When Mars appeared in his full splendor in 1877, he was for the first
time honored with the notice of instruments capable of doing him
justice. I do not, however, mean that in former apparitions he was
not also carefully observed, but a great improvement had recently
taken place in telescopes, and it was thus under specially favorable
auspices that his return was welcomed in 1877. This year will be always
celebrated in astronomical history for a beautiful discovery made by
Professor Asaph Hall, the illustrious astronomer at Washington.
Before I can explain what this discovery was, I must have a little talk
about moons, or satellites as they are often called. You know that
we have one moon, which is constantly revolving round the earth, and
accompanies the earth in its long voyage round the sun. But the earth
is only a planet, and there are many other planets which are worlds
like ours. It is natural to compare these worlds, and as we have one
moon, why should not the other planets also have moons? If there are
children in one house in a square, why should there not be children
in the other houses? We find that some of the other planets have
satellites, but they do not seem to be distributed very regularly. In
fact, they are almost as capriciously allotted as the children would be
in eight houses that you might take at random.
[Illustration: FIG. 63.--Mars and his Two Satellites.]
In Number One there lives an old bachelor, and in Number Two a single
lady. These are Mercury and Venus, and of course there are no children
in either of these houses. Number Three is inhabited by old mother
Earth, and she has got a fine big son, called the Moon. Number Four is
a nice little house inhabited by Mars. There are to be found a pair
of little twins, and nimble creatures they are too. Number Five is a
great mansion. A very big man lives here, called Jupiter, with four
robust sons and daughters that everybody knows. I fancy they must go
to many dancing parties, for every night they may be seen whirling
round and round. For three hundred years these four moons have been
known to astronomers, but in 1892 there was an addition to the family
in the shape of a tiny moon which had never been seen up to that time.
Number Six is also a fine big house, though not quite so big as Number
Five, but larger than any of the others. It is inhabited by Saturn,
and contains the biggest family of all. Up till the other day eight
sons and daughters were known to live here, but they are not nearly so
sturdy as Jupiter’s children; in fact, the young Saturns do not make
much display, and some of them are so delicate that they are hardly
ever seen. In this household, too, a new member has recently appeared.
For fifty years the family was known to consist of these eight sons
and daughters, but in August, 1898, when they were being photographed
in a group, it was discovered that a ninth moon had been added. Number
Seven is also a fine large house; but Uranus, who lives there, is such
a recluse that unless you carefully keep your eye on his house, you
will hardly ever catch a glimpse of him. There are four children in
that house, I believe, but we hardly know them. They move in circles
of their own, and apparently have seen a good deal of trouble. Only
one more house is to be mentioned, and that is Number Eight, inhabited
by Neptune. It contains one child, but we are hardly on visiting terms
with this household, and we know next to nothing about it.
Before 1877, Mars appeared to be in the same condition as Venus or
Mercury--that is, devoid of the dignity of attendants. There was,
however, good reason for thinking that there might be some satellites
to Mars, only that we had not seen them. You see that, as Number Three
had one child, and Numbers Five, Six, Seven, and Eight had each one, or
more than one, it seemed hard that poor Number Four should have none
at all. It was, however, certain that if there were any satellites to
Mars, they must be comparatively small things; for if Mars had even
one considerable moon, it must have been discovered long ago.
On the memorable occasion in 1877, Professor Hall discovered that the
ruddy planet Mars was attended, not alone by one moon, but by two.
Their behavior was most extraordinary. It appeared to him at first
almost as if one of these little moons was playing at hide-and-seek.
Sometimes it would peep out at one side of the planet, and sometimes
at the other side. I have here a picture (Fig. 63) which shows how
these moons of Mars revolve. That is the globe of the planet himself
in the middle, and he is turning round steadily in a period which is
nearly the same as our day. But the remarkable point is that the inner
of the moons of Mars runs round the planet in 7 hours 39 minutes. It
would seem very strange in our sky if we had a little moon which rose
in the west instead of in the east, and which galloped right across
the heavens three times every day--and this is what Mars has. The
outer moon takes a more leisurely journey, for he requires 30 hours 18
minutes to complete a circuit. If for no other reason than to see these
wonderful moons, it would be very interesting to visit Mars.
The satellites of this planet are in contrast to our moon. In the
first place, our moon takes 27 days to go round the earth, and is
comparatively a long way off. The moons of Mars are much nearer to
their planet, and they go round much more quickly. There is also
another difference. The moons of Mars are much smaller bodies than
our moon. If we represent Mars by a good-sized football, his moons,
on the same scale, would be hardly so big as the smallest-sized
grains of shot. Does it not speak well for the power of telescopes
in these modern days that objects so small as the satellites of Mars
should be seen at all? You remember, of course, that neither Mars
himself nor his moons have any light of their own. They shine solely
in consequence of the sunlight which falls upon them. They are merely
lighted like the earth itself, or like the moon. The difficulty about
observing the satellites is all the greater because they are seen in
the telescope close to such a brilliant body as Mars. The glare from
the bright planet is such that when we want to see faint objects like
the satellites we have to hide Mars, so as to get a comparatively dark
space in which to search.
Now that they know exactly what to look for, a good many astronomers
have observed the satellites of Mars. A superb telescope is
nevertheless required. And, in fact, you could not find a better
test for the excellence of an instrument than to try if it will show
these delicate objects. But do not imagine that merely having a
good telescope and a clear sky is all that is requisite for making
astronomical discoveries. You might just as well say that by putting
a first-rate cricket-bat in any man’s hands will ensure his making a
grand score. Every boy knows that the bat does not make the cricketer,
and I can assure him that neither will the telescope make the
astronomer. In both cases, no doubt, there is some element of luck. But
of this you may be certain: that as it is the man that makes the score,
and not the bat, so it is the astronomer that makes the discovery, and
not his telescope.
Deimos and Phobos were the names of the two personages, according to
Homer, whose duty it was to attend on the god Mars, and to yoke his
steeds. A conclave of classical scholars and astronomers appropriately
decided that Deimos and Phobos must be the names of the two satellites
to the planet which bears the name of Mars.