_THE PLANET MARS._
Every one who notices the stars at all,--and who that thinks and can see
does not?--must have observed during the autumn of 1877 two bright stars
in the southern heavens. One of these shone with a lustre which but for
its ruddy hue would have caused the star to be taken for the planet
Jupiter; the other shone with a somewhat yellowish light, and was much
fainter, though surpassing most of the _fixed_ stars in brightness. The
former was the planet Mars, the latter the ringed planet Saturn. The
motions of these two stars with respect to each other and to the
neighbouring stars were sufficiently conspicuous to attract attention.
During October these stars attracted still more attention, because they
drew nearer and nearer together, to all appearance, until on November
4th they were at their nearest, when the distance separating them was
about one-third the apparent diameter of the moon, so that in a
telescope showing at one view the whole disc of the moon, Mars and
Saturn on the night of November 4th appeared like a splendid double
star, the primary a fine red orb, the companion a smaller body, but
attended by a splendid ring system and companion moons.
It was strange when we looked at these two stars, the yellow one
apparently much smaller than the brighter, and the pair seemingly close
together, to consider how thoroughly the reality differed from these
appearances. The fainter and seemingly the smaller of the two stars was
in reality some four thousand times larger than the brighter, and had,
among eight orbs attending upon it, one nearly as large as the ruddy
planet which as actually seen so completely outshone Saturn himself.
Again, instead of being near each other, those two bodies were in
reality separated by a distance exceeding some sixteen times that which
separated us from the nearer of the two.
I propose now to consider some of the more interesting characteristics
of these two planets, presenting specially those features which mark
Saturn as the representative of one family of bodies, and Mars as the
representative of another and an entirely different family.
[Illustration: Fig. 16.--The paths of Mars and Saturn during the autumn
of 1877.]
It will be well to consider Mars first; for although, as will presently
be seen, Saturn came earlier of the two to the portion of his path
where he was most favourably seen, there was nothing specially
remarkable about the approach of Saturn on that occasion, whereas Mars
in the year 1877 made a nearer approach to the earth than he has for
thirty-two years past, or will for some forty-seven years to come.
In the first place, let us note the apparent paths on which the two
planets have been and are now travelling.
Fig. 16 presents that part of the zodiac along which lay the apparent
paths of Mars and Saturn in 1877. The stars marked with Greek letters
belong to the constellation Aquarius, or the Water-Bearer (his jar is
formed by the stars in the upper right-hand corner of the
picture),--with a single exception, the star marked κ, which, with
those close to it not lettered, belongs to the constellation Pisces, or
the Fishes. Thus the loops traversed by the two planets in 1877 both
fell in the constellation of the Water-Bearer; but, as will be seen from
the symbols on the ecliptic, these loops lie in the zodiacal sign
Pisces, which begins at κ and ends at γ. The signs have long
since passed away, in fact, from the constellations to which they
originally belonged.
It will be noticed that Mars described a wide loop ranging to a
considerable distance from the ecliptic (or sun's track). Saturn, on the
other hand, travelled on a narrow and shorter loop lying much nearer to
the ecliptic, his whole track, except just where he was turning,--his
stationary points,--lying nearly parallel to the ecliptic. It may be
well to mention the reason of this well-marked difference. Mars does not
in reality range even quite so widely from the plane of the ecliptic as
Saturn does. Nay, his path is even less inclined to the ecliptic. (This
may sound like repetition, but the inclination of a planet's path to the
ecliptic is one thing, the range of the planet north and south of the
ecliptic, in miles, is another. Mercury, for example, has of all planets
the path most inclined to the ecliptic, but Mercury never attains
anything like the same distance from the plane of the ecliptic which is
attained by the remote planet Uranus, whose path is of all others the
least inclined to the plane of the ecliptic. In fact, none of the
planets, except Venus and Mars, have so small a range from the ecliptic
in actual distance as Mercury has.) The reason why the range of Mars
from the ecliptic appeared so much greater than that of Saturn, in 1877,
is similar to the reason why Mars, though much smaller than Saturn,
largely outshone him. Mars looked larger because he was nearer, his loop
looked larger because his real path was nearer. For the same reason that
a hut close by seems to stand higher above the horizon than a palace at
a distance, or a mountain yet further away, so the displacement of Mars
from the ecliptic plane appeared greater than that of Saturn, though in
reality much less.
Let us consider how the paths of these planets are really situated. I
know of no better way of showing this than by drawing the paths of the
two families of planets separately. It is in fact utterly impossible to
give an accurate yet clear view of the solar system in a single picture;
and the student may take it for granted that every drawing or plate in
which this has ever been attempted is from one cause or another
misleading.
In figs. 17 and 18 the shape and position of the planetary paths are
correctly shown. Very little description is necessary, but it may be
mentioned that on each orbit the point nearest to the sun is indicated
by the initial letter of the planet, while the point farthest from the
sun is indicated by the same letter _accented_. The places where each
path crosses the plane of the earth's--which is supposed to be the plane
of the paper--are marked ☊ and ☋, the former sign marking
where the planet in travelling round in the direction shown by the
arrows crosses the plane of the earth's path from below upwards, while
the latter marks the place where the planet in travelling round crosses
the plane of the earth's path from above downwards.
[Illustration: Fig. 17.--The paths of Mercury, Venus, the Earth, and
Mars, around the Sun.]
Fig. 17 shows the paths of the inner family of planets of which our
earth is a member. Fig. 18 shows the outer family of planets, and
inside of it the ring of small planets called asteroids. Inside that
ring, again, we see the paths of the inner family of planets; but they
appear on a very small scale indeed. In fact, the scales appended to the
two figures show that a length which represents 50,000,000 miles in fig.
17, represents 1,000,000,000 miles in fig. 18; or, in other words, the
scale of fig. 18 is only one-twentieth of the scale of fig. 17. On the
scale of fig. 17 the sun would be fairly represented by an ordinary
pin-hole; on the scale of fig. 18 the sun would be scarcely visible. The
dots round the orbits show the planets' places at intervals of 10 days
in fig. 17, and of 1000 days in fig. 18, starting always from the left
side of orbit (on horizontal line through sun).
[Illustration: Fig. 18.--The paths of Jupiter, Saturn, Uranus, and
Neptune, around the ring of small planets.]
Now looking at fig. 18 and noting how small is the distance of the path
of Mars from the earth's path, compared with the distance of Saturn's
path, we understand why Saturn, despite his far superior size, shines
far less brightly in our skies than Mars does. In fact, in October,
1877, the Earth and Mars were on the parts of their tracks which lay
nearest together, that is, the parts occupying the lower right-hand
corner of fig. 17; and turning to fig. 18, we perceive that the distance
separating the two paths here is very small indeed compared with
Saturn's distance.
So that, when we looked at Mars and Saturn as they shone in conjoined
splendour in our skies, in 1877, we saw in the bright orb of Mars the
planet whose track lies nearest to us in that direction, whereas in
looking at Saturn the range of view passed athwart the track of Mars,
through the ring of asteroids, and past the orbit of Jupiter, before
entering the wide and barren region which separates the orbits of the
two giant members of the solar system.
We study Mars under much more favourable conditions than either Jupiter
or Saturn. And yet, at a first view, the telescopic aspect of this
interesting planet is exceedingly disappointing. Galileo, who quite
easily discovered the moons of Jupiter with his largest telescope,
could barely detect with it the fact that Mars is not quite round at all
times, but is seen sometimes in the shape of the moon two or three days
before or after full. "I dare not affirm," he wrote on December 30,
1610, to his friend Castelli, "that I can observe the phases of Mars;
yet, unless I mistake, I think I already perceive that he is not
perfectly round." But even in a large telescope one can see very little
except under very favourable conditions. It has only been by long and
careful study, and piecing together the information obtained at various
times, that astronomers have obtained a knowledge of the facts which
appear in our text-books of astronomy. The possessor of a telescope who
should expect, on turning the instrument towards Mars, to perceive what
he has read in descriptions of the planet, would be considerably
disappointed.
First noticed among the features of the planet were two white spots of
light occupying the northern and southern parts of his disc. These are
now known to be regions of snow and ice, like those which surround the
poles of our own earth. But how different the reality must be from what
we seem to see in the telescope! These two tiny white specks represent
hundreds of thousands of square miles covered over with great masses of
snow and ice, which doubtless are moved by disturbing forces similar to
those which make our arctic regions for the most part impassable even
for the most daring of our seamen.
The snow-caps of Mars change in size as the planet circuits round the
sun, completing his year of seasons (which lasts 687 of our days). They
are largest in the winter of Mars, smallest in the Martian summer; so
that, as it is winter for one hemisphere when it is summer for the
other, one of the snow-caps is larger than the other at the winter and
summer seasons. In the same way, our arctic snows extend more widely
during our winter, while the antarctic snows then retreat; whereas,
during our summer, when it is winter in the southern hemisphere, the
antarctic snows advance and our arctic snows retreat.
But we have still to learn why these white spots are _known_ to be
masses of snow. They might well from analogy be considered to be snows,
since they behave like the snows of our polar regions. Yet that would be
very different from proving them to be snow masses. I shall now show how
this has been done, and afterwards describe the lands and seas of the
planet, and give a short account of the recent interesting discovery of
two moons attending on the planet which Tennyson had called the
"moonless Mars."
* * * * *
Even before the poles of Mars had been discovered, observers had
perceived that the planet has marks upon its surface. Cassini, in 1666,
at Paris, found by observing these spots that the planet turns on its
axis once in about twenty-four hours forty minutes. In the same year Dr.
Hooke observed Mars. He was in doubt whether the planet turned once
round or twice round in about twenty-four hours; for with his imperfect
telescope two opposite faces of the planet seemed so much alike that he
was doubtful whether they really were two different faces or the same.
Fortunately he published two pictures of the planet, taken on the same
night in March, 1666, and we have been able to keep such good count of
Mars's turning on his axis, that we know exactly how many times he has
turned since that distant time. However, at present, we need not further
consider the turning motion of Mars, but rather what the telescope has
shown us about him. Only, let it be remembered that he has a day of
about twenty-four hours thirty-seven minutes, and is in this respect
much like our earth.
Maraldi, Cassini's nephew, early in the last century observed several
spots on Mars, and, in particular, one somewhat triangular dark spot,
which was one of Hooke's markings, but more clearly seen by Maraldi.
About this time it was seen that the darker markings have a somewhat
greenish colour; and towards the end of last century, or, more exactly,
about a hundred years ago, the idea was maintained by Sir W. Herschel
that the dark-greenish markings are seas, while the lighter parts of
Mars, to which the planet owes its somewhat ruddy colour, are lands. Sir
W. Herschel also was the first to show that Mars, like our earth, has
seasons. It had been supposed by Cassini, Maraldi, and others, that the
axis of Mars is upright to the level of the path in which he travels. Of
course, if this were so, the light of the sun would always fall on the
planet in the same way; for the sun is in that level. But the axis, like
that of our own earth, is bowed considerably from uprightness; so that
at one part of his year the sun's rays fall more fully on his northern
regions, and his southern regions are correspondingly turned away from
the sun; then it is summer in his northern regions, winter in his
southern. At the opposite season the reverse holds, and then winter
prevails over his northern and summer over his southern regions. Midway
between these two seasons, the sun's rays are equably distributed over
both hemispheres of Mars, and then the days and nights are equal, and it
is spring in that hemisphere which is passing from winter to summer, and
autumn in the other hemisphere which is passing from summer to winter.
All these changes are precisely like those which take place in the case
of our own earth. Only, the year of Mars, and therefore his seasons, are
longer. He takes 687 days in travelling round the sun, giving nearly
172 days, or more than five and a half of our months, for each season.
[Illustration:
Fig. 19. Fig. 20. Fig. 21.