shown in Fig. 278. The diameters of these moons range from about
twenty-two hundred to thirty-seven hundred miles. The second from the
planet is the smallest, and the third the largest. The smallest is about
the size of our moon; the largest considerably exceeds Mercury, and
almost rivals Mars, in bulk. The sizes of these moons, compared with
those of the earth and its moon, are shown in Fig. 279.
[Illustration: Fig. 279.]
The names of these satellites, in the order of their distance from the
planet, are _Io_, _Europa_, _Ganymede_, and _Callisto_. Their times of
revolution range from about a day and three-fourths up to about sixteen
days and a half. Their orbits are shown in Fig. 280.
[Illustration: Fig. 280.]
257. _The Variability of Jupiter's Satellites._--Remarkable variations
in the light of these moons have led to the supposition that violent
changes are taking place on their surfaces. It was formerly believed,
that, like our moon, they always present the same face to the planet,
and that the changes in their brilliancy are due to differences in the
luminosity of parts of their surface which are successively turned
towards us during a revolution; but careful measurements of their light
show that this hypothesis does not account for the changes, which are
sometimes very sudden. The satellites are too distant for examination of
their surfaces with the telescope: hence it is impossible to give any
certain explanation of these phenomena.
[Illustration: Fig. 281.]
258. _Eclipses of Jupiter's Satellites._--Jupiter, like the earth, casts
a shadow away from the sun, as shown in Fig. 281; and, whenever one of
his moons passes into this shadow, it becomes eclipsed. On the other
hand, whenever one of the moons throws its shadow on Jupiter, the sun is
eclipsed to that part of the planet which lies within the shadow.
To the inhabitants of Jupiter (if there are any, and if they can see
through the clouds) these eclipses must be very familiar affairs; for in
consequence of the small inclinations of the orbits of the satellites to
the planet's equator, and the small inclination of the latter to the
plane of Jupiter's orbit, all the satellites, except the most distant
one, are eclipsed in every revolution. A spectator on Jupiter might
therefore witness during the planetary year forty-five hundred eclipses
of the moons, and about the same number of the sun.
[Illustration: Fig. 282.]
259. _Transits of Jupiter's Satellites._--Whenever one of Jupiter's
moons passes in front of the planet, it is said to make a _transit_
across his disk. When a moon is making a transit, it presents its bright
hemisphere towards the earth, as will be seen from Fig. 282: hence it is
usually seen as a bright spot on the planet's disk; though sometimes, on
the brighter central portions of the disk, it appears dark.
[Illustration: Fig. 283.]
It will be seen from Fig. 282 that the shadow of a moon does not fall
upon the part of the planet's disk that is covered by the moon: hence we
may observe the transit of both the moon and its shadow. The shadow
appears as a small black spot, which will precede or follow the moon
according to the position of the earth in its orbit. Fig. 283 shows two
moons of Jupiter in transit.
260. _Occultations of Jupiter's Satellites._--The eclipse of a moon of
Jupiter must be carefully distinguished from the _occultation_ of a moon
by the planet. In the case of an eclipse, the moon ceases to be visible,
because the mass of Jupiter is interposed between the sun and the moon,
which ceases to be luminous, because the sun's light is cut off; but, in
the case of an occupation, the moon gets into such a position that the
body of Jupiter is interposed between it and the earth, thus rendering
the moon invisible to us. The third satellite, _m''_ (Fig. 282), is
invisible from the earth _E_, having become _occulted_ when it passed
behind the planet's disk; but it will not be _eclipsed_ until it passes
into the shadow of Jupiter.
261. _Jupiter without Satellites._--It occasionally happens that every
one of Jupiter's satellites will disappear at the same time, either by
being eclipsed or occulted, or by being in transit. In this event,
Jupiter will appear without satellites. This occurred on the 21st of
August, 1867. The position of Jupiter's satellites at this time is shown
in Fig. 284.
[Illustration: Fig. 284.]
Saturn.
THE PLANET AND HIS MOONS.
262. _The Orbit of Saturn._--The orbit of Saturn is rather more
eccentric than that of Jupiter, its eccentricity being somewhat more
than one-twentieth. Its inclination to the ecliptic is about two degrees
and a half. The mean distance of Saturn from the sun is about eight
hundred and eighty million miles. It is about a hundred million miles
nearer the sun at perihelion than at aphelion.
263. _Distance of Saturn from the Earth._--The mean distance of Saturn
from the earth at opposition is eight hundred and eighty million miles
_minus_ ninety-two million miles, or seven hundred and eighty-eight
million; and at conjunction, eight hundred and eighty million miles
_plus_ ninety-two million, or nine hundred and seventy-two million.
Owing to the eccentricity of the orbit of Saturn, his distance from the
earth at opposition and at conjunction varies by about a hundred million
miles at different times; but he is so immensely far away, that this is
only a small fraction of his mean distance.
264. _Apparent Size and Brightness of Saturn._--The apparent diameter of
Saturn varies from about twenty seconds to about fourteen seconds. His
apparent size at his extreme and mean distances from the earth is shown
in Fig. 285.
[Illustration: Fig. 285.]
The planet generally shines with the brilliancy of a moderate
first-magnitude star, and with a dingy, reddish light, as if seen
through a smoky atmosphere.
265. _Volume and Density of Saturn._--The real diameter of Saturn is
about seventy thousand miles, and its volume over seven hundred times
that of the earth. The comparative size of the earth and Saturn is shown
in Fig. 286. This planet is a little more than half as dense as Jupiter.
[Illustration: Fig. 286.]
266. _The Sidereal and Synodical Periods of Saturn._--Saturn makes a
complete revolution round the sun in a period of about twenty-nine years
and a half, moving in his orbit at the rate of about six miles a second.
The planet passes from opposition to opposition again in a period of
three hundred and seventy-eight days, or thirteen days over a year.
267. _Physical Constitution of Saturn._--The physical constitution of
Saturn seems to resemble that of Jupiter; but, being twice as far away,
the planet cannot be so well studied. The farther an object is from the
sun, the less it is illuminated; and, the farther it is from the earth,
the smaller it appears: hence there is a double difficulty in examining
the more distant planets. Under favorable circumstances, the surface of
Saturn is seen to be diversified with very faint markings; and, with
high telescopic powers, two or more very faint streaks, or belts, may be
discerned parallel to its equator. These belts, like those of Jupiter,
change their aspect from time to time; but they are so faint that the
changes cannot be easily followed. It is only on rare occasions that the
time of rotation can be determined from a study of the markings.
268. _Rotation of Saturn._--On the evening of Dec. 7, 1876, Professor
Hall, who had been observing the satellites of Saturn with the great
Washington telescope (18), saw a brilliant white spot near the equator
of the planet. It seemed as if an immense eruption of incandescent
matter had suddenly burst up from the interior. The spot gradually
spread itself out into a long light streak, of which the brightest point
was near the western end. It remained visible until January, when it
became faint and ill-defined, and the planet was lost in the rays of the
sun.
From all the observations on this spot, Professor Hall found the period
of Saturn to be ten hours fourteen minutes, reckoning by the brightest
part of the streak. Had the middle of the streak been taken, the time
would have been less, because the bright matter seemed to be carried
along in the direction of the planet's rotation. If this motion was due
to a wind, the velocity of the current must have been between fifty and
a hundred miles an hour. The axis of Saturn is inclined twenty-seven
degrees from the perpendicular to its orbit.
[Illustration: Fig. 287.]
269. _The Satellites of Saturn._--Saturn is accompanied by eight moons.
Seven of these are shown in Fig. 287. The names of these satellites, in
the order of their distances from the planet, are given in the
accompanying table:--
Number. Name. Distance Sidereal Discoverer.
from Period.
Planet
1 Mimas 120,800 0 22 37 0.94 Herschel
2 Enceladus 155,000 1 8 53 1.37 Herschel
3 Tethys 191,900 1 21 18 1.88 Cassini
4 Dione 245,800 2 17 41 2.73 Cassini
5 Rhea 343,400 4 12 25 4.51 Cassini
6 Titan 796,100 15 22 41 15.94 Huyghens
7 Hyperion 963,300 21 7 7 21.29 Bond
8 Japetus 2,313,800 79 7 53 79.33 Cassini
The apparent brightness or visibility of these satellites follows the
order of their discovery. The smallest telescope will show Titan, and
one of very moderate size will show Japetus in the western part of its
orbit. An instrument of four or five inches aperture will show Rhea, and
perhaps Tethys and Dione; while seven or eight inches are required for
Enceladus, even at its greatest elongation from the planet. Mimas can
rarely be seen except at its greatest elongation, and then only with an
aperture of twelve inches or more. Hyperion can be detected only with
the most powerful telescopes, on account of its faintness and the
difficulty of distinguishing it from minute stars.
_Japetus_, the outermost satellite, is remarkable for the fact, that
while, in one part of its orbit, it is the brightest of the satellites
except Titan, in the opposite part it is almost as faint as Hyperion,
and can be seen only in large telescopes. When west of the planet, it is
bright; when east of it, faint. This peculiarity has been accounted for
by supposing that the satellite, like our moon, always presents the same
face to the planet, and that one side of it is white and the other
intensely black; but it is doubtful whether any known substance is so
black as one side of the satellite must be to account for such
extraordinary changes of brilliancy.
[Illustration: Fig. 288.]
_Titan_, the largest of these satellites, is about the size of the
largest satellite of Jupiter. The relative sizes of the satellites are
shown in Fig. 288, and their orbits in Fig. 289.
[Illustration: Fig. 289.]
[Illustration: Fig. 290.]
Fig. 290 shows the transit of one of the satellites, and of its shadow,
across the disk of the planet.
THE RINGS OF SATURN.
270. _General Appearance of the Rings._--Saturn is surrounded by a thin
flat ring lying in the plane of its equator. This ring is probably less
than a hundred miles thick. The part of it nearest Saturn reflects
little sunlight to us; so that it has a dusky appearance, and is not
easily seen, although it is not quite so dark as the sky seen between it
and the planet. The outer edge of this dusky portion of the ring is at a
distance from Saturn of between two and three times the earth's
diameter. Outside of this dusky part of the ring is a much brighter
portion, and outside of this another, which is somewhat fainter, but
still so much brighter than the dusky part as to be easily seen. The
width of the brighter parts of the ring is over three times the earth's
diameter. To distinguish the parts, the outer one is called ring _A_,
the middle one ring _B_, and the dusky one ring _C_. Between _A_ and _B_
is an apparently open space, nearly two thousand miles wide, which looks
like a black line on the ring. Other divisions in the ring have been
noticed at times; but this is the only one always seen with good
telescopes at times when either side of the ring is in view from the
earth. The general telescopic appearance of the ring is shown in Fig.
291.
[Illustration: Fig. 291.]
[Illustration: Fig. 292.]
Fig. 292 shows the divisions of the rings as they were seen by Bond.
271. _Phases of Saturn's Ring._--The ring is inclined to the plane of
the planet's orbit by an angle of twenty-seven degrees. The general
aspect from the earth is nearly the same as from the sun. As the planet
revolves around the sun, the axis and plane of the ring keep the same
direction in space, just as the axis of the earth and the plane of the
equator do.
When the planet is in one part of its orbit, we see the upper or
northern side of the ring at an inclination of twenty-seven degrees, the
greatest angle at which the ring can ever be seen. This phase of the
ring is shown in Fig. 293.
[Illustration: Fig. 293.]
When the planet has moved through a quarter of a revolution, the edge of
the ring is turned towards the sun and the earth; and, owing to its
extreme thinness, it is visible only in the most powerful telescopes as
a fine line of light, stretching out on each side of the planet. This
phase of the ring is shown in Fig. 294.
[Illustration: Fig. 294.]
All the satellites, except Japetus, revolve very nearly in the plane of
the ring: consequently, when the edge of the ring is turned towards the
earth, the satellites seem to swing from one side of the planet to the
other in a straight line, running along the thin edge of the ring like
beads on a string. This phase affords the best opportunity of seeing the
inner satellites, Mimas and Enceladus, which at other times are obscured
by the brilliancy of the ring.
[Illustration: Fig. 295.]
Fig. 295 shows a phase of the ring intermediate between the last two.
When the planet has moved ninety degrees farther, we again see the ring
at an angle of twenty-seven degrees; but now it is the lower or southern
side which is visible. When it has moved ninety degrees farther, the
edge of the ring is again turned towards the earth and sun.
[Illustration: Fig. 295.]
The successive phases of Saturn's ring during a complete revolution are
shown in Fig. 296.
It will be seen that there are two opposite points of Saturn's orbit in
which the rings are turned edgewise to us, and two points half-way
between the former in which the ring is seen at its maximum inclination
of about twenty-seven degrees. Since the planet performs a revolution in
twenty-nine years and a half, these phases occur at average intervals of
about seven years and four months.
[Illustration: Fig. 297.]
[Illustration: Fig. 298.]
272. _Disappearance of Saturn's Ring._--It will be seen from Fig. 297
that the plane of the ring may not be turned towards the sun and the
earth at exactly the same time, and also that the earth may sometimes
come on one side of the plane of the ring while the sun is shining on
the other. In the figure, _E_, _E'_, _E''_, and _E'''_ is the orbit of
the earth. When Saturn is at _S'_, or opposite, at _F_, the plane of the
ring will pass through the sun, and then only the edge of the ring will
be illumined. Were Saturn at _S_, and the earth at _E'_, the plane of
the ring would pass through the earth. This would also be the case were
the earth at _E'''_, and Saturn at _S''_. Were Saturn at _S_ or at
_S''_, and the earth farther to the left or to the right, the sun would
be shining on one side of the ring while we should be looking on the
other. In all these cases the ring will disappear entirely in a
telescope of ordinary power. With very powerful telescopes the ring will
appear, in the first two cases, as a thin line of light (Fig. 298). It
will be seen that all these cases of disappearance must take place when
Saturn is in the parts of his orbit intercepted between the parallel
lines _AC_ and _BD_. These lines are tangent to the earth's orbit, which
they enclose, and are parallel to the plane of Saturn's ring. As Saturn
passes away from these two lines on either side, the rings appear more
and more open. When the dark side of the ring is in view, it appears as
a black line crossing the planet; and on such occasions the sunlight
reflected from the outer and inner edges of the rings _A_ and _B_
enables us to see traces of the ring on each side of Saturn, at least in
places where two such reflections come nearly together. Fig. 299
illustrates this reflection from the edges at the divisions of the
rings.
[Illustration: Fig. 299.]
273. _Changes in Saturn's Ring._--The question whether changes are going
on in the rings of Saturn is still unsettled. Some observers have
believed that they saw additional divisions in the rings from time to
time; but these may have been errors of vision, due partly to the
shading which is known to exist on portions of the ring.
Professor Newcomb says, "As seen with the great Washington equatorial in
the autumn of 1874, there was no great or sudden contrast between the
inner or dark edge of the bright ring and the outer edge of the dusky
ring. There was some suspicion that the one shaded into the other by
insensible gradations. No one could for a moment suppose, as some
observers have, that there was a separation between these two rings. All
these considerations give rise to the question whether the dusky ring
may not be growing at the expense of the inner bright ring."
Struve, in 1851, advanced the startling theory that the inner edge of
the ring was gradually approaching the planet, the whole ring spreading
inwards, and making the central opening smaller. The theory was based
upon the descriptions and drawings of the rings by the astronomers of
the seventeenth century, especially Huyghens, and the measures made by
later astronomers up to 1851. This supposed change in the dimension of
the ring is shown in Fig. 300.
[Illustration: Fig. 300.]
274. _Constitution of Saturn's Ring._--The theory now generally held by
astronomers is, that the ring is composed of a cloud of satellites too
small to be separately seen in the telescope, and too close together to
admit of visible intervals between them. The ring looks solid, because
its parts are too small and too numerous to be seen singly. They are
like the minute drops of water that make up clouds and fogs, which to
our eyes seem like solid masses. In the dusky ring the particles may be
so scattered that we can see through the cloud, the duskiness being due
to the blending of light and darkness. Some believe, however, that the
duskiness is caused by the darker color of the particles rather than by
their being farther apart.
Uranus.
275. _Orbit and Dimensions of Uranus._--Uranus, the smallest of the
outer group of planets, has a diameter of nearly thirty-two thousand
miles. It is a little less dense than Jupiter, and its mean distance
from the sun is about seventeen hundred and seventy millions of miles.
Its orbit has about the same eccentricity as that of Jupiter, and is
inclined less than a degree to the ecliptic. Uranus makes a revolution
around the sun in eighty-four years, moving at the rate of a little over
four miles a second. It is visible to the naked eye as a star of the
sixth magnitude.
As seen in a large telescope, the planet has a decidedly sea-green
color; but no markings have with certainty been detected on its disk, so
that nothing is really known with regard to its rotation. Fig. 301 shows
the comparative size of Uranus and the earth.
[Illustration: Fig. 301.]
276. _Discovery of Uranus._--This planet was discovered by Sir William
Herschel in March, 1781. He was engaged at the time in examining the
small stars of the constellation _Gemini_, or the Twins. He noticed that
this object which had attracted his attention had an appreciable disk,
and therefore could not be a star. He also perceived by its motion that
it could not be a nebula; he therefore concluded that it was a comet,
and announced his discovery as such. On attempting to compute its orbit,
it was soon found that its motions could be accounted for only on the
supposition that it was moving in a circular orbit at about twice the
distance of Saturn from the sun. It was therefore recognized as a new
planet, whose discovery nearly doubled the dimensions of the solar
system as it was then known.
277. _The Name of the Planet._--Herschel, out of compliment to his
patron, George III., proposed to call the new planet _Georgium Sidus_
(the Georgian Star); but this name found little favor. The name of
_Herschel_ was proposed, and continued in use in England for a time, but
did not meet with general approval. Various other names were suggested,
and finally that of _Uranus_ was adopted.
[Illustration: Fig. 302.]
278. _The Satellites of Uranus._--Uranus is accompanied by four
satellites, whose orbits are shown in Fig. 302. These satellites are
remarkable for the great inclination of their orbits to the plane of the
planet's orbit, amounting to about eighty degrees, and for their
_retrograde_ motion; that is, they move _from east to west_, instead of
from west to east, as in the case of all the planets and of all the
satellites previously discovered.
Neptune.
279. _Orbit and Dimensions of Neptune._--So far as known, Neptune is the
most remote member of the solar system, its mean distance from the sun
being twenty-seven hundred and seventy-five million miles. This distance
is considerably less than twice that of Uranus. Neptune revolves around
the sun in a period of a little less than a hundred and sixty-five
years. Its orbit has but slight eccentricity, and is inclined less than
two degrees to the ecliptic. This planet is considerably larger than
Uranus, its diameter being nearly thirty-five thousand miles. It is
somewhat less dense than Uranus. Neptune is invisible to the naked eye,
and no telescope has revealed any markings on its disk: hence nothing is
certainly known as to its rotation. Fig. 303 shows the comparative size
of Neptune and the earth.
[Illustration: Fig. 303.]
280. _The Discovery of Neptune._--The discovery of Neptune was made in
1846, and is justly regarded as one of the grandest triumphs of
astronomy.
Soon after Uranus was discovered, certain irregularities in its motion
were observed, which could not be explained. It is well known that the
planets are all the while disturbing each other's motions, so that none
of them describe perfect ellipses. These mutual disturbances are called
_perturbations_. In the case of Uranus it was found, that, after making
due allowance for the action of all the known planets, there were still
certain perturbations in its course which had not been accounted for.
This led astronomers to the suspicion that these might be caused by an
unknown planet. Leverrier in France, and Adams in England, independently
of each other, set themselves the difficult problem of computing the
position and magnitude of a planet which would produce these
perturbations. Both, by a most laborious computation, showed that the
perturbations were such as would be produced by a planet revolving about
the sun at about twice the distance of Uranus, and having a mass
somewhat greater than that of this planet; and both pointed out the same
part of the heavens as that in which the planet ought to be found at
that time. Almost immediately after they had announced the conclusion to
which they had arrived, the planet was found with the telescope. The
astronomer who was searching for the planet at the suggestion of
Leverrier was the first to recognize it: hence Leverrier has obtained
the chief credit of the discovery.
The observed planet is proved to be nearer than the one predicted by
Leverrier and Adams, and therefore of smaller magnitude.