, 1877-12-08
The Moons OF Mars.
srRONOMER PROCTOR SAYS THEIR Ey-
lsTESCE WAS KNOWN LONG AGO.

tinn of the discovery of Mnrtmn moons
ms well ms the discovery itself; at least
the prediction was made by men of sci
ence, though not in what is now rcgari-
ed Ms the true scientific spirit. when
Galileo announced the discovery of the,
Jovian Satellites, Keplpr, after cvngrat-
uhlins Kim, said, Il please myself with
the thought that you will discover two
moons attending on Mars, and many on
Saturn" Such, ut least, is my reculec.-
ton of Kcpler'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-
sini, in his "Elemcns d'As,r0nomien n
work which Votnire must, and Arbuth.
not may, have SEEM, urged from consil-
em'ions OF analogy that Mas should
have two moons, anG explained how these
might have escaped observation by be-
ing very close to their primary. The
view was certainly urged either by S.
Cassini or his cousin Maraldi. The elder
Cassini had disposed of an argument from
analogy advanced by Huyghens, who,
when he had discovered Titan, said there
could he no other moons. since now six
planets and six moons were known. CAS-
SIM discovered four more Saturnian
moons. Yet these arguments from ANAL
ogy long remained in favor. What Bu-
con sad OF astrological predictions was
tand is) trne OF them-"Men mark when
they hit, and never mark when they
miss" And arguments from analogy
were regarded by many rs equivalent To
scientific demonstrations. Even Newton.
after showing that Jupiter IS denser than
Saturn, and the earth than Jupiter, gues
on to say, on the strength Of analogy
only, 1--WE are not to doubt that the mat !,
ter Of Mercury is more dense than the '
EARTH which chances to be true, though
the analogy is unHound. Both Uranus
and Neptune are denser than Saturn,
whereas, according to Newtons argu--
ment, they should he rarer.
There are good reasons for believing
TINT Switt's idea about the Laputan dis
covery of Martisn satellites was elabcrat-
ed by ArbuthnOt. As Prof. De Morgan
points out in his Budget of Paradoxes," !"
We cannot credit q man who could not
understand leap year (Swiit had always
supposed, be tells Stella, that one year in
three was leap year. with the correct in
vention Of Marian 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 telc-
scopes in the world which can show
them. They will lie lost to view alter
October, even in the Washington relrac-
tor, the largest yet mounted. They may
be visible again during . few weeks in
Autumn, 1879, but after that it Is pro1a-
BILE no telescope will show them until
1909. There was bare possibility that
in lS45 they might have been detected
had diligent search been made, but up
to that date men had not opened their
telescOpic eyes sufficiently to see these
tiny orbs.
In have men toned the peculiarly
of the inner satellites in moving from
west to east across the Martian sky.
The outer also has its peculiarities. For
instance, paradoxical though 1t seems,
this moon which circles Mars In 30t
hours, aud in that time circles also the
stella heavens, takes abons twice that
time in passing from the eastern to the
western horizon. In oiheI words, the
lunar day is about four times as long as
the lunar month.
Ina paper which appeared in week
y contemporary shortly before the de-
tails OF the discovery arrived estimate
ed the diameter ot the other satellite as
most probably not exceeding teHmiles.
Prof Newcomb, Of the Washington OBJ
servatory, in u communication to tho
New York Tribune, says that the diam
ter cannot be much over ten miles, and
may be less.