NOTE III
As the statement has been widely circulated that recent spectroscopic
observations negative an atmosphere on Mars, it may be well to mention in a
note that the observations in question neither affirm nor deny its presence, as
their self-disclosed measure of precision, 1/4 of an atmosphere, proves them
incapable of it. They simply concur in showing that atmosphere to be thin. As a
matter of fact, if spectroscopic observations did deny the existence of an
atmosphere on Mars, such assertion would be fatal, not to the atmosphere, but
to the observer or his instrument, as the existence of an atmosphere is
demonstrated by the fundamental laws of physics, inasmuch as no change could
take place on the planet's surface without it, and that changes do take place
is undeniable. (See page 31 et seq)
Mars has two satellites, discovered by Hall in 1877, and known as Deimos
(Dread) and Phobos (Fear), named in keeping with the God of War.
Deimos, at a distance of 14,600 miles from the planet's centre, makes his
circuit in 30 hours and 18 minutes; Phobos, at a distance of 5,800, in 7 hours
and 39 minutes. As Mars himself rotates in 24 hours and 39 minutes, Phobos goes
round the planet faster than the planet turns upon itself, and, in consequence,
would appear to any observers on the planet's surface to break the otherwise
universal conformity of stellar motions by rising in the west and setting in
the east. Deimos, too, is just as unconventional in its way, for it remains for
two days at a time above the horizon. Furthermore, with each, owing to its
nearness to the planet, its distance from any place on the surface varies at
different times, and with its distance varies its apparent size in a somewhat
startling manner.
As for themselves, they are very minute bodies, though not so difficult to see
as is commonly stated. In the clear air of Arizona, both were conspicuous
objects. They appear as stars of about the 12th and 10th magnitudes
respectively; Phobos being much larger, relatively to Deimos, than its hitherto
accepted value would indicate. Observations at Flagstaff by both Mr. Douglass
and by me agree in making its relative brilliancy such as to give it a diameter
about 3.6 times that of Deimos. It is not usually so conspicuous as Deimos, in
spite of its size, because of its proximity to the planet, and the consequent
much greater illumination of the field upon which it is seen. Considering their
most probable albedoes as somewhat less than that of our moon, we find from
their stellar magnitudes, taking the stellar magnitude found for Deimos by
Pickering in 1877 as basis, their diameters to be,--
Phobos, about 36 miles.
Phobos would thus, at its closest approach to the surface of the planet, that
is, when it was in the zenith, just show a disk like the Moon. Otherwise both
satellites would appear as stars.
Neither satellite shares the red tint of the planet.
NOTE IV
NOTE V.
As the means employed in any astronomical observation are of interest, I may
add that the telescope used in these researches was an 18-inch refractor, made
by Brashear, of Alleghany, Pa., the largest he has yet made. The powers used
varied from 320 to 1305 diameters, the usual ones being, for visual purposes,
440 and 617, and, for micrometric measurements, 862. There is, not unnaturally,
much misconception prevalent as to the magnification possible in a telescope.
The highest powers of a glass can never be used on planetary detail, as the
tremors of the air blur the image. Thus we come back again to the question of
atmosphere, which is indeed the crux observationis. With regard to work on
the planets, the important point about an observatory is not so much what is
its lens as what is its location.
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