SCENES ON THE PLANETS
"These starry globes far surpassed the earth in grandeur, and the
latter looked so diminutive that our empire, which appeared only as
a point on its surface, awoke my pity."--CICERO, THE DREAM OF
SCIPIO.
Although amateurs have played a conspicuous part in telescopic discovery
among the heavenly bodies, yet every owner of a small telescope should
not expect to attach his name to a star. But he certainly can do
something perhaps more useful to himself and his friends; he can follow
the discoveries that others, with better appliances and opportunities,
have made, and can thus impart to those discoveries that sense of
reality which only comes from seeing things with one's own eyes. There
are hundreds of things continually referred to in books and writings on
astronomy which have but a misty and uncertain significance for the mere
reader, but which he can easily verify for himself with the aid of a
telescope of four or five inches aperture, and which, when actually
confronted by the senses, assume a meaning, a beauty, and an importance
that would otherwise entirely have escaped him. Henceforth every
allusion to the objects he has seen is eloquent with intelligence and
suggestion.
Take, for instance, the planets that have been the subject of so many
observations and speculations of late years--Mars, Jupiter, Saturn,
Venus. For the ordinary reader much that is said about them makes very
little impression upon his mind, and is almost unintelligible. He reads
of the "snow patches" on Mars, but unless he has actually seen the
whitened poles of that planet he can form no clear image in his mind of
what is meant. So the "belts of Jupiter" is a confusing and misleading
phrase for almost everybody except the astronomer, and the rings of
Saturn are beyond comprehension unless they have actually been seen.
It is true that pictures and photographs partially supply the place of
observation, but by no means so successfully as many imagine. The most
realistic drawings and the sharpest photographs in astronomy are those
of the moon, yet I think nobody would maintain that any picture in
existence is capable of imparting a really satisfactory visual
impression of the appearance of the lunar globe. Nobody who has not seen
the moon with a telescope--it need not be a large one--can form a
correct and definite idea of what the moon is like.
The satisfaction of viewing with one's own eyes some of the things the
astronomers write and talk about is very great, and the illumination
that comes from such viewing is equally great. Just as in foreign travel
the actual seeing of a famous city, a great gallery filled with
masterpieces, or a battlefield where decisive issues have been fought
out illuminates, for the traveler's mind, the events of history, the
criticisms of artists, and the occurrences of contemporary life in
foreign lands, so an acquaintance with the sights of the heavens gives a
grasp on astronomical problems that can not be acquired in any other
way. The person who has been in Rome, though he may be no archaeologist,
gets a far more vivid conception of a new discovery in the Forum than
does the reader who has never seen the city of the Seven Hills; and the
amateur who has looked at Jupiter with a telescope, though he may be no
astronomer, finds that the announcement of some change among the
wonderful belts of that cloudy planet has for him a meaning and an
interest in which the ordinary reader can not share.
[Illustration: JUPITER SEEN WITH A FIVE-INCH TELESCOPE.
Shadow of a satellite visible.]
Jupiter is perhaps the easiest of all the planets for the amateur
observer. A three-inch telescope gives beautiful views of the great
planet, although a four-inch or a five-inch is of course better. But
there is no necessity for going beyond six inches' aperture in any case.
For myself, I should care for nothing better than my Byrne five-inch of
fifty-two inches' focal distance. With such a glass more details are
visible in the dark belts and along the bright equatorial girdle than
can be correctly represented in a sketch before the rotation of the
planet has altered their aspect, while the shadows of the satellites
thrown upon the broad disk, and the satellites themselves when in
transit, can be seen sometimes with exquisite clearness. The contrasting
colors of various parts of the disk are also easily studied with a glass
of four or five inches' aperture.
There is a charm about the great planet when he rides high in a clear
evening sky, lording it over the fixed stars with his serene,
unflickering luminousness, which no possessor of a telescope can resist.
You turn the glass upon him and he floats into the field of view, with
his _cortege_ of satellites, like a yellow-and-red moon, attended by
four miniatures of itself. You instantly comprehend Jupiter's mastery
over his satellites--their allegiance is evident. No one would for an
instant mistake them for stars accidentally seen in the same field of
view. Although it requires a very large telescope to magnify their disks
to measurable dimensions, yet the smallest glass differentiates them at
once from the fixed stars. There is something almost startling in their
appearance of companionship with the huge planet--this sudden
verification to your eyes of the laws of gravitation and of central
forces. It is easy, while looking at Jupiter amid his family, to
understand the consternation of the churchmen when Galileo's telescope
revealed that miniature of the solar system, and it is gratifying to
gaze upon one of the first battle grounds whereon science gained a
decisive victory for truth.
The swift changing of place among the satellites, as well as the
rapidity of Jupiter's axial rotation, give the attraction of visible
movement to the Jovian spectacle. The planet rotates in four or five
minutes less than ten hours--in other words, it makes two turns and four
tenths of a third turn while the earth is rolling once upon its axis. A
point on Jupiter's equator moves about twenty-seven thousand miles, or
considerably more than the entire circumference of the earth, in a
single hour. The effect of this motion is clearly perceptible to the
observer with a telescope on account of the diversified markings and
colors of the moving disk, and to watch it is one of the greatest
pleasures that the telescope affords.
It would be possible, when the planet is favorably situated, to witness
an entire rotation of Jupiter in the course of one night, but the
beginning and end of the observation would be more or less interfered
with by the effects of low altitude, to say nothing of the tedium of so
long a vigil. But by looking at the planet for an hour at a time in the
course of a few nights every side of it will have been presented to
view. Suppose the first observation is made between nine and ten o'clock
on any night which may have been selected. Then on the following night
between ten and eleven o'clock Jupiter will have made two and a half
turns upon his axis, and the side diametrically opposite to that seen on
the first night will be visible. On the third night between eleven and
twelve o'clock Jupiter will have performed five complete rotations, and
the side originally viewed will be visible again.
Owing to the rotundity of the planet, only the central part of the disk
is sharply defined, and markings which can be easily seen when centrally
located become indistinct or disappear altogether when near the limb.
Approach to the edge of the disk also causes a foreshortening which
sometimes entirely alters the aspect of a marking. It is advisable,
therefore, to confine the attention mainly to the middle of the disk. As
time passes, clearly defined markings on or between the cloudy belts
will be seen to approach the western edge of the disk, gradually losing
their distinctness and altering their appearance, while from the region
of indistinct definition near the eastern edge other markings slowly
emerge and advance toward the center, becoming sharper in outline and
more clearly defined in color as they swing into view.
Watching these changes, the observer is carried away by the reflection
that he actually sees the turning of another distant world upon its axis
of rotation, just as he might view the revolving earth from a standpoint
on the moon. Belts of reddish clouds, many thousands of miles across,
are stretched along on each side of the equator of the great planet he
is watching; the equatorial belt itself, brilliantly lemon-hued, or
sometimes ruddy, is diversified with white globular and balloon-shaped
masses, which almost recall the appearance of summer cloud domes hanging
over a terrestrial landscape, while toward the poles shadowy expanses of
gradually deepening blue or blue-gray suggest the comparative coolness
of those regions which lie always under a low sun.
[Illustration: ECLIPSES AND TRANSITS OF JUPITER'S SATELLITES.
Satellite I and the shadow of III are seen in transit. IV is about to be
eclipsed.]
After a few nights' observation even the veriest amateur finds himself
recognizing certain shapes or appearances--a narrow dark belt running
slopingly across the equator from one of the main cloud zones to the
other, or a rift in one of the colored bands, or a rotund white mass
apparently floating above the equator, or a broad scallop in the edge of
a belt like that near the site of the celebrated "red spot," whose
changes of color and aspect since its first appearance in 1878, together
with the light it has thrown on the constitution of Jupiter's disk, have
all but created a new Jovian literature, so thoroughly and so frequently
have they been discussed.
And, having noticed these recurring features, the observer will begin to
note their relations to one another, and will thus be led to observe
that some of them gradually drift apart, while others drift nearer; and
after a time, without any aid from books or hints from observatories, he
will discover for himself that there is a law governing the movements on
Jupiter's disk. Upon the whole he will find that the swiftest motions
are near the equator, and the slowest near the poles, although, if he is
persistent and has a good eye and a good instrument, he will note
exceptions to this rule, probably arising, as Professor Hough suggests,
from differences of altitude in Jupiter's atmosphere. Finally, he will
conclude that the colossal globe before him is, exteriorly at least, a
vast ball of clouds and vapors, subject to tremendous vicissitudes,
possibly intensely heated, and altogether different in its physical
constitution, although made up of similar elements, from the earth.
Then, if he chooses, he can sail off into the delightful cloud-land of
astronomical speculation, and make of the striped and spotted sphere of
Jove just such a world as may please his fancy--for a world of some
kind it certainly is.
For many observers the satellites of Jupiter possess even greater
attractions than the gigantic ball itself. As I have already remarked,
their movements are very noticeable and lend a wonderful animation to
the scene. Although they bear classical names, they are almost
universally referred to by their Roman numbers, beginning with the
innermost, whose symbol is I, and running outward in regular order II,
III, and IV.[5] The minute satellite much nearer to the planet than any
of the others, which Mr. Barnard discovered with the Lick telescope in
1892, is called the fifth, although in the order of distance it would be
the first. In size and importance, however, it can not rank with its
comparatively gigantic brothers. Of course, no amateur's telescope can
afford the faintest glimpse of it.
[5] Their names, in the same order as their numbers, are Io, Europa,
Ganymede, and Callisto.
Satellite I, situated at a mean distance of 261,000 miles from Jupiter's
center--about 22,000 miles farther than the moon is from the earth--is
urged by its master's overpowering attraction to a speed of 320 miles
per minute, so that it performs a complete revolution in about forty-two
hours and a half. The others, of course, move more slowly, but even the
most distant performs its revolution in several hours less than sixteen
days. The plane of their orbits is presented edgewise toward the earth,
from which it follows that they appear to move back and forth nearly in
straight lines, some apparently approaching the planet, while others are
receding from it. The changes in their relative positions, which can be
detected from hour to hour, are very striking night after night, and
lead to a great variety of arrangements always pleasing to the eye.
The most interesting phenomena that they present are their transits and
those of their round, black shadows across the face of the planet; their
eclipses by the planet's shadow, when they disappear and afterward
reappear with astonishing suddenness; and their occultations by the
globe of Jupiter. Upon the whole, the most interesting thing for the
amateur to watch is the passage of the shadows across Jupiter. The
distinctness with which they can be seen when the air is steady is
likely to surprise, as it is certain to delight, the observer. When it
falls upon a light part of the disk the shadow of a satellite is as
black and sharply outlined as a drop of ink; on a dark-colored belt it
can not so easily be seen.
It is more difficult to see the satellites themselves in transit. There
appears to be some difference among them as to visibility in such
circumstances. Owing to their luminosity they are best seen when they
have a dark belt for a background, and are least easily visible when
they appear against a bright portion of the planet. Every observer
should provide himself with a copy of the American Ephemeris for the
current year, wherein he will find all the information needed to enable
him to identify the various satellites and to predict, by turning
Washington mean time into his own local time, the various phenomena of
the transits and eclipses.
While a faithful study of the phenomena of Jupiter is likely to lead the
student to the conclusion that the greatest planet in our system is not
a suitable abode for life, yet the problem of its future, always
fascinating to the imagination, is open; and whosoever may be disposed
to record his observations in a systematic manner may at least hope to
render aid in the solution of that problem.
Saturn ranks next to Jupiter in attractiveness for the observer with a
telescope. The rings are almost as mystifying to-day as they were in
the time of Herschel. There is probably no single telescopic view that
can compare in the power to excite wonder with that of Saturn when the
ring system is not so widely opened but that both poles of the planet
project beyond it. One returns to it again and again with unflagging
interest, and the beauty of the spectacle quite matches its singularity.
When Saturn is in view the owner of a telescope may become a recruiting
officer for astronomy by simply inviting his friends to gaze at the
wonderful planet. The silvery color of the ball, delicately chased with
half-visible shadings, merging one into another from the bright
equatorial band to the bluish polar caps; the grand arch of the rings,
sweeping across the planet with a perceptible edging of shadow; their
sudden disappearance close to the margin of the ball, where they go
behind it and fall straightway into night; the manifest contrast of
brightness, if not of color, between the two principal rings; the fine
curve of the black line marking the 1,600-mile gap between their
edges--these are some of the elements of a picture that can never fade
from the memory of any one who has once beheld it in its full glory.
[Illustration: SATURN SEEN WITH A FIVE-INCH TELESCOPE.]
Saturn's moons are by no means so interesting to watch as are those of
Jupiter. Even the effect of their surprising number (raised to nine by
Professor Pickering's discovery in 1899 of a new one which is almost at
the limit of visibility, and was found only with the aid of photography)
is lost, because most of them are too faint to be seen with ordinary
telescopes, or, if seen, to make any notable impression upon the eye.
The two largest--Titan and Japetus--are easily found, and Titan is
conspicuous, but they give none of that sense of companionship and
obedience to a central authority which strikes even the careless
observer of Jupiter's system. This is owing partly to their more
deliberate movements and partly to the inclination of the plane of their
orbits, which seldom lies edgewise toward the earth.
[Illustration: POLAR VIEW OF SATURN'S SYSTEM.
The orbits of the five nearest satellites are shown. The dotted line
outside the rings shows Roche's limit.]
But the charm of the peerless rings is abiding, and the interest of the
spectator is heightened by recalling what science has recently
established as to their composition. It is marvelous to think, while
looking upon their broad, level surfaces--as smooth, apparently, as
polished steel, though thirty thousand miles across--that they are in
reality vast circling currents of meteoritic particles or dust, through
which run immense waves, condensation and rarefaction succeeding one
another as in the undulations of sound. Yet, with all their inferential
tumult, they may actually be as soundless as the depths of interstellar
space, for Struve has shown that those spectacular rings possess no
appreciable mass, and, viewed from Saturn itself, their (to us) gorgeous
seeming bow may appear only as a wreath of shimmering vapor spanning the
sky and paled by the rivalry of the brighter stars.
In view of the theory of tidal action disrupting a satellite within a
critical distance from the center of its primary, the thoughtful
observer of Saturn will find himself wondering what may have been the
origin of the rings. The critical distance referred to, and which is
known as Roche's limit, lies, according to the most trustworthy
estimates, just outside the outermost edge of the rings. It follows that
if the matter composing the rings were collected into a single body that
body would inevitably be torn to pieces and scattered into rings; and
so, too, if instead of one there were several or many bodies of
considerable size occupying the place of the rings, all of these bodies
would be disrupted and scattered. If one of the present moons of
Saturn--for instance, Mimas, the innermost hitherto discovered--should
wander within the magic circle of Roche's limit it would suffer a
similar fate, and its particles would be disseminated among the rings.
One can hardly help wondering whether the rings have originated from the
demolition of satellites--Saturn devouring his children, as the ancient
myths represent, and encircling himself, amid the fury of destruction,
with the dust of his disintegrated victims. At any rate, the amateur
student of Saturn will find in the revelations of his telescope the
inspirations of poetry as well as those of science, and the bent of his
mind will determine which he shall follow.
Professor Pickering's discovery of a ninth satellite of Saturn, situated
at the great distance of nearly eight million miles from the planet,
serves to call attention to the vastness of the "sphere of activity"
over which the ringed planet reigns. Surprising as the distance of the
new satellite appears when compared with that of our moon, it is yet far
from the limit where Saturn's control ceases and that of the sun becomes
predominant. That limit, according to Prof. Asaph Hall's calculation, is
nearly 30,000,000 miles from Saturn's center, while if our moon were
removed to a distance a little exceeding 500,000 miles the earth would
be in danger of losing its satellite through the elopement of Artemis
with Apollo.
Although, as already remarked, the satellites of Saturn are not
especially interesting to the amateur telescopist, yet it may be well to
mention that, in addition to Titan and Japetus, the satellite named
Rhea, the fifth in order of distance from the planet, is not a difficult
object for a three-or four-inch telescope, and two others considerably
fainter than Rhea--Dione (the fourth) and Tethys (the third)--may be
seen in favorable circumstances. The others--Mimas (the first),
Enceladus (the second), and Hyperion (the seventh)--are beyond the reach
of all but large telescopes. The ninth satellite, which has received the
name of Ph[oe]be, is much fainter than any of the others, its stellar
magnitude being reckoned by its discoverer at about 15.5.
Mars, the best advertised of all the planets, is nearly the least
satisfactory to look at except during a favorable opposition, like those
of 1877 and 1892, when its comparative nearness to the earth renders
some of its characteristic features visible in a small telescope. The
next favorable opposition will occur in 1907.
When well seen with an ordinary telescope, say a four-or five-inch
glass, Mars shows three peculiarities that may be called fairly
conspicuous--viz., its white polar cap, its general reddish, or
orange-yellow, hue, and its dark markings, one of the clearest of which
is the so-called Syrtis Major, or, as it was once named on account of
its shape, "Hourglass Sea." Other dark expanses in the southern
hemisphere are not difficult to be seen, although their outlines are
more or less misty and indistinct. The gradual diminution of the polar
cap, which certainly behaves in this respect as a mass of snow and ice
would do, is a most interesting spectacle. As summer advances in the
southern hemisphere of Mars, the white circular patch surrounding the
pole becomes smaller, night after night, until it sometimes disappears
entirely even from the ken of the largest telescopes. At the same time
the dark expanses become more distinct, as if the melting of the polar
snows had supplied them with a greater depth of water, or the advance of
the season had darkened them with a heavier growth of vegetation.
[Illustration: MARS SEEN WITH A FIVE-INCH TELESCOPE.]
The phenomena mentioned above are about all that a small telescope will
reveal. Occasionally a dark streak, which large instruments show is
connected with the mysterious system of "canals," can be detected, but
the "canals" themselves are far beyond the reach of any telescope except
a few of the giants handled by experienced observers. The conviction
which seems to have forced its way into the minds even of some
conservative astronomers, that on Mars the conditions, to use the
expression of Professor Young, "are more nearly earthlike than on any
other of the heavenly bodies which we can see with our present
telescopes," is sufficient to make the planet a center of undying
interest notwithstanding the difficulties with which the amateur is
confronted in his endeavors to see the details of its markings.
THE ILLUMINATION OF VENUS'S ATMOSPHERE AT THE BEGINNING OF HER TRANSIT
ACROSS THE SUN.
In Venus "the fatal gift of beauty" may be said, as far as our
observations are concerned, to be matched by the equally fatal gift of
brilliance. Whether it be due to atmospheric reflection alone or to the
prevalence of clouds, Venus is so bright that considerable doubt exists
as to the actual visibility of any permanent markings on her surface.
The detailed representations of the disk of Venus by Mr. Percival
Lowell, showing in some respects a resemblance to the stripings of Mars,
can not yet be accepted as decisive. More experienced astronomers than
Mr. Lowell have been unable to see at all things which he draws with a
fearless and unhesitating pencil. That there are some shadowy features
of the planet's surface to be seen in favourable circumstances is
probable, but the time for drawing a "map of Venus" has not yet come.
The previous work of Schiaparelli lends a certain degree of probability
to Mr. Lowell's observations on the rotation of Venus. This rotation,
according to the original announcement of Schiaparelli, is probably
performed in the same period as the revolution around the sun. In other
words, Venus, if Schiaparelli and Lowell are right, always presents the
same side to the sun, possessing, in consequence, a day hemisphere and a
night hemisphere which never interchange places. This condition is so
antagonistic to all our ideas of what constitutes habitability for a
planet that one hesitates to accept it as proved, and almost hopes that
it may turn out to have no real existence. Venus, as the twin of the
earth in size, is a planet which the imagination, warmed by its sunny
aspect, would fain people with intelligent beings a little fairer than
ourselves; but how can such ideas be reconciled with the picture of a
world one half of which is subjected to the merciless rays of a
never-setting sun, while the other half is buried in the fearful gloom
and icy chill of unending night?
Any amateur observer who wishes to test his eyesight and his telescope
in the search of shades or markings on the disk of Venus by the aid of
which the question of its rotation may finally be settled should do his
work while the sun is still above the horizon. Schiaparelli adopted that
plan years ago, and others have followed him with advantage. The
diffused light of day serves to take off the glare which is so serious
an obstacle to the successful observation of Venus when seen against a
dark sky. Knowing the location of Venus in the sky, which can be
ascertained from the Ephemeris, the observer can find it by day. If his
telescope is not permanently mounted and provided with "circles" this
may not prove an easy thing to do, yet a little perseverance and
ingenuity will effect it. One way is to find, with a star chart, some
star whose declination is the same, or very nearly the same, as that of
Venus, and which crosses the meridian say twelve hours ahead of her.
Then set the telescope upon that star, when it is on the meridian at
night, and leave it there, and the next day, twelve hours after the star
crossed the meridian, look into your telescope and you will see Venus,
or, if not, a slight motion of the tube will bring her into view.
For many amateurs the phases of Venus will alone supply sufficient
interest for telescopic observation. The changes in her form, from that
of a round full moon when she is near superior conjunction to the
gibbous, and finally the half-moon phase as she approaches her eastern
elongation, followed by the gradually narrowing and lengthening
crescent, until she is a mere silver sickle between the sun and the
earth, form a succession of delightful pictures.
Not very much can be said for Mercury as a telescopic object. The little
planet presents phases like those of Venus, and, according to
Schiaparelli and Lowell, it resembles Venus in its rotation, keeping
always the same side to the sun. In fact, Schiaparelli's discovery of
this peculiarity in the case of Mercury preceded the similar discovery
in the case of Venus. There are markings on Mercury which have reminded
some astronomers of the moon, and there are reasons for thinking that
the planet can not be a suitable abode for living beings, at least for
beings resembling the inhabitants of the earth.
Uranus and Neptune are too far away to present any attraction for
amateur observers.