Not long ago, the planet Jupiter came among the stars of our southern
evening skies. Those who noted down his track found that he first
advanced from west to east, then receded along a track near his
advancing one, then advanced again, still running on a track side by
side with his former advancing track, and so passed away from the
scene, toward the part of the sky where the sun's light prevents our
tracking him.
That was a useful and rather easy first lesson about the motions of
the bodies called planets.
We have now to consider a rather less simple case, but one a great
deal more interesting. Two planets intrude among our evening stars,
each following a looped track, but the tracks are unlike; the two
planets are unlike in appearance, and they are also very unlike in
reality.
I hope many of my young readers have already found out for themselves
that these intrusive bodies have been wandering among our fixed stars.
I purposely said nothing about the visitors last August, so that those
who try to learn the star-groups from my maps may have had a chance of
discovering the two planets for themselves. If they have done so, they
have in fact repeated a discovery which was made many, many years ago.
Ages before astronomy began to be a science, men found out that some
of the stars move about among the rest, and they also noticed the kind
of path traveled in the sky by each of those moving bodies. It was
long, indeed, before they found out the kind of path traveled _really_
by the planets. In fact, they supposed our earth to be fixed; and if
our earth were fixed, the paths of the planets about her as a center
would be twisted and tangled in the most perplexing way. So that folks
in those old times, seeing the planets making all manner of loops and
twistings round the sky, and supposing they made corresponding loops
and twistings in traveling round the earth, thought the planets were
living creatures, going round the earth to watch it and rule over it,
each according to his own fashion. So they worshiped the planets as
gods, counting seven of them, including the sun and moon. Some they
thought good to men, others evil. The two planets now twisting their
way along the southern skies were two of the evil sort, viz.: Mars,
called the Lesser Infortune, and Saturn, called the Greater Infortune.
In the old system of star-worship, Mars ruled over Tuesday, and Saturn
over Saturday,--the Sabbath of olden times,--a day which the Chaldean
and Egyptian astrologers regarded as the most unlucky in the whole
week.
[Illustration: FIG. 1. THE PATHS OF MARS AND SATURN.]
The actual paths traveled among the stars by these two planets, this
fall, are shown in Fig. 1. You will see how wildly the fiery Mars, the
planet of war, careers round his great loop, while old Saturn, "heavy,
dull, and slow" (as Armado says that lead is--the metal dedicated to
Saturn), plods slowly and wearily along. Between August 6 and October
1, Mars traversed his entire backward track,--Saturn, you notice, only
a small portion of his much smaller loop. On the sky, too, you will
see that while Mars shines with a fierce ruddy glow, well suited
to his warlike character, Saturn shines with a dull yellow light,
suggestive of the evil qualities which the astrologers of old assigned
to him. "My loking," says Saturn, in Chaucer's "Canterbury Tales," "is
the fader of pestilence:
"Min ben also the maladies colde,
The derke treasons, and the costes olde;
Min is the drenching in the see so wan,
Min is the prison in the derke cote,[1]
Min is the strangel and hanging by the throte,
The murmure, and the cherles[2] rebelling,
The groyning and the prine empoysoning."
[Footnote 1: _Dark or gloomy coast_. This line was amusingly
rendered, by the printer of my "Saturn and its System," in which I
quoted Chaucer's lines, "Mine is the prison, and the dirty coat."]
[Footnote 2: _Churl's._ Notice this word. It is the same as the
word rendered _Charles's_ in the common English name for the
Dipper. One should always say Charles's Wain, not Charles' (as is
the way Tennyson does in the "May Queen ").]
For the present, however, let us consider the planet Mars, leaving
slow Saturn to wait for us another month.
It has always seemed to me one of the most useful lessons in astronomy
to follow the line by which, long ago, great discoveries were made.
Thus, if the young reader went out on every fine night and noted the
changing position of Mars, he traced out the track shown in Fig. 1.
He noted, also, that the planet, which shone at its brightest about
September 5, gradually grew less and less bright as it traveled off,
after rounding the station near October 5 (really on Oct. 7), toward
the east. He observed, then, that the seeming loop followed by the
planet was a real looped track (so far, at least, as our observer on
the earth was concerned). Fig. 2 shows the apparent shape of Mars's
loop, the dates corresponding to those shown in Fig. 1. Only it does
not lie flat, as shown on the paper, but must be supposed to lie
somewhat under the surface of the paper, as shown by the little
upright _a, b,_ which, indeed, gives the distance under the paper at
which the part of the loop is supposed to lie where lowest at _m_. The
other similar uprights at M_1, M_2, and M_3 show the depression at
these places. You perceive that the part M_1, M_2, lies higher than
the part M_2, M_3. If the loop were flat, and, like E, the earth,
were in the level of the paper, it would be seen edgewise, and the
advancing, receding, and advancing parts of the planet's course would
all lie on the same line upon the sky. But being thus out of the
level, we see through the loop, so to speak, and it has the seeming
shape shown in Fig. 1.[3]
[Footnote 3: I must re-mention that though this explanation is
made as simple as I possibly can make it, so far as words are
concerned, the figures present the result of an exact geometrical
investigation. Every dot, for instance, in Fig. 2, has had its
place separately determined by me.]
[Illustration: FIG. 2. ONE OF MARS'S LOOPS.]
This is one loop, you will understand, out of an immense number which
Mars makes in journeying round the earth, regarded as fixed. He
retreats to a great distance, swoops inward again toward the earth,
making a loop as in Fig. 2, and retreating again. Then he comes
again, makes another swoop, and a loop on another side, and so on.
He behaves, in fact, like that "little quiver fellow," a right
martialist, no doubt, who, as Justice Shallow tells us, "would about
and about, and come you in, and come you in,--and away again would a
go, and again would a come." The loops are not all of the same size.
The one shown in Fig. 2 is one of the smallest. I have before me a
picture which I have made of all this planet's loops from 1875 to
1892, and it forms the most curiously intertwined set of curves you
can imagine,--rather pretty, though not regular, the loops on one side
being much larger than those on the other. I would show the picture
here, but it is too large. One of these days, it will be given in a
book I am going to write about Mars, who is quite important enough to
have a book all to himself. I want you, now, to understand me that
Mars really does travel in a most complicated path, when you consider
the earth as at rest. If a perfect picture of all his loopings and
twistings since astronomy began could be drawn,--even on a sheet of
paper as large as the floor of a room,--the curves would so interlace
that you would not be able to track them out, but be always leaving
the true track and getting upon one crossing it slightly aslant,--just
like the lines by which trains are made to run easily off one
track on to another.
The unfortunate astronomers of old times, who had to explain, _if they
could_, this complicated behavior of Mars (and of other planets, too),
were quite beaten. The more carefully they made their observations,
the more peculiar the motions seemed. One astronomer gave up the work
in despair, just like that unfortunate Greek philosopher who, because
he could not understand the tides of the Euboean Sea, drowned
himself in it. So this astronomer, who was a king,--Alphonsus of
Portugal,--unable to unravel the loops of the planets, said, in his
wrath, that if he had been called on by the Creator to assign the
planets their paths, he would have managed the matter a great deal
better. The plates of the old astronomical books became more and more
confusing, and cost more and more labor, as astronomers continued to
... "Build, unbuild, contrive
To save appearances, to gird the sphere
With centric and eccentric scribbled o'er,
Cycle and epicycle, orb in orb."
It was to the study of Mars, the wildest wanderer of all, that we owe
the removal of all these perplexities. The idea had occurred to the
great astronomer, Copernicus, that the complexities of the planets'
paths are not real, but are caused by the constant moving about of the
place from whence we watch the planets. If a fly at rest at the middle
of a clock face watched the ends of the two hands, they would seem to
go round him in circles; but if, instead, he was on the end of one of
the hands (and was not knocked off as the other passed), the end of
this other hand would not move round the fly in the same simple way.
When the two hands were together it would be near, when they were
opposite it would be far away, and, without entering into any
particular description of the way in which it would seem to move, you
can easily see that the motion would seem much more complicated
than if the fly watched it from the middle of the clock face. Now,
Copernicus _did_ enter into particulars, and showed by mathematical
reasoning that nearly all the peculiarities of the planets' motions
could be explained by supposing that the sun, not the earth, was the
body round which the planets move, and that they go round him nearly
in circles.
[Illustration: FIG. 3. THE PATHS OF MARS, THE EARTH, VENUS, AND
MERCURY.]
But Copernicus could not explain _all_ the motions. And Tycho Brahe,
another great astronomer, who did not believe at all in the new ideas
of Copernicus, made a number of observations on our near neighbor
Mars, to show that Copernicus was wrong. He gave these to Kepler,
another great astronomer, enjoining him to explain them in such a way
as to overthrow the Copernican ideas. But Kepler behaved like Balaam
the son of Beor; for, called on to curse (or at least to denounce) the
views of Copernicus, he altogether blessed them three times. First,
he found from the motions of Mars that the planets do not travel in
circles, but in ovals, very nearly circular in shape, but not having
the sun exactly at the center. Secondly, he discovered the law
according to which they move, now faster now slower, in their oval
paths; and thirdly, he found a law according to which the nearer
planets travel more quickly and the farther planets more slowly,
every distance having its own proper rate. These three laws of Kepler
constitute the Magna Charta of the solar system.
Afterward, Newton showed _how_ it happens that the planets obey these
laws, but as his part of the work had no particular reference to Mars,
I say no more about it in this place.
Here, in Fig. 3, are the real paths of Mars and the Earth, and also of
Venus and Mercury. No loops, you see, in any of them, simply because
we have set the sun in the middle. Set the earth in the middle, and
each planet would have its own set of loops, each set enormously
complicated, and all three sets mixed together in the most confusing
way. It is well to remember this when you see, as in many books of
astronomy, the old theory illustrated with a set of circles looking
almost as neat and compact as the set truly representing the modern
theory. For the idea is suggested by this simple picture of the old
theory that the theory itself was simple, whereas it had become so
confusing that not merely young learners, but the most profound
mathematicians, were baffled when they tried to unravel the motions of
the planets.
I think the figure pretty well explains itself. All I need mention is,
that while the shape and position of each path is correctly shown, the
size of the sun at center is immensely exaggerated. A mere pin point,
but shining with star-like splendor, would properly represent him.
As for the figures of the earth and Mars, they are still more
tremendously out of proportion. The cross-breadth of the lines
representing these planets' tracks is _many times_ greater than the
breadth of either planet on the scale of the chart.
On September 5 the earth and Mars came to the position shown at E and
M. You observe that they could not be much nearer. It is indeed very
seldom that Mars is so well placed for observation. His illuminated
face was turned toward the dark or night half of the earth, so that he
shone brightly in the sky at midnight, and can be well studied with
the telescope.
When Galileo turned toward Mars the telescope with which he had
discovered the moons of Jupiter, the crescent form of Venus, and many
other wonders in the heavens, he was altogether disappointed. His
telescope was indeed too small to show any features of interest in
Mars, though the planet of war is much nearer to us than Jupiter. Mars
is but a small world. The diameter of the planet is about 4,400 miles,
that of our earth being nearly 8,000. Jupiter, though much farther
away, has an immense diameter of more than 80,000 miles to make
up, and much more than make up, for the effect of distance. With his
noble system of moons he appears a remarkable object even with a
small telescope, while Mars shows no feature of interest even with
telescopes of considerable size.
It was not, then, till very powerful telescopes had been constructed
that astronomers learned what we now know about Mars.[4]
[Footnote 4: See the "Moons of Mars" in "Letter Box" Department]
It is found that his surface is divided into land and water, like the
surface of our own earth. But his seas and oceans are not nearly so
large compared with his continents and lands. You know that on our own
earth the water covers so much larger a surface than the land that
the great continents are in reality islands. Europe, Asia and Africa
together form one great island; North and South America another, not
quite so large; then come Australia, Greenland, Madagascar, and so
forth; all the lands being islands, larger or smaller. On the other
hand, except the Caspian Sea and the Sea of Aral, there are no large
seas entirely land-bound. In the case of Mars a very different state
of things prevails, as you will see from the three accompanying
pictures (hitherto unpublished), drawn by the famous English observer,
Dawes (called the Eagle-eyed). The third and best was drawn with
a telescope constructed by your famous optician, Alvan Clark, of
Cambridge, Massachusetts. The dark parts are the seas, the light parts
being land, or in some cases cloud or snow. But in these pictures most
of the lighter portions represent land; for they have been seen often
so shaped, whereas clouds, of course, would change in shape.
The planet Mars, like our earth, turns on its axis, so that it has day
and night as we have. The length of its day is not very different from
that of our own day. Our earth turns once on its axis in ---- but
before reading on, try to complete this sentence for yourself. Every
one knows that the earth's turning on its axis produces day and night,
and nine persons out of ten, if asked how long the earth takes in
turning round her axis, will answer, 24 hours; and if asked how many
times she turns on her axis in a year, will say 365 times, or if
disposed to be very exact, "about 365-1/4 times." But neither answer
is correct. The earth turns on her axis about 366-1/4 times in each
year, and each turning occupies 23 hours 56 minutes and 4 seconds and
1 tenth of a second. We, taking the ordinary day as the time of a
turning or rotation, lose count of one rotation each year. It is
necessary to mention this, in order that when I tell you how long the
day of Mars is, you may be able correctly to compare it with our own
day. Mars, then, turns on his axis in 24 hours 37 minutes 22 seconds
and 7 tenth-parts of a second. So that Mars requires 41 minutes 18
seconds and 6-tenths of a second longer to turn his small body once
round than our earth requires to turn round her much larger body. The
common day of Mars is, however, only about 39 minutes longer than our
common day.
Mars has a long year, taking no less than 687 of our days to complete
his circuit round the sun, so that his year lasts only about one month
and a half less than two of ours.
[Illustration: APPEARANCE OF MARS, 1852, MARCH 23, 5 H. 45 M.,
Greenwich Mean Time. Power of Telescope, 358; 6-1/3 inch object-glass]
[Illustration: APPEARANCE OF MARS, 1852, FEBRUARY 3, 6 H. 50 M.,
Greenwich Mean Time. Power of Telescope, 242 and 358 on 6-1/3 inch
object-glass.]
[Illustration: APPEARANCE OF MARS, 1860, JULY 6, 11 H. 33 M.,
Greenwich Mean Time. Power of Telescope, 201; 8-1/4 inch object-glass.
Planet very low, yet pretty distinct.]
Like the earth, Mars has seasons, for his polar axis, like that of
the earth, is aslant, and at one part of his year brings his northern
regions more fully into sunlight, at which time summer prevails there
and winter in his southern regions; while at the opposite part of his
year his southern regions are turned more fully sunward and have their
summer, while winter prevails over his northern regions.
Around his poles, as around the earth's, there are great masses of
ice, insomuch that it is very doubtful whether any inhabitants of Mars
have been able to penetrate to his poles, any more than Kane or Hayes
or Nares or Parry, despite their courage and endurance, have been
able to reach our northern pole, or Cook or Wilkes or James Ross our
antarctic pole.
In the summer of either hemisphere of Mars, the north polar snows
become greatly reduced in extent, as is natural, while in winter
they reach to low latitudes, showing that in parts of the planet
corresponding to the United States, or mid-Europe, as to latitude,
bitter cold must prevail for several weeks in succession.
The land regions of Mars can be distinguished from the seas by their
ruddy color, the seas being greenish. But here, perhaps, you will be
disposed to ask how astronomers can be sure that the greenish regions
are seas, the ruddy regions land, the white spots either snow or
cloud. Might not materials altogether unlike any we are acquainted
with exist upon that remote planet?
The spectroscope answers this question in the clearest way. You
may remember what I told you in October, 1876, about Venus, how
astronomers have learned that the vapor of water exists in
her atmosphere. The same method has been applied, even more
satisfactorily, to the planet of war, and it has been found that he
also has his atmosphere at times laden with moisture. This being so,
it is clear we have not to do with a planet made of materials utterly
unlike those forming our earth. To suppose so, when we find that the
air of Mars, formed like our own (for if it contained other gases the
spectroscope would tell us), contains often large quantities of the
vapor of water, would be as absurd as to believe in the green cheese
theory of the moon, or in another equally preposterous, advanced
lately by an English artist--Mr. J.T. Brett--to the effect that the
atmosphere of Venus is formed of glass.
There is another theory about Mars, certainly not so absurd as
either of those just named, but scarcely supported by evidence at
present--the idea, namely, advanced by a French astronomer, that the
ruddy color of the lands and seas of Mars is due to red trees and a
generally scarlet vegetation. Your poet Holmes refers to this in those
lines of his, "Star-clouds and Wind-clouds" (to my mind among the most
charming of his many charming poems):
"The snows that glittered on the disc of Mars
Have melted, and the planet's fiery orb
Rolls in the crimson summer of its year."
It is quite possible, of course, that such colors as are often seen
in American woods in the autumn-time may prevail in the forests and
vegetation of Mars during the fullness of the Martian summer. The fact
that during this season the planet looks ruddier than usual, in some
degree corresponds with this theory. But it is much better explained,
to my mind, by the greater clearness of the Martian air in the
summer-time. That would enable us to see the color of the soil better.
If our earth were looked at from Venus during the winter-time, the
snows covering large parts of her surface, and the clouds and mists
common in the winter months, would hide the tints of the surface,
whereas these would be very distinct in clear summer weather.
I fear my own conclusion about Mars is that his present condition
is very desolate. I look on the ruddiness of tint to which I have
referred as one of the signs that the planet of war has long since
passed its prime. There are lands and seas in Mars, the vapor of water
is present in his air, clouds form, rains and snows fall upon his
surface, and doubtless brooks and rivers irrigate his soil, and carry
down the moisture collected on his wide continents to the seas whence
the clouds had originally been formed. But I do not think there is
much vegetation on Mars, or that many living creatures of the higher
types of Martian life as it once existed still remain. All that is
known about the planet tends to show that the time when it attained
that stage of planetary existence through which our earth is now
passing must be set millions of years, perhaps hundreds of millions of
years, ago. He has not yet, indeed, reached that airless and waterless
condition, that extremity of internal cold, or in fact that utter
unfitness to support any kind of life, which would seem to prevail
in the moon. The planet of war in some respects resembles a desolate
battle-field, and I fancy that there is not a single region of the
earth now inhabited by man which is not infinitely more comfortable as
an abode of life than the most favored regions of Mars at the present
time would be for creatures like ourselves.
But there are other subjects besides astronomy that the readers of the
ST. NICHOLAS want to learn about. I do not wish you to have to say to
me what a little daughter of mine said the other day. She had asked me
several questions about the sun, and after I had answered them I went
on to tell her several things which she had not asked. She listened
patiently for quite a long time,--fully five minutes, I really
believe,--and then she said: "Don't you think, papa, that that's
enough about the sun? Come and play with us on the lawn." So, as it
was holiday time, we went and played in the sun, instead of talking
about him.
* * * * *