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 archæologist,
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 think I should care for nothing better than my
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 _cortége_ 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 turning 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.
[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.]
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.
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. 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
show the faintest glimpse of it.
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.
[Illustration: SATURN SEEN WITH A FIVE-INCH TELESCOPE.]
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.
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 last spring 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 Phœbe, 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.
[Illustration: MARS SEEN WITH A FIVE-INCH TELESCOPE.]
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.
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.
[Illustration: 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 favorable 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 one way or another 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 becomes a mere silver sickle as she swings in
between the sun and the earth, form a succession of delightful pictures
for the eye.
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 perceptible markings on Mercury which
have reminded some astronomers of the appearance of the moon, and
there are various 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 observation.