to the Earth ; which gives us about 12 
millions of miles per minute, or 200,000 
miles per second, for the velocity of light. 
A discovery that was first made by M. 
Roemer. 
The third and greatest advantage derived 
from the eclipses of the satellites, is the 
knowledge of the longitudes of places 
on the Earth. Suppose two observers of an 
eclipse, the one, for example, at London, 
the other at the Canaries ; it is certain that 
the eclipse will appear at the same moment 
to both observers ; but as they are situated 
under different meridians, they count dif- 
ferent hours, being perhaps 9 o’clock to the 
one, when it is only 8 to the other ; by 
which observations of the true time of the 
eclipse, on communication, they find the 
difference of their longitudes to be one 
hour in time, which answers to 15 degrees 
of longitude. 
Saturn has seven secondary planets re- 
volving about him. One of them, which 
till lately was reckoned the fourth in order 
from Saturn, was discovered by Huygens, 
the 25th of March, 1655, by means of a te- 
lescope 12 feet long ; and the first, second, 
third, and fifth, at different times, by Cas- 
sini, viz. the fifth in October, 1671, by a te- 
lescope of 17 feet *, the third in December, 
1672, by a telescope of Campani’s, 35 feet 
long ; and the first and second in March, 
1684, by help of Campani’s glasses, of 100 
and 136 feet. Finally, the sixth and seventh 
satellites were discovered by Dr. Herschel, 
with his 40 feet reflecting telescope, viz. the 
sixth on the 19th of August, 1787, and the 
seventh on the 17th of September, 1788. 
These two he has called the sixth and se- 
venth satellites, though they are nearer to 
the planet Saturn than any of the former 
five, that the names or numbers of these 
might not be mistaken or confounded, 
with regard to former observations of 
them. 
Moreover, the great distance between 
the fourth and fifth satellite, gave occasion 
