e major planets are
continually shifting.

THE PLANET JUPITER

Jupiter is the greatest of the major planets. It has been justly called
the "Giant" planet, for both in volume and in mass it exceeds all the
other planets put together. When seen through the telescope it exhibits
a surface plentifully covered with markings, the most remarkable being a
series of broad parallel belts. The chief belt lies in the central parts
of the planet, and is at present about 10,000 miles wide. It is bounded
on either side by a reddish brown belt of about the same width. Bright
spots also appear upon the surface of the planet, last for a while, and
then disappear. The most notable of the latter is one known as the
"Great Red Spot." This is situated a little beneath the southern red
belt, and appeared for the first time about thirty years ago. It has
undergone a good many changes in colour and brightness, and is still
faintly visible. This spot is the most permanent marking which has yet
been seen upon Jupiter. In general, the markings change so often that
the surface which we see is evidently not solid, but of a fleeting
nature akin to cloud (see Plate XIV., p. 230).

[Illustration: PLATE XIV. THE PLANET JUPITER

The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908,
with a 12-1/2-inch reflecting telescope. The extensive oval marking in
the upper portion of the disc is the "Great Red Spot." The South is at
the top of the picture, the view being the _inverted_ one given by an
astronomical telescope. From a drawing by the Rev. Theodore E.R.
Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British
Astronomical Association.

(Page 231)]

Observations of Jupiter's markings show that on an average the planet
rotates on its axis in a period of about 9 hours 54 minutes. The mention
here of _an average_ with reference to the rotation will, no doubt,
recall to the reader's mind the similar case of the sun, the different
portions of which rotate with different velocities. The parts of Jupiter
which move quickest take 9 hours 50 minutes to go round, while those
which move slowest take 9 hours 57 minutes. The middle portions rotate
the fastest, a phenomenon which the reader will recollect was also the
case with regard to the sun.

Jupiter is a very loosely packed body. Its density is on an average only
about 1-1/2 times that of water, or about one-fourth the density of the
earth; but its bulk is so great that