The Republican. [volume] (Oakland, Md.) 1877-current, 1877-10-20
The Moons Of Mars.
the Editor of the London Times:

To the Editor of the London Times: N
think science must claim the predic-
ton of the discovery of Marian moons
as well as the discovery itself at least,
the prediction was made by men of sci
ence, though not in what is now regarded
as the true scientific spirit. When Cali
leo announced the discovery of the Jo-
Wan satellites, Kepler, after congratulate
ing him, said; " l please myself with the
thought that you will discover two moons
attending on Mars and many attending
on Saturn" Such at least is my recollec-
ton of Kepier's words. In works on
astronomy, published in the seventeenth
century, the idea was generally advanced
that Mars must have two satellites and,
if l remember rightly the younger Cas-
SIM, in his clemens d'Astronomie" Ga
work which Voltaire must, and Arbuth-
not may have seem, urged from consider
atoms of analogy that Mars should have
two moons, and explained how these
might have escaped observation by being
very close to their primary. The view
was certainly urged either by J. Cassini
or his cousin Maraldi. The elder Cassini
had disposed of an argument from anal
ogy advanced by Huyghens, who, when
he had discovered Titan, said there could
be no other moons, since now six new
| planets and six moons were known. Cas-
SIM discovered four more Saturnian
moons. Yet these arguments from anal
ogy long remained in favor. What Ba-
con said of astrological predictions was
(and isl true of them "Men mark when
they hit and never mark when they miss"
And arguments from analogy were re-
garded by many as equivalent to scien-
tifc demonstrations. Even Newton, aft
er showing that Jupiter is denser than
Saturn, and the earth than Jupiter, goes
on to say ION the strength of analogy
only: We are not to doubt that the
matter of Mercury is more dense than
the matter of the EARTH" which chances
to be true, though the analogy is unsound.
Both Uranus and Neptune are denser
than Saturn, whereas, according to New
tons argument they should be rarer.
There are good reasons for believing
that Swift's idea about the Laputan dis
covery of Marian satellites was elabo-
rated by Arbuthnot. AS Prof. de Nor
gan poixts out in his Budget of Para
doxes," we cannot credit man who
could not understand leap year Swift
had always supposed, he tells Stella, that
one year in three was leap year, with the
correct invention of Marian satellites
according to Keplers third law.
Your readers may take it for granted
that the discovery of the moons of Mars
is altogether new. Even now that these
bodies are known, there are not ten tele-
scopes in the world that will show them
They will be lost to view after October,
even in the Washington refractor, the
largest yet mounted. They may be visi-
bic again during few weeks in autumn,
1879; but after that it is probable that
no telescope will show them until 1892.
There was bare possibility that in 1845
they might have been detected had dill
gent search been made; but up to that
date men had not opened their telescope
eyes sufficiently to see these tiny orbs.
l have mentioned the popularity of the
inner satellite in moving from west to
east across the Marian sky. The outer
also has its peculiarities. For instance,
paradoxical though it may seem, this
moon, which circuits Mars in thirty
and one-fourth hours, and in that time
circuits also the stellar heavens, takes
about twice that time in passing from
the eastern to the western horizon. In
other words, the lunar day is about four
times as long as the lunar month.
In paper which appeared in a week
ly contemporary shortly before the de-
tails of the discovery arrived, l estimated
the diameter of the outer satellite as
most probably not exceeding ten miles.
Prof. Newcomb, of the Washington OBJ
servatory, in communication to the
New York Tribune, says that the diane
ter cannot be much over ten miles, and
may be less. RICHARD A. PRoCToR.