THE SUPERIOR PLANETS--_continued_
The planets, so far, have been divided into inferior and superior. Such
a division, however, refers merely to the situation of their orbits with
regard to that of our earth. There is, indeed, another manner in which
they are often classed, namely, according to size. On this principle
they are divided into two groups; one group called the _Terrestrial
Planets_, or those which have characteristics like our earth, and the
other called the _Major Planets_, because they are all of very great
size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
and Neptune. As the earth's orbit is the boundary which separates the
inferior from the superior planets, so does the asteroidal belt divide
the terrestrial from the major planets. We found the division into
inferior and superior useful for emphasising the marked difference in
aspect which those two classes present as seen from our earth; the
inferior planets showing phases like the moon when viewed in the
telescope, whereas the superior planets do not. But the division into
terrestrial and major planets is the more far-reaching classification of
the two, for it includes the whole number of planets, whereas the other
arrangement necessarily excludes the earth. The members of each of
these classes have many definite characteristics in common. The
terrestrial planets are all of them relatively small in size,
comparatively near together, and have few or no satellites. They are,
moreover, rather dense in structure. The major planets, on the other
hand, are huge bodies, circulating at great distances from each other,
and are, as a rule, provided with a number of satellites. With respect
to structure, they may be fairly described as being loosely put
together. Further, the markings on the surfaces of the terrestrial
planets are permanent, whereas those on the 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 the gravitation at that surface
which we see is about 2-1/2 times what it is on the surface of the
earth. In accordance, therefore, with the kinetic theory, we may expect
the planet to retain an extensive layer of gases around it; and this is
confirmed by the spectroscope, which gives evidence of the presence of a
dense atmosphere.
All things considered, it may be safely inferred that the interior of
Jupiter is very hot, and that what we call its surface is not the actual
body of the planet, but a voluminous layer of clouds and vapours driven
upwards from the heated mass underneath. The planet was indeed formerly
thought to be self-luminous; but this can hardly be the case, for those
portions of the surface which happen to lie at any moment in the
shadows cast by the satellites appear to be quite black. Again, when a
satellite passes into the great shadow cast by the planet it becomes
entirely invisible, which would not be the case did the planet emit any
perceptible light of its own.
In its revolutions around the sun, Jupiter is attended, so far as we
know, by seven[22] satellites. Four of these were among the first
celestial objects which Galileo discovered with his "optick tube," and
he named them the "Medicean Stars" in honour of his patron, Cosmo de
Medici. Being comparatively large bodies they might indeed just be seen
with the naked eye, were it not for the overpowering glare of the
planet.
It was only in quite recent times, namely, in 1892, that a fifth
satellite was added to the system of Jupiter. This body, discovered by
Professor E.E. Barnard, is very small. It circulates nearer to the
planet than the innermost of Galileo's moons; and, on account of the
glare, is a most difficult object to obtain a glimpse of, even in the
best of telescopes. In December 1904 and January 1905 respectively, two
more moons were added to the system, these being found by _photography_,
by the American astronomer, Professor C.D. Perrine. Both the bodies in
question revolve at a greater distance from the planet than the
outermost of the older known satellites.
Galileo's moons, though the largest bodies of Jupiter's satellite
system, are, as we have already pointed out, very small indeed when
compared with the planet itself. The diameters of two of them, Europa
and Io, are, however, about the same as that of our moon, while those of
the other two, Callisto and Ganymede, are more than half as large again.
The recently discovered satellites are, on the other hand,
insignificant; that found by Barnard, for example, being only about 100
miles in diameter.
Of the four original satellites Io is the nearest to Jupiter, and, seen
from the planet, it would show a disc somewhat larger than that of our
moon. The others would appear somewhat smaller. However, on account of
the great distance of the sun, the entire light reflected to Jupiter by
all the satellites should be very much less than what we get from our
moon.
Barnard's satellite circles around Jupiter at a distance less than our
moon is from us, and in a period of about 12 hours. Galileo's four
satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days
respectively, at distances lying roughly between a quarter of a million
and one million miles. Perrine's two satellites are at a distance of
about seven million miles, and take about nine months to complete their
revolutions.
The larger satellites, when viewed in the telescope, exhibit certain
defined markings; but the bodies are so far away from us, that only
those details which are of great extent can be seen. The satellite Io,
according to Professor Barnard, shows a darkish disc, with a broad white
belt across its middle regions. Mr. Douglass, one of the observers at
the Lowell Observatory, has noted upon Ganymede a number of markings
somewhat resembling those seen on Mars, and he concludes, from their
movement, that this satellite rotates on its axis in about seven days.
Professor Barnard, on the other hand, does not corroborate this, though
he claims to have discovered bright polar caps on both Ganymede and
Callisto.
In an earlier chapter we dealt at length with eclipses, occultations,
and transits, and endeavoured to make clear the distinction between
them. The system of Jupiter's satellites furnishes excellent examples of
all these phenomena. The planet casts a very extensive shadow, and the
satellites are constantly undergoing obscuration by passing through it.
Such occurrences are plainly comparable to our lunar eclipses. Again,
the satellites may, at one time, be occulted by the huge disc of the
planet, and at another time seen in transit over its face. A fourth
phenomenon is what is known as an _eclipse of the planet by a
satellite_, which is the exact equivalent of what we style on the earth
an eclipse of the sun. In this last case the shadow, cast by the
satellite, appears as a round black spot in movement across the planet's
surface.
In the passages of these attendant bodies behind the planet, into its
shadow, or across its face, respectively, it occasionally happens that
Galileo's four satellites all disappear from view, and the planet is
then seen for a while in the unusual condition of being apparently
without its customary attendants. An instance of this phenomenon took
place on the 3rd of October 1907. On that occasion, the satellites known
as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say,
obscured by passing into the planet's shadow; Satellite IV. (Callisto)
was occulted by the planet's disc; while Satellite II. (Europa), being
at the same moment in transit across the planet's face, was invisible
against that brilliant background. A number of instances of this kind of
occurrence are on record. Galileo, for example, noted one on the 15th of
March 1611, while Herschel observed another on the 23rd of May 1802.
It was indirectly to Jupiter's satellites that the world was first
indebted for its knowledge of the velocity of light. When the periods of
revolution of the satellites were originally determined, Jupiter
happened, at the time, to be at his nearest to us. From the periods thus
found tables were made for the prediction of the moments at which the
eclipses and other phenomena of the satellites should take place. As
Jupiter, in the course of his orbit, drew further away from the earth,
it was noticed that the disappearances of the satellites into the shadow
of the planet occurred regularly later than the time predicted. In the
year 1675, Roemer, a Danish astronomer, inferred from this, not that the
predictions were faulty, but that light did not travel instantaneously.
It appeared, in fact, to take longer to reach us, the greater the
distance it had to traverse. Thus, when the planet was far from the
earth, the last ray given out by the satellite, before its passage into
the shadow, took a longer time to cross the intervening space, than when
the planet was near. Modern experiments in physics have quite confirmed
this, and have proved for us that light does not travel across space in
the twinkling of an eye, as might hastily be supposed, but actually
moves, as has been already stated, at the rate of about 186,000 miles
per second.
THE PLANET SATURN
Seen in the telescope the planet Saturn is a wonderful and very
beautiful object. It is distinguished from all the other planets, in
fact from all known celestial bodies, through being girt around its
equator by what looks like a broad, flat ring of exceeding thinness.
This, however, upon closer examination, is found to be actually composed
of three concentric rings. The outermost of these is nearly of the same
brightness as the body of the planet itself. The ring which comes
immediately within it is also bright, and is separated from the outer
one all the way round by a relatively narrow space, known as "Cassini's
division," because it was discovered by the celebrated French
astronomer, J.D. Cassini, in the year 1675. Inside the second ring, and
merging insensibly into it, is a third one, known as the "crape ring,"
because it is darker in hue than the others and partly transparent, the
body of Saturn being visible through it. The inner boundary of this
third and last ring does not adjoin the planet, but is everywhere
separated from it by a definite space. This ring was discovered
_independently_[23] in 1850 by Bond in America and Dawes in England.
[Illustration: PLATE XV. THE PLANET SATURN
From a drawing made by Professor Barnard with the Great Lick Telescope.
The black band fringing the outer ring, where it crosses the disc, is
portion of the _shadow which the rings cast upon the planet_. The black
wedge-shaped mark, where the rings disappear behind the disc at the
left-hand side, is portion of the _shadow which the planet casts upon
the rings_.
(Page 237)]
As distinguished from the crape ring, the bright rings must have a
considerable closeness of texture; for the shadow of the planet may be
seen projected upon them, and their shadows in turn projected upon the
surface of the planet (see Plate XV., p. 236).
According to Professor Barnard, the entire breadth of the ring system,
that is to say, from one side to the other of the outer ring, is 172,310
miles, or somewhat more than double the planet's diameter.
In the varying views which we get of Saturn, the system of the rings is
presented to us at very different angles. Sometimes we are enabled to
gaze upon its broad expanse; at other times, however, its thin edge is
turned exactly towards us, an occurrence which takes place after
intervals of about fifteen years. When this happened in 1892 the rings
are said to have disappeared entirely from view in the great Lick
telescope. We thus get an idea of their small degree of thickness, which
would appear to be only about 50 miles. The last time the system of
rings was exactly edgewise to the earth was on the 3rd of October 1907.
The question of the composition of these rings has given rise to a good
deal of speculation. It was formerly supposed that they were either
solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
structure of this kind would not be able to stand. He showed, however,
that they could be fully explained by supposing them to consist of an
immense number of separate solid particles, or, as one might otherwise
put it, extremely small satellites, circling in dense swarms around the
middle portions of the planet. It is therefore believed that we have
here the materials ready for the formation of a satellite or satellites;
but that the powerful gravitative action, arising through the planet's
being so near at hand, is too great ever to allow these materials to
aggregate themselves into a solid mass. There is, as a matter of fact, a
minimum distance from the body of any planet within which it can be
shown that a satellite will be unable to form on account of
gravitational stress. This is known as "Roche's limit," from the name of
a French astronomer who specially investigated the question.
There thus appears to be a certain degree of analogy between Saturn's
rings and the asteroids. Empty spaces, too, exist in the asteroidal
zone, the relative position of one of which bears a striking resemblance
to that of "Cassini's division." It is suggested, indeed, that this
division had its origin in gravitational disturbances produced by the
attraction of the larger satellites, just as the empty spaces in the
asteroidal zone are supposed to be the result of perturbations caused by
the Giant Planet hard by.
It has long been understood that the system of the rings must be
rotating around Saturn, for if they were not in motion his intense
gravitational attraction would quickly tear them in pieces. This was at
length proved to be the fact by the late Professor Keeler, Director of
the Lick Observatory, who from spectroscopic observations found that
those portions of the rings situated near to the planet rotated faster
than those farther from it. This directly supports the view that the
rings are composed of satellites; for, as we have already seen, the
nearer a satellite is to its primary the faster it will revolve. On the
other hand, were the rings solid, their outer portions would move the
fastest; as we have seen takes place in the body of the earth, for
example. The mass of the ring system, however, must be exceedingly
small, for it does not appear to produce any disturbances in the
movements of Saturn's satellites. From the kinetic theory, therefore,
one would not expect to find any atmosphere on the rings, and the
absence of it is duly shown by spectroscopic observations.
The diameter of Saturn, roughly speaking, is about one-fifth less than
that of Jupiter. The planet is very flattened at the poles, this
flattening being quite noticeable in a good telescope. For instance, the
diameter across the equator is about 76,470 miles, while from pole to
pole it is much less, namely, 69,770.
The surface of Saturn bears a strong resemblance to that of Jupiter. Its
markings, though not so well defined, are of the same belt-like
description; and from observation of them it appears that the planet
rotates _on an average_ in a little over ten hours. The rotation is in
fact of the same peculiar kind as that of the sun and Jupiter; but the
difference of speed at which the various portions of Saturn go round are
even more marked than in the case of the Giant Planet. The density of
Saturn is less than that of Jupiter; so that it must be largely in a
condition of vapour, and in all probability at a still earlier stage of
planetary evolution.
Up to the present we know of as many as ten satellites circling around
Saturn, which is more than any other planet of the solar system can lay
claim to. Two of these, however, are very recent discoveries; one,
Phoebe, having been found by photography in August 1898, and the other,
Themis, in 1904, also by the same means. For both of these we are
indebted to Professor W.H. Pickering. Themis is said to be _the faintest
object in the solar system_. It cannot be _seen_, even with the largest
telescope in existence; a fact which should hardly fail to impress upon
one the great advantage the photographic plate possesses in these
researches over the human eye.
The most important of the whole Saturnian family of satellites are the
two known as Titan and Japetus. These were discovered respectively by
Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
as our moon; while the diameter of Titan, the largest of the satellites,
is about half as much again. Titan takes about sixteen days to revolve
around Saturn, while Japetus takes more than two months and a half. The
former is about three-quarters of a million miles distant from the
planet, and the latter about two and a quarter millions. To Sir William
Herschel we are indebted for the discovery of two more satellites, one
of which he found on the evening that he used his celebrated 40-foot
telescope for the first time. The ninth satellite, Phoebe, one of the
two discovered by Professor Pickering, is perhaps the most remarkable
body in the solar system, for all the other known members of that system
perform their revolutions in one fixed direction, whereas this satellite
revolves in the _contrary_ direction.
In consequence of the great distance of Saturn, the sun, as seen from
the planet, would appear so small that it would scarcely show any disc.
The planet, indeed, only receives from the sun about one-ninetieth of
the heat and light which the earth receives. Owing to this diminished
intensity of illumination, the combined light reflected to Saturn by the
whole of its satellites must be very small.
With the sole exception of Jupiter, not one of the planets circulating
nearer to the sun could be seen from Saturn, as they would be entirely
lost in the solar glare. For an observer upon Saturn, Jupiter would,
therefore, fill much the same position as Venus does for us, regularly
displaying phases and being alternately a morning and an evening star.
It is rather interesting to consider the appearances which would be
produced in our skies were the earth embellished with a system of rings
similar to those of Saturn. In consequence of the curving of the
terrestrial surface, they would not be seen at all from within the
Arctic or Antarctic circles, as they would be always below the horizon.
From the equator they would be continually seen edgewise, and so would
appear merely as line of light stretching right across the heaven and
passing through the zenith. But the dwellers in the remaining regions
would find them very objectionable, for they would cut off the light of
the sun during lengthy periods of time.
Saturn was a sore puzzle to the early telescopic observers. They did not
for a long time grasp the fact that it was surrounded by a ring--so slow
is the human mind to seek for explanations out of the ordinary course of
things. The protrusions of the ring on either side of the planet, at
first looked to Galileo like two minor globes placed on opposite sides
of it, and slightly overlapping the disc. He therefore informed Kepler
that "Saturn consists of three stars in contact with one another." Yet
he was genuinely puzzled by the fact that the two attendant bodies (as
he thought them) always retained the same position with regard to the
planet's disc, and did not appear to revolve around it, nor to be in any
wise shifted as a consequence of the movements of our earth.
About a year and a half elapsed before he again examined Saturn; and, if
he was previously puzzled, he was now thoroughly amazed. It happened
just then to be one of those periods when the ring is edgewise towards
the earth, and of course he only saw a round disc like that of Jupiter.
What, indeed, had become of the attendant orbs? Was some demon mocking
him? Had Saturn devoured his own children? He was, however, fated to be
still more puzzled, for soon the minor orbs reappeared, and, becoming
larger and larger as time went on, they ended by losing their globular
appearance and became like two pairs of arms clasping the planet from
each side! (see Plate XVI., p. 242).
Galileo went to his grave with the riddle still unsolved, and it
remained for the famous Dutch astronomer, Huyghens, to clear up the
matter. It was, however, some little time before he hit upon the real
explanation. Having noticed that there were dark spaces between the
strange appendages and the body of the planet, he imagined Saturn to be
a globe fitted with handles at each side; "ansæ" these came to be
called, from the Latin _ansa_, which means a handle. At length, in the
year 1656, he solved the problem, and this he did by means of that
123-foot tubeless telescope, of which mention has already been made. The
ring happened then to be at its edgewise period, and a careful study of
the behaviour of the ansæ when disappearing and reappearing soon
revealed to Huyghens the true explanation.
[Illustration: PLATE XVI. EARLY REPRESENTATIONS OF SATURN
From an illustration in the _Systema Saturnium_ of Christian Huyghens.
(Page 242)]
THE PLANETS URANUS AND NEPTUNE
We have already explained (in Chapter II.) the circumstances in which
both Uranus and Neptune were discovered. It should, however, be added
that after the discovery of Uranus, that planet was found to have been
already noted upon several occasions by different observers, but always
without the least suspicion that it was other than a mere faint star.
Again, with reference to the discovery of Neptune, it may here be
mentioned that the apparent amount by which that planet had pulled
Uranus out of its place upon the starry background was exceedingly
small--so small, indeed, that no eye could have detected it without the
aid of a telescope!
Of the two predictions of the place of Neptune in the sky, that of Le
Verrier was the nearer. Indeed, the position calculated by Adams was
more than twice as far out. But Adams was by a long way the first in the
field with his results, and only for unfortunate delays the prize would
certainly have fallen to him. For instance, there was no star-map at
Cambridge, and Professor Challis, the director of the observatory there,
was in consequence obliged to make a laborious examination of the stars
in the suspected region. On the other hand, all that Galle had to do was
to compare that part of the sky where Le Verrier told him to look with
the Berlin star-chart which he had by him. This he did on September 23,
1846, with the result that he quickly noted an eighth magnitude star
which did not figure in that chart. By the next night this star had
altered its position in the sky, thus disclosing the fact that it was
really a planet.
Six days later Professor Challis succeeded in finding the planet, but of
course he was now too late. On reviewing his labours he ascertained that
he had actually noted down its place early in August, and had he only
been able to sift his observations as he made them, the discovery would
have been made then.
Later on it was found that Neptune had only just missed being discovered
about fifty years earlier. In certain observations made during 1795, the
famous French astronomer, Lalande, found that a star, which he had
mapped in a certain position on the 8th of May of that year, was in a
different position two days later. The idea of a planet does not appear
to have entered his mind, and he merely treated the first observation as
an error!
The reader will, no doubt, recollect how the discovery of the asteroids
was due in effect to an apparent break in the seemingly regular sequence
of the planetary orbits outwards from the sun. This curious sequence of
relative distances is usually known as "Bode's Law," because it was
first brought into general notice by an astronomer of that name. It had,
however, previously been investigated mathematically by Titius in 1772.
Long before this, indeed, the unnecessarily wide space between the
orbits of Mars and Jupiter had attracted the attention of the great
Kepler to such a degree, that he predicted that a planet would some day
be found to fill the void. Notwithstanding the service which the
so-called Law of Bode has indirectly rendered to astronomy, it has
strangely enough been found after all not to rest upon any scientific
foundation. It will not account for the distance from the sun of the
orbit of Neptune, and the very sequence seems on the whole to be in the
nature of a mere coincidence.
Neptune is invisible to the naked eye; Uranus is just at the limit of
visibility. Both planets are, however, so far from us that we can get
but the poorest knowledge of their condition and surroundings. Uranus,
up to the present, is known to be attended by four satellites, and
Neptune by one. The planets themselves are about equal in size; their
diameters, roughly speaking, being about one-half that of Saturn. Some
markings have, indeed, been seen upon the disc of Uranus, but they are
very indistinct and fleeting. From observation of them, it is assumed
that the planet rotates on its axis in a period of some ten to twelve
hours. No definite markings have as yet been seen upon Neptune, which
body is described by several observers as resembling a faint planetary
nebula.
With regard to their physical condition, the most that can be said about
these two planets is that they are probably in much the same vaporous
state as Jupiter and Saturn. On account of their great distance from the
sun they can receive but little solar heat and light. Seen from
Neptune, in fact, the sun would appear only about the size of Venus at
her best, though of a brightness sufficiently intense to illumine the
Neptunian landscape with about seven hundred times our full moonlight.
[22] Mr. P. Melotte, of Greenwich Observatory, while examining a
photograph taken there on February 28, 1908, discovered upon it a very
faint object which it is firmly believed will prove to be an _eighth_
satellite of Jupiter. This object was afterwards found on plates exposed
as far back as January 27. It has since been photographed several times
at Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the Lick
Observatory. Its movement is probably _retrograde_, like that of Phoebe
(p. 240).
[23] In the history of astronomy two salient points stand out.
The first of these is the number of "independent" discoveries which have
taken place; such, for instance, as in the cases of Le Verrier and Adams
with regard to Neptune, and of Lockyer and Janssen in the matter of the
spectroscopic method of observing solar prominences.
The other is the great amount of "anticipation." Copernicus, as we have
seen, was anticipated by the Greeks; Kepler was not actually the first
who thought of elliptic orbits; others before Newton had imagined an
attractive force.
Both these points furnish much food for thought!