Phobos, the nearest moon, is only 4,000 miles from the surface of Mars,
and is obliged to move with such great velocity to prevent falling, that
it actually makes a circuit about its primary in only seven hours and
thirty-eight minutes. But Mars turns on _its_ axis in twenty-four hours
and thirty-seven minutes, so the moon goes round three times, while Mars
does once, hence it rises in the west and sets in the east, making one
day of Mars equal three of its months. This moon changes every two
hours, passing all phases in a single martial night; is anomalous in
the solar system, and tends to subvert that theory of cosmic evolution
wherein a rotating gaseous sun cast off concentric rings, afterward
becoming planets. Astronomers were not satisfied with the telescope;
true, they beheld the phenomena of the solar system; planets rotating on
axes, and satellites revolving about them. They saw sunspots, faculae,
and solar upheaval; watched eclipses, transits, and the alternations of
summer and winter on Mars, and detected the laws of gravity and motion
in the system to which the earth belongs. They then devised the
micrometer. This is a complex mechanism placed in the focus of a
telescope, and by its use any object, providing it shows a disk, no
matter what its distance, can be measured. It consists of spider webs
set within a graduated metallic circle, the webs movable by screws, and
the whole instrument capable of rotating about the collimation axis of
the telescope. The screw head is a circle ruled to degrees and minutes,
and turns in front of a fixed vernier in the field of a reading
microscope. One turn of the screw moves the web a certain number
of seconds; then as there are 360 deg. in a circle,
one-three-hundred-and-sixtieth of a turn moves the web one-three-hundred
and-sixtieth of the amount, and so on. Thus, when two stars are seen in
the field, one web is moved by the screw until the fixed line and the
movable one are parallel, each bisecting a star. By reading with the
microscope the number of degrees turned, the distance apart of the stars
becomes known; the distance being learned, position is then sought; the
observance of which led to one of the greatest discoveries ever made by
man. The permanent line of the micrometer is placed in the line joining
the north and south poles of the heavens, and brought across one of the
stars; the movable web is then rotated until it bisects the other, and
then the angle between the webs is recorded. Double stars are thus
measured, first in distance, and second, their position. After this, if
any movement of the stars takes place, the tell tale micrometer at once
detects it.
In 1780, Sir Wm. Herschel measured double stars and made catalogues with
distances and positions. Within twenty years, he startled intellectual
man with the statement that many of the fixed stars actually move--one
great sun revolving around another, and both rotating about their common
center of gravity. If we look at a double star with a small telescope,
it looks just like any other; using a little larger glass, it changes
appearance and looks elongated; with a still better telescope, they
become distinctly separated and appear as two beautiful stars whose
elements are measured and carefully recorded, in order to see if they
move. Herschel detected the motion of fifty of these systems, and
revolutionized modern astronomy. Astronomers soared away from the little
solar system, and began a minute search throughout the whole sidereal
heavens. Herschel's catalogue contained four hundred double suns, only
fifty of which were known to be in revolution. Since then, enormous
advance has been made. The micrometer has been improved into an
instrument of great delicacy, and the number of doubles has swelled to
ten thousand; six hundred and fifty of them being known to be binary,
or revolving on orbits--Prof. S. W. Burnham, the distinguished young
astronomer of the Dearborn Observatory, Chicago, having discovered eight