68. JUPITER, the biggest of all the Planets, is still higher in the
System, being about 424 millions of miles from the Sun: and going at the
rate of 25 thousand miles every hour in his Orbit, as in the circle ♃
finishes his annual period in eleven of our years 314 days and 18 hours.
He is above 1000 times as big as the Earth, for his diameter is 81,000
miles; which is more than ten times the diameter of the Earth.
[Sidenote: The number of days in his year.]
69. Jupiter turns round his Axis in 9 hours 56 minutes; so that his year
contains 10 thousand 464 days; and the diurnal velocity of his
equatoreal parts is greater than the swiftness with which he moves in
his annual Orbit; a singular circumstance, as far as we know. By this
prodigious quick Rotation, his equatoreal inhabitants are carried 25
thousand 920 miles every hour (which is 920 miles an hour more than an
inhabitant of our Earth moves in twenty-four hours) besides the 25
thousand above-mentioned, which is common to all parts of his surface,
by his annual motion.
[Sidenote: His Belts and spots.]
70. Jupiter is surrounded by faint substances, called _Belts_, in which
so many changes appear, that they are generally thought to be clouds:
for some of them have been first interrupted and broken, and then have
vanished entirely. They have sometimes been observed of different
breadths, and afterwards have all become nearly of the same breadth.
Large spots have been seen in these Belts; and when a Belt vanishes, the
contiguous spots disappear with it. The broken ends of some Belts have
been generally observed to revolve in the same time with the spots; only
those nearer the Equator in somewhat less time than those near the
Poles; perhaps on account of the Sun’s greater heat near the Equator,
which is parallel to the Belts and course of the spots. Several large
spots, which appear round at one time, grow oblong by degrees, and then
divide into two or three round spots. The periodical time of the spots
near the Equator is 9 hours 50 minutes, but of those near the Poles 9
hours 56 minutes. _See Dr._ SMITH_’s Optics_, § 1004 _& seq._
[Sidenote: He has no change of seasons;]
71. The Axis of Jupiter is so nearly perpendicular to his Orbit, that he
has no sensible change of seasons; which is a great advantage, and
wisely ordered by the Author of Nature. For, if the Axis of this Planet
were inclined any considerable number of degrees, just so many degrees
round each Pole would in their turn be almost six of our years together
in darkness. And, as each degree of a great Circle on Jupiter contains
706 of our miles at a mean rate, it is easy to judge what vast tracts of
land would be rendered uninhabitable by any considerable inclination of
his Axis.
[Sidenote: But has four Moons.]
72. The Sun appears but 1/28 part so big to Jupiter as to us; and his
light and heat are in the same small proportion, but compensated by the
quick returns thereof, and by four Moons (some bigger and some less than
our Earth) which revolve about him: so that there is scarce any part of
this huge Planet but what is during the whole night enlightened by one
or more of these Moons, except his Poles, whence only the farthest Moons
can be seen, and where their light is not wanted, because the Sun
constantly circulates in or near the Horizon, and is very probably kept
in view of both Poles by the Refraction of Jupiter’s Atmosphere, which,
if it be like ours, has certainly refractive power enough for that
purpose.
[Sidenote: Their periods round Jupiter.