JUPITER.
The planet Jupiter occupies, in one sense, the first position in the
planetary world, it being the largest of all the planets. Moreover, with
the exception of Venus, it is the brightest of the planets. As with
Mars, and for the like reason, Jupiter, when in the positions known as
the Quadratures (or near thereto), exhibits a slight phase, but owing to
the far greater distance from the Sun of Jupiter, compared with Mars,
the deviation of the illuminated surface from that of a complete circle
is very small; it is, however, perceptible at or near the time of
quadrature, a slight shading off of the limb farthest from the Sun being
traceable.
Jupiter is noteworthy on account of two features, both of them more or
less familiar, at least by name, to most people—its belts and its
satellites,—both of which will be described in due course.
The belts are dusky streaks, which vary from time to time both in
breadth and number: most commonly two broad belts will be seen with two
or three narrower ones on either side; but sometimes all are rather
narrow, and their narrowness is made up for by an increase in their
number.
[Illustration: Fig. 14.—Jupiter, November 27, 1857 (Dawes).]
Under all circumstances they lie practically parallel, or nearly so, to
the planet’s equator. It is generally thought that the planet, whatever
may be its actual structure or constitution, is surrounded by a dense
cloudy envelope, and that the shaded streaks which we call belts are
rifts in this atmosphere, which expose to view the solid body of the
planet underneath. Whether, however, the term “solid body” is an
accurate one to be used in this connection is thought by some to be open
to doubt. The laws which regulate the existence of these belts are quite
unknown; indeed it seems doubtful whether any laws exist at all, for the
belts at one time appear to undergo constant change, whilst at another
time they remain almost unchanged for several months. It has been
suggested that when the changes are rapid it must be presumed that great
atmospheric storms are to be considered as in progress, and possibly
this may be the true explanation. Belts are commonly non-existent
immediately under the equator; whilst north and south of this void space
it most usually happens that there is one broad belt and several
narrower ones in each hemisphere. At each pole the planet’s brightness
is less than the average brightness, but it cannot exactly be said that
this is due to the existence there of belts properly so called.
It was formerly considered that no tinges of colour could be traced on
Jupiter except a silvery gray of different degrees of intensity; but
during the last thirty years there can be no doubt that shades of brown,
red, and orange, of no great depth, but yet quite definite have been
traceable. Many observers concur in this opinion. Whether this detection
of colour is due to an absolute development of colour during the period
in question; or whether its detection is merely the result of more
careful scrutiny with better instruments is a matter as to which the
evidence is not clear. Though the general position of the belts is such
that they are parallel to the planet’s equator, yet there are sometimes
exceptions to this rule, for in a few very rare instances a streak in
the nature of a narrow belt has been seen, inclined to the equator at a
decided angle, perhaps 20° or even more.
It occasionally happens that spots are seen on Jupiter’s belts.
Sometimes these remain visible for a considerable period. They are
either dark or luminous, and their origin is unknown. Besides these
casual spots, which are always small in size, there was visible during
many years following 1878 a very remarkable and conspicuous large spot,
strongly red in colour for several years, though it afterwards became
much fainter. This spot exhibited an oval outline and was about 27,000
miles long and 8000 miles broad. For about 4 years it maintained its
intense red colour and its shape almost unaltered; but after 1882, the
shape remaining, the colour sensibly faded. The observations which were
made on this spot during 1886 by Professor Hough at Chicago, U. S., with
an 18-inch refractor, led him to the opinion that the persistence of the
red spot for so many years rendered untenable the generally accepted
theory that the phenomena seen on the surface of the planet are due to
atmospheric causes.
Some astronomers have thought that a relationship subsists between the
spots on the Sun and the spots on Jupiter. There certainly seems an
apparent identity in point of time between the two classes of spots, and
on the assumption that the spots on Jupiter are indicative of
disturbances on the planet, Ranyard broached the idea that both classes
of phenomena are dependent on some extraneous cosmical change; and are
not related as cause and effect. Browning suggested many years ago that
the red colour of the belts is a periodical phenomenon coinciding with
the epoch of the greatest display of sunspots, but this thought does not
appear to have been followed up by any one. Spots on Jupiter seem to
have been first recorded by Robert Hooke in 1664. In the following year
Cassini saw a spot which he found to be in motion, and by following it
attentively he inferred that the planet rotated on its axis in 9h. 56m.
It is a remarkable illustration of the great care bestowed by Cassini on
his astronomical work that the best modern determinations of Jupiter’s
rotation-period differ from Cassini’s estimate by only half a minute.
Bearing in mind the enormous size of Jupiter compared with the Earth,
whilst its period of rotation is considerably less than half the
Earth’s, it will be at once seen that the velocity of matter at the
planet’s equator is immensely great—466 miles per minute against the
Earth’s 17 miles per minute. One result of this is the great intensity
of the centrifugal force at the equator, and likewise the greatness of
the compression of the planet’s body at the poles. Hind has suggested
that the great velocity which thus evidently exists may have the effect,
by reason of the development of the heat which it gives rise to, of
compensating the planet for the small amount of heat which owing to its
distance it receives from the Sun.
On favourable occasions the brilliancy of Jupiter is very considerable;
so much so that it rivals Venus and Mars. And besides this, there
appears to be something special in the nature of Jupiter’s surface, for
not only does it seem to radiate a much larger proportion of the solar
light which falls on it than do the planets generally, but some
observers have expressed the opinion that it possesses inherent light of
its own. Speculations, however, such as this must always be received
with reserve, because of the evident difficulty of making sure of the
facts on which they must be based. One thing, however, seems less open
to doubt. Bearing in mind the small amount of heat which reaches Jupiter
from the Sun, there is reason to infer that the clouds which certainly
exist on Jupiter must owe their origin to the influence of some other
heat than solar heat; in other words that Jupiter possesses sources of
heat within itself.
Jupiter has satellites, 5 in number. The discovery of four of these, was
one of the first fruits of the invention of the telescope, for they were
found by Galileo in January, 1610. The 5th satellite is so small that it
escaped notice until as recently as 1892, having been discovered on
September 9 of that year by Professor Barnard, with the great Lick
telescope in California. It is, however, so minute that one can count on
one’s fingers the telescopes capable of showing it.
The four old satellites of Jupiter shine as stars of about the 7th
magnitude; in other words, they are sufficiently bright to be visible
with telescopes however small: indeed several instances are on record of
persons gifted with very good sight, having been able to see them with
the naked eye. For the study of their physical appearance very powerful
optical assistance is necessary, but their movements are so rapid, and
the phenomena which result from those movements are so interesting, that
these bodies may be considered to occupy the first place in the
stock-in-trade of every amateur astronomer, who lays himself out for
planet-gazing, with the object of profiting himself or his friends. The
phenomena here alluded to are known as eclipses, transits, and
occultations.
The four old satellites do not bear any names, but are numbered from the
innermost outwards, and are always alluded to by their numbers as I, II,
III and IV.
An eclipse of a Jovian satellite is identical in principle with an
eclipse of the Moon; that is to say, just as an eclipse of the Moon
happens when the Moon passes into and is lost in the Earth’s shadow, so
an eclipse of a Jovian satellite happens when such satellite becomes
lost in the shadow cast by the planet into space. The Ist IInd and IIIrd
satellites in consequence of the smallness of the inclination of their
orbits, undergo eclipse once in every revolution round their primary,
but the IVth is less often eclipsed, owing to the joint effect of its
considerable orbital inclination, and of the distance to which it
recedes from its primary.
An occultation of a Jovian satellite is akin in principle to an
occultation of a star by the Moon. As the Moon moving forwards suddenly
covers a star, so the planet, on occasions, suddenly covers one of its
satellites. If the satellite in question is the IVth, its disappearance
behind the planet and its reappearance from behind the planet will both
be visible in due succession. This is often true also of the IIIrd
satellite, but for reasons connected with the proximity to their primary
of the Ist and IInd satellites, only their disappearance _or_
reappearance (not both) can, as a rule, be observed on the same
occasion. The most interesting, by far, however, of the phenomena
connected with Jupiter’s satellites are their transits in front of, that
is across, the visible disc of the planet. Though these transits are of
frequent occurrence, yet they are always interesting because of the
diverse appearances which the satellites exhibit at different times, and
which cannot be said to be in accordance with any recognised laws.
Moreover, in observing the transit of a satellite, we may often see the
black shadow cast by the satellite on the planet’s disc; and this shadow
will sometimes precede and sometimes follow the satellite itself. From
the fact that the satellite generally appears as a bright spot on a
bright background whilst the shadow is black, or blackish, an
inexperienced observer is apt to look at the shadow and think he is
seeing the satellite.
Jupiter revolves round the Sun in not quite 12 years at a mean distance
of 483 millions of miles. Its apparent diameter varies between 50″ and
30″ according to its position with respect to the Earth. Its true
diameter is about 88,000 miles. Owing to its large size and rapid
rotation, as has already been mentioned, Jupiter is very much flattened
at the poles. The amount of this (the polar “compression” as it is
called) is about 1/16.