Jupiter is the center of a superb system of satellites, eight in
number. Four of them were first seen in 1610, and have the honor to
be the first heavenly bodies discovered by means of the telescope.
The fifth one was not discovered until 1892. The sixth was first seen
in 1904, and the seventh in 1905. After three years an eighth was
discovered (in 1908).
When the first four satellites were discovered they were named
respectively, in the order of their distances from Jupiter, Io, Europa,
Ganymede, and Callisto. Ganymede is not only the largest of the four,
but is also the largest satellite in the solar system. It is larger
than Mercury, and not much smaller than Mars. Callisto is next to
Ganymede in size, and is about the size of Mercury. Io is about the
size of our moon, and Europa is not much smaller. Under very favorable
conditions Ganymede and Callisto can be seen by the naked eye; but
a good many persons think they see the moons of Jupiter when they
see only some small stars in that region. They are invisible to most
people, but probably could be seen oftener if it were not for the
glaring light of the planet, which more or less obscures anything so
near it.
After the discovery of Jupiter’s fifth satellite, astronomers seem
to have become possessed with that dull spirit of orderliness such
as is sometimes exhibited by city councils in substituting numbers
for historic and beautiful names in designating streets. No more of
Jupiter’s satellites were given names such as might be appropriate for
members of this Jovian family; but all were given numbers--the first
four in order of their distance from Jupiter, the others in order of
their discovery. Io, Europa, Ganymede, and Callisto are now designated,
respectively, I, II, III, and IV, while V, VI, VII, and VIII have never
had any designation other than these numbers.
The fifth satellite, discovered in 1892, is the nearest to Jupiter,
and the smallest of all his satellites. Its diameter is probably not
more than one hundred and twenty miles, but its exact size can be
estimated only by the amount of light it reflects. It is too small to
show a measurable disc, and cannot even be seen when it makes a transit
across the planet. It would seem then a mere speck, if we could see
it at all. It makes one revolution about Jupiter in less than twelve
hours (eleven hours and fifty-seven minutes), and is only a little more
than twenty-two thousand miles from the surface of the planet at the
equator. It appears to us as a star of about the thirteenth magnitude,
and cannot be seen except with a large telescope. Owing to the great
curvature of the planet, and to the satellite’s being so near him, it
cannot be seen from the surface of Jupiter beyond sixty-five degrees
of latitude. It moves faster than any other satellite in the solar
system, going at the rate of sixteen and a half miles a second. It
does not make a revolution in as short a time as Phobos, the little
satellite of Mars, does, but it has a much longer distance to travel
and goes at a faster rate. The fact that Jupiter rotates in ten hours
and the satellite makes a revolution around him in twelve hours results
in the satellite’s taking five of Jupiter’s days to cross from the
eastern horizon to the western. It would go through all its phases four
times during that period if it were not that, being so near the planet,
his huge form cuts off the sunlight from the little satellite for
nearly one-fifth of the time, and it is never seen “full.”
This satellite is very difficult for us to see on account of its
diminutive size and its nearness to the shining disc of Jupiter; yet
it was discovered by means of the telescope, and not by photography,
as so many small bodies are discovered nowadays, and by a man who thus
far has not been able to see the fine line markings on Mars, which some
other astronomers think they can see--a fact that is very interesting
as showing the difference between observers even of great keenness of
vision. From this satellite Jupiter would seem an enormous body, nearly
eighty-five times larger than our sun appears to us, and, no doubt, a
splendid object. But the little satellite pays rather dearly for the
view by suffering numerous and long-continued eclipses.
The sixth and seventh satellites are also very minute bodies, measuring
probably less than one hundred miles in diameter. They circle about
Jupiter at a distance nearly thirty times more remote than our moon
is from us. They are about seven million miles from the planet, and
probably not more than barely visible from it. It takes them two
hundred and sixty-five days to make one revolution, which is more
than five hundred times as long as the period of Jupiter’s nearest
satellite. These two satellites are so nearly of one size and revolve
so nearly in the same time and at the same distance from Jupiter that
they are thought to have had a common origin. Just what their relation
is has not yet been determined.
The eighth satellite, discovered in January, 1908, is certainly
no larger, and is perhaps still more tiny, than the sixth and the
seventh, though it is a little brighter than either one of them. It is
about three times farther away from Jupiter than the seventh satellite,
and with eyes such as ours would not be visible from Jupiter. It
shows to us as about a seventeenth-magnitude star, which is almost
at the limit of our vision with even the largest telescope. It seems
to revolve about Jupiter in a direction exactly opposite to that of
the other satellites--a retrograde motion that appears in the solar
system in only two or three other cases and has not yet been entirely
accounted for.
Jupiter’s satellites have played an important part in astronomical
discoveries and investigations. It was through observation of their
transits that it was discovered that light occupied time in passing
through space. When Jupiter was near us in his orbit, the eclipses
occurred too soon for their calculated time; when he was farther
away, they occurred too late. It was found that these irregularities
were due to the fact that light is not transmitted through space
instantaneously, and further investigation showed that it travels at
the rate of 186,400 miles a second. The eclipses of Jupiter’s moons are
carefully computed and recorded in the _Nautical Almanac_, and it is
through observations of them that chronometers are corrected at sea.
Ganymede and Callisto have been found to keep always the same face
toward the planet, as our moon keeps always the same face toward us;
and it is thought that all of Jupiter’s satellites probably do this.
The symbol of Jupiter is ♃, a hieroglyph for the eagle, which
was the bird of Jove.