The Eaton Democrat. (Eaton, Ohio) 1875-1903, 1877-10-04
Prof. Proctor on the Discovery Of the
Two Satellites of Mars.

think science must claim the pre-
diction of the discovery of Marian
moons as well as the discovery itself; at
least, the prediction was made by men
of science, though not in what is now
regarded as the true scientific spirit.
When Galileo announced the discovery
of the Jovian satellites, Kepler, after
congratulating him, said, +" please my
self with the thought that you will dis
cover two moons attending on Mars and
many attending on Saturn" Such at
least ismy recollection of Kepler's words.
In works on astronomy, published in
the seventeenth century, the idea was
generally advanced that Mars must have
two satellites, and, if remember rightly,
the youngest Cassini, in his clemens
d. Astronomie" "a work which Voltaire
must, and Arbuthnot may, have seen),
urged from considerations 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 ]. Cassini or his cousin Mar
aldi. The elder Cassini had disposed of
an argument from analogy advanced by
Huyghens, who when he had discovered
Titan, said there could be no other
moons, since now six planets and. six
moons were known. Cassini discovered
four more Saturnian moons. Yet these
arguments from analogy long remained
in favor. What Bacon said of astro-
logical predictions was land is) true of
them: "Men mark when they hit and
never mark when they miss" And
arguments from analogy were regarded
by many as equivalent to scientific DEM
onstrations. Even Newton, after show
ing that Jupiter is denser than Saturn,
and the earth than Jupiter, goes on to
say (on the strength of analogy only:
4 We are not to doubt that the matter
of Mercury is more dense than the mat
ter 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
toms argument, they should be rarer.
There are good reasons for believing
that Swift's idea about the Laputan dis
covery of Martian satellites was elabor-
ated by Arbuthnot. As Professor De
Morgan points out IN his Budget of
Paradoxes," we can not credit a 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
Martian satellites according to Kepler's
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 Wis
ible again during a few weeks in
autumn, 1879; but after that it is prob
able that no telescope will show them
until 1892. There was bare possibility
that in 1845 they might have been de-
tected had diligent search been made,
but up to that date men had not opened
their telescope eyes sufficiently to see
these tiny orbs.
I have mentioned the popularity of
the inner satellite in moving from west
to east across the Martian sky. The
outer also has its peculiarities. For in
stance, paradoxical though it may seem,
this moon, which circuits Mars in thirty
and 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.
Ina paper which appeared in weekly
contemporary shortly before the details
of the discovery arrived, 1 estimated the
diameter of the outer satellite as most
probably not exceeding ten miles. Pro-
fessor Newcomb, of the Washington
Observatory, in communication to the
New York Tribune, says that the diane
ter can not be much over ten miles, and
NAV be less