THE GIANT PLANET
I can never forget as a young boy my first glimpse of the planet Jupiter
and his moons; it was through a bit of a telescope that I had put
together with my own hands; a tube of pasteboard, and a pair of old
spectacle lenses that chanced to be lying about the house.
In the field of view I saw five objects; four of them looking quite
alike, and as if they were stars merely (they were Jupiter's moons),
while the fifth was vastly larger and brighter. It was circular in
shape, and I thought I could see a faint darkish line across the middle
of it.
This experience encouraged me immensely, and I availed myself eagerly of
the first chance to see Jupiter through a bigger and better glass. Then
I saw at once that I had observed nothing wrongly, but that I had seen
only the merest fraction of what there was to see.
In the first place, the planet's disk was not perfectly circular, but
slightly oval. Inquiring into the cause of this, we must remember that
Jupiter is actually not a flat disk but a huge ball or globe, more than
ten times the diameter of the earth, which turns swiftly round on its
axis once every ten hours as against the earth's turning round in
twenty-four hours. Then it is easy to see how the centrifugal force
bulges outward the equatorial regions of Jupiter, so that the polar
regions are correspondingly drawn inward, thereby making the polar
diameter shorter than the equatorial one, which is in line with the
moons or satellites. The difference between the two diameters is very
marked, as much as one part in fifteen. All the planets are slightly
flattened in this way, but Jupiter is the most so of all except Saturn.
The little darkish line across the planet's middle region or equator was
found to be replaced by several such lines or irregular belts and spots,
often seen highly colored, especially with reflecting telescopes; and
they are perpetually changing their mutual relation and shapes, because
they are not solid territory or land on Jupiter, but merely the outer
shapes of atmospheric strata, blown and torn and twisted by atmospheric
circulation on this planet, quite the same as clouds in the atmosphere
on the earth are.
Besides this the axial turning of Jupiter brings an entirely different
part of the planet into view every two or three hours; so that in making
a map or chart of the planet, an arbitrary meridian must be selected.
Even then the process is not an easy one, and it is found that spots on
Jupiter's equator turn round in 9 hours 50 minutes, while other regions
take a few minutes longer, the nearer the poles are approached. The
Great Red Spot, about 30,000 miles long and a quarter as much in breadth
has been visible for about half a century. Bolton, an English observer,
has made interesting studies of it very recently.
The four moons, or satellites, which a small telescope reveals, are
exceedingly interesting on many accounts. They were the first heavenly
bodies seen by the aid of the telescope, Galileo having discovered them
in 1610. They travel round Jupiter much the same as the moon does round
the earth, but faster, the innermost moon about four times per week, the
second moon about twice a week, the third or largest moon (larger than
the planet Mercury) once a week, and the outermost in about sixteen
days. The innermost is about 260,000 miles from Jupiter, and the
outermost more than a million miles. From their nearness to the huge and
excessively hot globe of Jupiter, some astronomers, Proctor especially,
have inclined to the view that these little bodies may be inhabited.
Jupiter has other moons; a very small one, close to the planet, which
goes round in less than twelve hours, discovered by Barnard in 1892.
Four others are known, very small and faint and remote from the planet,
which travel slowly round it in orbits of great magnitude. The ninth, or
outermost, is at a distance of fifteen and one-half million miles from
Jupiter, and requires nearly three years in going round the planet. It
was discovered by Nicholson at the Lick Observatory in 1914. The eighth
was discovered by Melotte at Greenwich in 1908, and is peculiar in the
great angle of 28 degrees, at which its orbit is inclined to the equator
of Jupiter. The sixth and seventh satellites revolve round Jupiter
inside the eighth satellite, but outside the orbit of IV; and they were
discovered by photography at the Lick Observatory in 1905 by Perrine,
now director of the Argentine National Observatory at Cordoba.
The ever-changing positions of the Medicean moons, as Galileo called the
four satellites that he discovered--their passing into the shadow in
eclipse, their transit in front of the disk, and their occultation
behind it--form a succession of phenomena which the telescopist always
views with delight. The times when all these events take place are
predicted in the "Nautical Almanac," many thousand of them each year,
and the predictions cover two or three years in advance.
Jupiter, as the naked eye sees him high up in the midnight sky, is the
brightest of all the planets except Venus; indeed, he is five times
brighter than Sirius, the brightest of all the fixed stars. His stately
motion among the stars will usually be visible by close observation from
day to day, and his distance from the earth, at times when he is best
seen, is usually about 400 million miles. Jupiter travels all the way
round the sun in twelve years; his motion in orbit is about eight miles
a second.
The eclipses of Jupiter's moons, caused by passing into the shadow of
the planet, would take place at almost perfectly regular intervals, if
our distance from Jupiter were invariable. But it was early found out
that while the earth is approaching Jupiter the eclipses take place
earlier and earlier, but later and later when the earth is moving away.
The acceleration of the earliest eclipse added to the retardation of the
latest makes 1,000 seconds, which is the time that light takes in
crossing a diameter of the earth's orbit round the sun. Now the velocity
of light is well known to be 186,300 miles per second, so we calculate
at once and very simply that the sun's distance from the earth, which is
half the diameter of the orbit, equals 500 times 186,300, or 93,000,000
miles.