MARS.
Our nearer Neighbours in the Heavens--Surface of Mars can be
Examined in the Telescope--Remarkable Orbit of Mars--Resemblance of
Mars to a Star--Meaning of Opposition--The Eccentricity of the
Orbit of Mars--Different Oppositions of Mars--Apparent Movements of
the Planet--Effect of the Earth's Movement--Measurement of the
Distance of Mars--Theoretical Investigation of the Sun's
Distance--Drawings of the Planet--Is there Snow on Mars?--The
Rotation of the Planet--Gravitation on Mars--Has Mars any
Satellites?--Prof. Asaph Hall's great Discovery--The Revolutions of
the Satellites--Deimos and Phobos--"Gulliver's Travels."
The special relation in which we stand to one planet of our system has
necessitated a somewhat different treatment of that globe from the
treatment appropriate to the others. We discussed Mercury and Venus as
distant objects known chiefly by telescopic research, and by
calculations of which astronomical observations were the foundation. Our
knowledge of the earth is of a different character, and attained in a
different way. Yet it was necessary for symmetry that we should discuss
the earth after the planet Venus, in order to give to the earth its true
position in the solar system. But now that the earth has been passed in
our outward progress from the sun, we come to the planet Mars; and here
again we resume, though in a somewhat modified form, the methods that
were appropriate to Venus and to Mercury.
Venus and Mars have, from one point of view, quite peculiar claims on
our attention. They are our nearest planetary neighbours, on either
side. We may naturally expect to learn more of them than of the other
planets farther off. In the case of Venus, however, this anticipation
can hardly be realised, for, as we have already pointed out, its dense
atmosphere prevents us from making a satisfactory telescopic
examination. When we turn to our other planetary neighbour, Mars, we are
enabled to learn a good deal with regard to his appearance. Indeed, with
the exception of the moon, we are better acquainted with the details of
the surface of Mars than with those of any other celestial body.
This beautiful planet offers many features for consideration besides
those presented by its physical structure. The orbit of Mars is one of
remarkable proportions, and it was by the observations of this orbit
that the celebrated laws of Kepler were discovered. During the
occasional approaches of Mars to the earth it has been possible to
measure its distance with accuracy, and thus another method of finding
the sun's distance has arisen which, to say the least, may compete in
precision with that afforded by the transit of Venus. It must also be
observed that the greatest achievement in pure telescopic research which
this century has witnessed was that of the discovery of the satellites
of Mars.
To the unaided eye this planet generally appears like a star of the
first magnitude. It is usually to be distinguished by its ruddy colour,
but the beginner in astronomy cannot rely on its colour only for the
identification of Mars. There are several stars nearly, if not quite, as
ruddy as this globe. The bright star Aldebaran, the brightest star in
the constellation of the Bull, has often been mistaken for the planet.
It often resembles Betelgeuze, a brilliant point in the constellation of
Orion. Mistakes of this kind will be impossible if the learner has first
studied the principal constellations and the more brilliant stars. He
will then find great interest in tracing out the positions of the
planets, and in watching their ceaseless movements.
[Illustration: Fig. 48.--The Orbits of the Earth and of Mars, showing
the Favourable Opposition of 1877.]
The position of each orb can always be ascertained from the almanac.
Sometimes the planet will be too near the sun to be visible. It will
rise with the sun and set with the sun, and consequently will not be
above the horizon during the night. The best time for seeing one of the
planets situated like Mars will be during what is called its opposition.
This state of things occurs when the earth intervenes directly between
the planet and the sun. In this case, the distance from Mars to the
earth is less than at any other time. There is also another advantage in
viewing Mars during opposition. The planet is then at one side of the
earth and the sun at the opposite side, so that when Mars is high in the
heavens the sun is directly beneath the earth; in other words, the
planet is then at its greatest elevation above the horizon at midnight.
Some oppositions of Mars are, however, much more favourable than others.
This is distinctly shown in Fig. 48, which represents the orbit of Mars
and the orbit of the Earth accurately drawn to scale. It will be seen
that while the orbit of the earth is very nearly circular, the orbit of
Mars has a very decided degree of eccentricity; indeed, with the
exception of the orbit of Mercury, that of Mars has the greatest
eccentricity of any orbit of the larger planets in our system.
The value of an opposition of Mars for telescopic purposes will vary
greatly according to circumstances. The favourable oppositions will be
those which occur as near as possible to the 26th of August. The other
extreme will be found in an opposition which occurs near the 22nd of
February. In the latter case the distance between the planet and the
earth is nearly twice as great as the former. The last opposition which
was suitable for the highest class of work took place in the year 1877.
Mars was then a magnificent object, and received much, and deserved,
attention. The favourable oppositions follow each other at somewhat
irregular intervals; the last occurred in the year 1892, and another
will take place in the year 1909.
The apparent movements of Mars are by no means simple. We can imagine
the embarrassment of the early astronomer who first undertook the task
of attempting to decipher these movements. The planet is seen to be a
brilliant and conspicuous object. It attracts the astronomer's
attention; he looks carefully, and he sees how it lies among the
constellations with which he is familiar. A few nights later he observes
the same body again; but is it exactly in the same place? He thinks not.
He notes more carefully than before the place of the planet. He sees how
it is situated with regard to the stars. Again, in a few days, his
observations are repeated. There is no longer a trace of doubt about the
matter--Mars has decidedly changed his position. It is veritably a
wanderer.
Night after night the primitive astronomer is at his post. He notes the
changes of Mars. He sees that it is now moving even more rapidly than it
was at first. Is it going to complete the circuit of the heavens? The
astronomer determines to watch the orb and see whether this surmise is
justified. He pursues his task night after night, and at length he
begins to think that the body is not moving quite so rapidly as at
first. A few nights more, and he is sure of the fact: the planet is
moving more slowly. Again a few nights more, and he begins to surmise
that the motion may cease; after a short time the motion does cease, and
the object seems to rest; but is it going to remain at rest for ever?
Has its long journey been finished? For many nights this seems to be the
case, but at length the astronomer suspects that the planet must be
commencing to move backwards. A few nights more, and the fact is
confirmed beyond possibility of doubt, and the extraordinary discovery
of the direct and the retrograde movement of Mars has been accomplished.
[Illustration: Fig. 49.--The Apparent Movements of Mars In 1877.]
In the greater part of its journey around the heavens Mars seems to move
steadily from the west to the east. It moves backwards, in fact, as the
moon moves and as the sun moves. It is only during a comparatively small
part of its path that those elaborate movements are accomplished which
presented such an enigma to the primitive observer. We show in the
adjoining picture (Fig. 49) the track of the actual journey which Mars
accomplished in the opposition of 1877. The figure only shows that part
of its path which presents the anomalous features; the rest of the orbit
is pursued, not indeed with uniform velocity, but with unaltered
direction.
This complexity of the apparent movements of Mars seems at first sight
fatal to the acceptance of any simple and elementary explanation of the
planetary motion. If the motion of Mars were purely elliptic, how, it
may well be said, could it perform this extraordinary evolution? The
elucidation is to be found in the fact that the earth on which we stand
is itself in motion. Even if Mars were at rest, the fact that the earth
moves would make the planet appear to move. The apparent movements of
Mars are thus combined with the real movements. This circumstance will
not embarrass the geometer. He is able to disentangle the true movement
of the planet from its association with the apparent movement, and to
account completely for the complicated evolutions exhibited by Mars.
Could we transfer our point of view from the ever-shifting earth to an
immovable standpoint, we should then see that the shape of the orbit of
Mars was an ellipse, described around the sun in conformity with the
laws which Kepler discovered by observations of this planet.
Mars takes 687 days to travel round the sun, its average distance from
that body being 141,500,000 miles. Under the most favourable
circumstances the planet, at the time of opposition, may approach the
earth to a distance not greater than about 35,500,000 miles. No doubt
this seems an enormous distance, when estimated by any standard adapted
for terrestrial measurements; it is, however, hardly greater than the
distance of Venus when nearest, and it is much less than the distance
from the earth to the sun.
We have explained how the _form_ of the solar system is known from
Kepler's laws, and how the absolute size of the system and of its
various parts can be known when the direct measurement of any one part
has been accomplished. A close approach of Mars affords a favourable
opportunity for measuring his distance, and thus, in a different way,
solving the same problem as that investigated by the transit of Venus.
We are thus led a second time to a knowledge of the distance of the sun
and the distances of the planets generally, and to many other numerical
facts about the solar system.
On the occasion of the opposition of Mars in 1877 a successful attempt
was made to apply this refined process to the solution of the problem of
celestial measurement. It cannot be said to have been the first occasion
on which this method was suggested, or even practically attempted. The
observations of 1877 were, however, conducted with such skill and with
such minute attention to the necessary precautions as to render them an
important contribution to astronomy. Dr. David Gill, now her Majesty's
Astronomer at the Cape of Good Hope, undertook a journey to the Island
of Ascension for the purpose of observing the parallax of Mars in 1877.
On this occasion Mars approached to the earth so closely as to afford an
admirable opportunity for the application of the method. Dr. Gill
succeeded in obtaining a valuable series of measurements, and from them
he concluded the distance of the sun with an accuracy somewhat superior
to that attainable by the transit of Venus.
There is yet another method by which Mars can be made to give us
information as to the distance of the sun. This method is one of some
delicacy, and is interesting from its connection with the loftiest
enquiries in mathematical astronomy. It was foreshadowed in the
Dynamical theory of Newton, and was wrought to perfection by Le Verrier.
It is based upon the great law of gravitation, and is intimately
associated with the splendid discoveries in planetary perturbation which
form so striking a chapter in modern astronomical discovery.
There is a certain relation between two quantities which at first sight
seems quite independent. These quantities are the mass of the earth and
the distance of the sun. The distance of the sun bears to a certain
distance (which can be calculated when we know the intensity of
gravitation at the earth's surface, the size of the earth and the length
of the year) the same proportion that the cube root of the sun's mass
bears to the cube root of that of the earth. There is no uncertainty
about this result, and the consequence is obvious. If we have the means
of weighing the earth in comparison with the sun, then the distance of
the sun can be immediately deduced. How are we to place our great earth
in the weighing scales? This is the problem which Le Verrier has shown
us how to solve, and he does so by invoking the aid of the planet Mars.
If Mars in his revolution around the sun were solely swayed by the
attraction of the sun, he would, in accordance with the well-known laws
of planetary motion, follow for ever the same elliptic path. At the end
of one century, or even of many centuries, the shape, the size, and the
position of that ellipse would remain unaltered. Fortunately for our
present purpose, a disturbance in the orbit of Mars is produced by the
earth. Although the mass of our globe is so much less than that of the
sun, yet the earth is still large enough to exercise an appreciable
attraction on Mars. The ellipse described by the planet is consequently
not invariable. The shape of that ellipse and its position gradually
change, so that the position of the planet depends to some extent upon
the mass of the earth. The place in which the planet is found can be
determined by observation; the place which the planet would have had if
the earth were absent can be found by calculation. The difference
between the two is due to the attraction of the earth, and, when it has
been measured, the mass of the earth can be ascertained. The amount of
displacement increases from one century to another, but as the rate of
growth is small, ancient observations are necessary to enable the
measures to be made with accuracy.
A remarkable occurrence which took place more than two centuries ago
fortunately enables the place of Mars to be determined with great
precision at that date. On the 1st of October, 1672, three independent
observers witnessed the occultation of a star in Aquarius by the ruddy
planet. The place of the star is known with accuracy, and hence we are
provided with the means of indicating the exact point in the heavens
occupied by Mars on the day in question. From this result, combined with
the modern meridian observations, we learn that the displacement of Mars
by the attraction of the earth has, in the lapse of two centuries, grown
to about five minutes of arc (294 seconds). It has been maintained that
this cannot be erroneous to the extent of more than a second, and hence
it would follow that the earth's mass is determined to about one
three-hundredth part of its amount. If no other error were present, this
would give the sun's distance to about one nine-hundredth part.
[Illustration: Fig. 50.--Relative Sizes of Mars and the Earth.]
Notwithstanding the intrinsic beauty of this method, and the very high
auspices under which it has been introduced, it is, we think, at present
hardly worthy of reliance in comparison with some of the other methods.
As the displacement of Mars, due to the perturbing influence of the
earth, goes on increasing continually, it will ultimately attain
sufficient magnitude to give a very exact value of the earth's mass, and
then this method will give us the distance of the sun with great
precision. But interesting and beautiful though this method may be, we
must as yet rather regard it as a striking confirmation of the law of
gravitation than as affording an accurate means of measuring the sun's
distance.
[Illustration: Fig. 51.--Drawing of Mars (July 30th, 1894).]
[Illustration: Fig. 52.--Drawing of Mars (August 16th, 1894).]
[Illustration: Fig. 53.--Elevations and Depressions on the "Terminator"
of Mars (August 24th, 1894).]
[Illustration: Fig. 54.--The Southern Polar Cap on Mars (July 1, 1894).]
The close approaches of Mars to the earth afford us opportunities for
making a careful telescopic scrutiny of his surface. It must not be
expected that the details on Mars could be inspected with the same
minuteness as those on the moon. Even under the most favourable
circumstances, Mars is still more than a hundred times as far as the
moon, and, therefore, the features of the planet have to be at least one
hundred times as large if they are to be seen as distinctly as the
features on the moon. Mars is much smaller than the earth. The diameter
of the planet is 4,200 miles, but little more than half that of the
earth. Fig. 50 shows the comparative sizes of the two bodies. We here
reproduce two of the remarkable drawings[16] of Mars made by Professor
William H. Pickering at the Lowell Observatory, Flagstaff A.T. Fig. 51
was taken on the 30th of July, 1894, and Fig. 52 on the 16th of August,
1894.
The southern polar cap on Mars, as seen by Professor William H.
Pickering at Lowell Observatory on the 1st of July, 1894, is represented
in Fig. 54.[17] The remarkable black mark intruding into the polar area
will be noticed. In Fig. 53 are shown a series of unusually marked
elevations and depressions upon the "terminator" of the planet, drawn as
accurately as possible to scale by the same skilful hand on the 24th of
August, 1894.
In making an examination of the planet it is to be observed that it does
not, like the moon, always present the same face towards the observer.
Mars rotates upon an axis in exactly the same manner as the earth. It is
not a little remarkable that the period required by Mars for the
completion of one rotation should be only about half an hour greater
than the period of rotation of the earth. The exact period is 24 hours,
37 minutes, 22-3/4 seconds. It therefore follows that the aspect of the
planet changes from hour to hour. The western side gradually sinks from
view, the eastern side gradually assumes prominence. In twelve hours the
aspect of the planet is completely changed. These changes, together with
the inevitable effects of foreshortening, render it often difficult to
correlate the objects on the planet with those on the maps. The latter,
it must be confessed, fall short of the maps of the moon in definiteness
and in certainty; yet there is no doubt that the main features of the
planet are to be regarded as thoroughly established, and some
astronomers have given names to all the prominent objects.
The markings on the surface of Mars are of two classes. Some of them are
of an iron-grey hue verging on green, while the others are generally
dark yellow or orange, occasionally verging on white. The former have
usually been supposed to represent the tracts of ocean, the latter the
continental masses on the ruddy planet. We possess a great number of
drawings of Mars, the earliest being taken in the middle of the
seventeenth century. Though these early sketches are very rough, and are
not of much value for the solution of questions of topography, they have
been found very useful in aiding us to fix the period of rotation of the
planet on its axis by comparison with our modern drawings.
Early observers had already noticed that each of the poles of Mars is
distinguished by a white spot. It is, however, to William Herschel that
we owe the first systematic study of these remarkable polar caps. This
illustrious astronomer was rewarded by a very interesting discovery. He
found that these arctic tracts on Mars vary both in extent and
distinctness with the seasons of the hemisphere on which they are
situated. They attain a maximum development from three to six months
after the winter solstice on that planet, and then diminish until they
are smallest about three to six months after the summer solstice. The
analogy with the behaviour of the masses of snow and ice which surround
our own poles is complete, and there has until lately been hardly any
doubt that the white polar spots of Mars are somewhat similarly
constituted.
As the period of revolution of Mars around the sun is so much longer
than our year, 687 days instead of 365, the seasons of the planet are,
of course, also much longer than the terrestrial seasons. In the
northern hemisphere of Mars the summer lasts for no fewer than 381 days,
and the winter must be 306 days. In both hemispheres the white polar cap
in the course of the long winter season increases until it reaches a
diameter of 45 deg. to 50 deg., while the long summer reduces it to a small area
only 4 deg. or 5 deg. in diameter. It is remarkable that one of these white
regions--that at the south pole--seems not to be concentric with the
pole, but is placed so much to one side that the south pole of Mars
appears to be quite free from ice or snow once a year.
Although many valuable observations of Mars were made in the course of
the nineteenth century, it is only since the very favourable opposition
of 1877 that the study of the surface of Mars has made that immense
progress which is one of the most remarkable features of modern
astronomy. Among the observers who produced valuable drawings of the
planet in 1877 we may mention Mr. Green, whose exquisite pictures were
published by the Royal Astronomical Society, and Professor Schiaparelli,
of Milan, who almost revolutionised our knowledge of this planet.
Schiaparelli had a refractor of only eight inches aperture at his
disposal, but he was doubtless much favoured by the purity of the
Italian sky, which enabled him to detect in the bright portions of the
surface of Mars a considerable number of long, narrow lines. To these he
gave the name of canals, inasmuch as they issued from the so-called
oceans, and could be traced across the reputed continents for
considerable distances, which sometimes reached thousands of miles.
The canals seemed to form a kind of network, which connected the various
seas with each other. A few of the more conspicuous of these so-called
canals appeared indeed on some of the drawings made by Dawes and others
before Schiaparelli's time. It was, however, the illustrious Italian
astronomer who detected that these narrow lines are present in such
great numbers as to form a notable feature of the planet. Some of these
remarkable features are shown in Figs. 51 and 52, which are copied from
drawings made by Professor William H. Pickering at the Lowell
Observatory in 1894.
Great as had been the surprise of astronomers when Schiaparelli first
proclaimed the discovery of these numerous canals, it was, perhaps,
surpassed by the astonishment with which his announcement was received
in 1882 that most of the canals had become double. Between December,
1881, and February, 1882, thirty of these duplications appear to have
taken place. Nineteen of these were cases of a well-traced parallel line
being formed near a previously existing canal. The remaining canals were
less certainly established, or were cases where the two lines did not
seem to be quite parallel. A copy of the map of Mars which Schiaparelli
formed from his observations of 1881-82 is given in Plate XVIII. It
brings out clearly these strange double canals, so unlike any features
that we know on any other globe.
[Illustration: PLATE XVIII.
SCHIAPARELLI'S MAP OF MARS IN 1881-82.]
Subsequent observations by Schiaparelli and several other observers
seem to indicate that this phenomenon of the duplication of the canals
is of a periodic character. It is produced about the times when Mars
passes through its equinoxes. One of the two parallel lines is often
superposed as exactly as possible upon the track of the old canal. It
does, however, sometimes happen that both the lines occupy opposite
sides of the former canal and are situated on entirely new ground. The
distance between the two lines varies from about 360 miles as a maximum
down to the smallest limit distinguishable in our large telescopes,
which is something less than thirty miles. The breadth of each of these
remarkable channels may range from the limits of visibility, say, up to
more than sixty miles.
The duplication of the canals is perhaps the most difficult problem
which Mars offers to us for solution. Even if we admit that the canals
themselves represent inlets or channels through which the melted polar
snow makes its way across the equatorial continents, it is not easy to
see how the duplicate canals can arise. This is especially true in those
cases where the original channel seems to vanish and to be replaced by
two quite new canals, each about the breadth of the English Channel, and
lying one on each side of the course of the old one. The very obvious
explanation that the whole duplication is an optical illusion has been
brought forward more than once, but never in a conclusive manner. We
must, perhaps, be content to let the solution of this matter rest for
the present, in the hope that the extraordinary attention which this
planet is now receiving will in due time explain the present enigma.
The markings on the surface of this planet are, generally speaking, of a
permanent character, so that when we compare drawings made one or two
hundred years ago with drawings made more recently we can recognise in
each the same features. This permanence is, however, not nearly so
absolute as it is in the case of the moon. In addition to the canals
which we have already considered, many other parts of the surface of
Mars alter their outlines from time to time. This is particularly the
case with those dark spots which we call oceans, the contours of which
sometimes undergo modifications in matters of detail which are quite
unmistakable. Changes of colour are often observed on parts of the
planet, and though some of these observations may perhaps be attributed
to the influence of our own atmosphere on the planet's appearance, they
cannot be all thus accounted for. Some of the phenomena must certainly
be due to actual changes which have taken place on the surface of Mars.
As an example of such changes, we may refer to the north-western part of
the notable feature, to which Schiaparelli has given the name of _Syrtis
major_.[18] This has at various times been recorded as grey, green,
blue, brown, and even violet. When this region (about the time of the
autumnal equinox of the northern hemisphere) is situated in the middle
of the visible disc, the eastern part is distinctly greener than the
western. As the season progresses this characteristic colour gets
feebler, until the green tint is to be perceived only on the shores of
the Syrtis. The atmosphere of Mars is usually very transparent, and
fortunately allows us to scrutinise the surface of the planet without
putting obstacles in the way m the shape of Martian clouds. Such clouds,
however, are not invariably absent. Our view of the surface is
occasionally obstructed in such a manner as to make it certain that
clouds or mist in the atmosphere of Mars must be the cause of the
trouble.
Would we form an idea of the physical constitution of the surface of
Mars, then the question as to the character of the atmosphere of the
planet is among the first to be considered. Spectroscopic observations
do not in this case render us much assistance. Of course, we know that
the planet has no intrinsic light. It merely shines by reflected
sunlight. The hemisphere which is turned towards the sun is bright, and
the hemisphere which is turned away from the sun is dark. The spectrum
ought, therefore, like that of the moon, to be an exact though faint
copy of the solar spectrum, unless the sun's rays, by passing twice
through the atmosphere of Mars, suffered some absorption which could
give rise to additional dark lines. Some of the earlier observers
thought that they could distinctly make out some such lines due, as was
supposed, to water vapour. The presence of such lines is, however,
denied by Mr. Campbell, of the Lick Observatory, and Professor Keeler,
at the Allegheny Observatory,[19] who, with their unrivalled
opportunities, both instrumental and climatic, could find no difference
between the spectra of Mars and the moon. If Mars had an atmosphere of
appreciable extent, its absorptive effect should be noticeable,
especially at the limb of the planet; but Mr. Campbell's observations do
not show any increased absorption at the limb. It would therefore seem
that Mars cannot have an extensive atmosphere, and this conclusion is
confirmed in several other ways.
The distinctness with which we see the surface of this planet tends to
show that the atmosphere must be very thin as compared with our own.
There can hardly be any doubt that an observer on Mars with a good
telescope would be unable to distinguish much of the features of the
earth's surface. This would be the case not only by reason of the strong
absorption of the light during the double passage through our
atmosphere, but also on account of the great diffusion of the light
caused by this same atmosphere. Also, it is needless to say, the great
amount of cloud generally floating over the earth would totally obscure
many parts of our planet from a Martian observer. But though, as already
mentioned, we occasionally find parts of Mars rendered indistinct, it
must be acknowledged that the clouds on Mars are very slight. We should
expect that the polar caps, if composed of snow, would, when melting,
produce clouds which would more or less hide the polar regions from our
inspection; yet nothing of the kind has ever been seen.
We have seen that there are very grave doubts as to the existence of
water on Mars. No doubt we have frequently spoken of the dark markings
as "oceans" and of the bright parts as "continents." That this language
was just has been the opinion of astronomers for a very long time. A few
years ago Mr. Schaeberle, of the Lick Observatory, came to the very
opposite conclusion. He contended that the dark parts were the
continents and the bright ones were the oceans of water, or some other
fluid. He pointed to the irregular shading of the dark parts, which does
not suggest the idea of light reflected from a spherical surface of
water, especially as the contrasts between light and shade are strongest
about the middle of the disc.
It is also to be noticed that the dark regions are not infrequently
traversed by still darker streaks, which can be traced for hundreds of
miles almost in straight lines, while the so-called canals in the bright
parts often seem to be continuations of these same lines. Mr. Schaeberle
therefore suggests that the canals may be chains of mountains stretching
over sea and land! The late Professor Phillips and Mr. H.D. Taylor have
pointed out that if there were lakes or seas in the tropical regions of
Mars we should frequently see the sun directly reflected from them, thus
producing a bright, star-like point which could not escape observation.
Even moderately disturbed water would make its presence known in this
manner, and yet nothing of the kind has ever been recorded.
On the question as to the possibility of life on Mars a few words may be
added. If we could be certain of the existence of water on Mars, then
one of the fundamental conditions would be fulfilled; and even though
the atmosphere on Mars had but few points of resemblance either in
composition or in density to the atmosphere of the earth, life might
still be possible. Even if we could suppose that a man would find
suitable nutriment for his body and suitable air for his respiration, it
seems very doubtful whether he would be able to live. Owing to the small
size of Mars and the smallness of its mass in comparison with the
earth, the intensity of the gravitation on the neighbouring planet would
be different from the attraction on the surface of the earth. We have
already alluded to the small gravitation on the moon, and in a lesser
degree the same remarks will apply to Mars. A body which weighs on the
earth two pounds would on the surface of Mars weigh rather less than one
pound. Nearly the same exertion which will raise a 56-lb. weight on the
earth would lift two similar weights on Mars.
The earth is attended by one moon. Jupiter is attended by four
conspicuous moons. Mars is a planet revolving between the orbits of the
earth and of Jupiter. It is a body of the same general type as the earth
and Jupiter. It is ruled by the same sun, and all three planets form
part of the same system; but as the earth has one moon and Jupiter four
moons, why should not Mars also have a moon? No doubt Mars is a small
body, less even than the earth, and much less than Jupiter. We could not
expect Mars to have large moons, but why should it be unlike its two
neighbours, and not have any moon at all? So reasoned astronomers, but
until modern times no satellite of Mars could be found. For centuries
the planet has been diligently examined with this special object, and as
failure after failure came to be recorded, the conclusion seemed almost
to be justified that the chain of analogical reasoning had broken down.
The moonless Mars was thought to be an exception to the rule that all
the great planets outside Venus were dignified by an attendant retinue
of satellites. It seemed almost hopeless to begin again a research which
had often been tried, and had invariably led to disappointment; yet,
fortunately, the present generation has witnessed still one more attack,
conducted with perfect equipment and with consummate skill This attempt
has obtained the success it so well merited, and the result has been the
memorable detection of two satellites of Mars.
This discovery was made by Professor Asaph Hall, the distinguished
astronomer at the observatory of Washington. Mr. Hall was provided with
an instrument of colossal proportions and of exquisite workmanship,
known as the great Washington refractor. It is the product of the
celebrated workshop of Messrs. Alvan Clark and Sons, from which so many
large telescopes have proceeded, and in its noble proportions far
surpassed any other telescope ever devoted to the same research. The
object-glass measures twenty-six inches in diameter, and is hardly less
remarkable for the perfection of its definition than for its size. But
even the skill of Mr. Hall, and the space-penetrating power of his
telescope, would not have been able on ordinary occasions to discover
the satellites of Mars. Advantage was accordingly taken of that
memorable opposition of Mars in 1877, when, as we have already
described, the planet came unusually near the earth.
Had Mars been attended by a moon one-hundredth part of the bulk of our
moon it must long ago have been discovered. Mr. Hall, therefore, knew
that if there were any satellites they must be extremely small bodies,
and he braced himself for a severe and diligent search. The
circumstances were all favourable. Not only was Mars as near as it well
could be to the earth; not only was the great telescope at Washington
the most powerful refractor then in existence; but the situation of
Washington is such that Mars was seen from the observatory at a high
elevation. It was while the British Association were meeting at
Plymouth, in 1877, that a telegram flashed across the Atlantic.
Brilliant success had rewarded Mr. Hall's efforts. He had hoped to
discover one satellite. The discovery of even one would have made the
whole scientific world ring; but fortune smiled on Mr. Hall. He
discovered first one satellite, and then he discovered a second; and, in
connection with these satellites, he further discovered a unique fact in
the solar system.
Deimos, the outer of the satellites, revolves around the planet in the
period of 30 hours, 17 mins., 54 secs.; it is the inner satellite,
Phobos, which has commanded the more special attention of every
astronomer in the world. Mars turns round on his axis in a Martial day,
which is very nearly the same length as our day of twenty-four hours.
The inner satellite of Mars moves round in 7 hours, 39 mins., 14 secs.
Phobos, in fact, revolves three times round Mars in the same time that
Mars can turn round once. This circumstance is unparalleled in the solar
system; indeed, as far as we know, it is unparalleled in the universe.
In the case of our own planet, the earth rotates twenty-seven times for
one revolution of the moon. To some extent the same may be said of
Jupiter and of Saturn; while in the great system of the sun himself and
the planets, the sun rotates on his axis several times for each
revolution of even the most rapidly moving of the planets. There is no
other known case where the satellite revolves around the primary more
quickly than the primary rotates on its axis. The anomalous movement of
the satellite of Mars has, however, been accounted for. In a subsequent
chapter we shall again allude to this, as it is connected with an
important department of modern astronomy.
The satellites are so small that we are unable to measure their
diameters directly, but from observations of their brightness it is
evident that their diameters cannot exceed twenty or thirty miles, and
may be even smaller. Owing to their rapid motion the two satellites must
present some remarkable peculiarities to an observer on Mars. Phobos
rises in the west, passes across the heavens, and sets in the east after
about five and a half hours, while Deimos rises in the east and remains
more than two days above the horizon.
As the satellites revolve in paths vertically above the equator of their
primary, the one less than 4,000 miles and the other only some 14,500
miles above the surface, it follows that they can never be visible from
the poles of Mars; indeed, to see Phobos, the observer's planetary
latitude must not be above 68-3/4 deg.. If it were so, the satellite would
be hidden by the body of Mars, just as we, in the British Islands, would
be unable to see an object revolving round the earth a few hundred miles
above the equator.
Before passing from the attractive subject of the satellites, we may
just mention two points of a literary character. Mr. Hall consulted his
classical friends as to the designation to be conferred on the two
satellites. Homer was referred to, and a passage in the "Iliad"
suggested the names of Deimos and Phobos. These personages were the
attendants of Mars, and the lines in which they occur have been thus
construed by my friend Professor Tyrrell:--
"Mars spake, and called Dismay and Rout
To yoke his steeds, and he did on his harness sheen."
A curious circumstance with respect to the satellites of Mars will be
familiar to those who are acquainted with "Gulliver's Travels." The
astronomers on board the flying Island of Laputa had, according to
Gulliver, keen vision and good telescopes. The traveller says that they
had found two satellites to Mars, one of which revolved around him in
ten hours, and the other in twenty-one and a half. The author has thus
not only made a correct guess about the number of the satellites, but he
actually stated the periodic time with considerable accuracy! We do not
know what can have suggested the latter guess. A few years ago any
astronomer reading the voyage to Laputa would have said this was absurd.
There might be two satellites to Mars, no doubt; but to say that one of
them revolves in ten hours would be to assert what no one could believe.
Yet the truth has been even stranger than the fiction.
And now we must bring to a close our account of this beautiful and
interesting planet. There are many additional features over which we are
tempted to linger, but so many other bodies claim our attention in the
solar system, so many other bodies which exceed Mars in size and
intrinsic importance, that we are obliged to desist. Our next step will
not, however, at once conduct us to the giant planets. We find outside
Mars a host of objects, small indeed, but of much interest; and with
these we shall find abundant occupation for the following chapter.