Every owner of a telescope delights to turn it on the planet Jupiter,
both for the spectacle the globe itself affords him, and for a view of
the wonderful system of moons by which the giant planet is attended.
Fortunately the four satellites of Jupiter lie within reach of even
the most modest telescope, and their incessant changes relative to
Jupiter and each other give them a never-ending interest for the
astronomer. Compared with the torpid performance of our moon, which
requires a month to complete a circuit around the earth, Jupiter’s
moons are wonderfully brisk and lively. Nor are they small bodies like
the satellites of Mars, for the second of Jupiter’s satellites is quite
as big as our moon, and the other three are very much larger. It is,
however, true that his satellites appear insignificant when compared
with Jupiter’s own enormous bulk.
The innermost of these little bodies flies right round in a period of
one day and eighteen or nineteen hours, while the outermost of them
takes a little more than a fortnight--that is, rather more than half
the time that our moon demands for a complete revolution. Jupiter’s
satellites are too far off for us to see much with respect to their
structure or appearance even with mighty telescopes. It is, of course,
their great distance from us that makes them look insignificant. They
would, however, be bright enough to be seen like small stars were
it not that, being so close to Jupiter, his overpowering brightness
renders such faint objects in his vicinity invisible.
It was by means of the satellites of Jupiter that one of the most
beautiful scientific discoveries was made. As a satellite revolves
round the giant planet it often happens that the little body enters
into the shadow of the great planet. No sunlight will then fall upon
the satellite, and as it has no light of its own, it disappears from
sight until it has passed through the shadow and again receives
sunlight on the other side. We can watch these eclipses with our
telescopes, and there can be no more interesting employment for
a small telescope. The movements of these bodies are now known so
thoroughly that the occurrence of the eclipses can be predicted. The
almanacs will tell when the satellite is calculated to disappear, and
when it ought again to return to visibility. When astronomers first
began to make these computations a couple of hundred years ago, the
little satellites gave a great deal of trouble. They would not keep
their time. Sometimes they were a quarter of an hour too soon, and
sometimes a quarter of an hour too late. At last, however, the reason
for these irregularities was discovered, and a wonderful reason it was.
Suppose there were a number of cannons all over Hyde Park, and that
these cannons were fired at the same moment by electricity. Though the
sounds would all be produced simultaneously, yet, no matter where you
stood, you would not hear them altogether; the noise from the cannons
close at hand would reach your ears first, and the more distant reports
would come in subsequently. You can calculate the distance of a flash
of lightning if you allow a mile for every five seconds that elapse
between the time you saw the flash and the time you heard the peal
of thunder which followed it. The light and the noise were produced
simultaneously, but the sound takes five seconds to pass over every
mile, while the light, in comparison to sound, may be said to move
instantaneously. That sound travelled with a limited velocity was
always obvious, but never until the discrepancies arose about Jupiter’s
satellites was it learned that light also takes time to travel. It is
true that light travels much more quickly than sound--indeed, about
a million times as fast. Light goes so quickly, that it would rush
more than seven times around the earth in a single second. So far as
terrestrial distances are concerned, the velocity of light is such
that the time required for a journey is inappreciable. The distances,
however, between one celestial body and another are so enormous, that
even a ray of light, moving as quickly as it alone can move, will
occupy a measurable time on the way. Our moon is comparatively so near
us, that light takes little more than a second to cover that short
distance. Eight minutes are, however, required for light to travel from
the sun to the earth; in fact, the sunbeams that now come into our
eyes left the sun eight minutes ago. If the sun were to be suddenly
extinguished, it would still seem to shine as brightly as ever in the
eyes of the inhabitants of this earth for eight minutes longer. As
Jupiter is five times as far from the sun as we are, it follows that
the light from the sun to Jupiter will spend forty minutes on the
journey, and the light from Jupiter to the earth will take a somewhat
similar time. When we look at Jupiter and his moons, we do not see him
as he is now, we see him as he was more than half an hour ago, but
the interval will vary somewhat according to our different distances
from the planet. Sometimes the light from Jupiter will reach us in as
little as thirty-two minutes, while sometimes it will take as much as
forty-eight--that is, the light sometimes requires for its journey a
quarter of an hour more than is sufficient at other times.
We can therefore understand that irregularity of Jupiter’s satellites
which puzzled the early astronomers. An eclipse sometimes appeared a
quarter of an hour before it was expected; because the earth was then
as near as it could be to Jupiter, while the calculations had been
made from observations when Jupiter was at his greatest distance.
It was these eclipses of the satellites which first suggested the
possibility that light must have a measurable speed. When this was
taken into account, then the occasional delay of the eclipses was found
to be satisfactorily explained. Confirmation flowed in from other
sources, and thus the discovery of the velocity of light was completely
established.
Professor Barnard, when studying Jupiter in 1892 with the splendid
refractor at the Lick Observatory, saw a very small point of light
nearer to the planet than the nearest of the four satellites already
known. Further examination showed that this little object was indeed
another satellite. Thus Jupiter has a fifth moon in addition to the
four which have been known so long. This little body is so small and
faint that it can only be discerned under the most favorable conditions
by the most powerful telescopes.