twenty-six inches in diameter, and is hardly less
remarkable for the perfection of its definition than for its size. But
even the skill of Mr. Hall, and the space-penetrating power of his
telescope, would not have been able on ordinary occasions to discover
the satellites of Mars. Advantage was accordingly taken of that
memorable opposition of Mars in 1877, when, as we have already
described, the planet came unusually near the earth.

Had Mars been attended by a moon one-hundredth part of the bulk of our
moon it must long ago have been discovered. Mr. Hall, therefore, knew
that if there were any satellites they must be extremely small bodies,
and he braced himself for a severe and diligent search. The
circumstances were all favourable. Not only was Mars as near as it well
could be to the earth; not only was the great telescope at Washington
the most powerful refractor then in existence; but the situation of
Washington is such that Mars was seen from the observatory at a high
elevation. It was while the British Association were meeting at
Plymouth, in 1877, that a telegram flashed across the Atlantic.
Brilliant success had rewarded Mr. Hall's efforts. He had hoped to
discover one satellite. The discovery of even one would have made the
whole scientific world ring; but fortune smiled on Mr. Hall. He
discovered first one satellite, and then he discovered a second; and, in
connection with these satellites, he further discovered a unique fact in
the solar system.

Deimos, the outer of the satellites, revolves around the planet in the
period of 30 hours, 17 mins., 54 secs.; it is the inner satellite,
Phobos, which has commanded the more special attention of every
astronomer in the world. Mars turns round on his axis in a Martial day,
which is very nearly the same length as our day of twenty-four hours.
The inner satellite of Mars moves round in 7 hours, 39 mins., 14 secs.
Phobos, in fact, revolves three times round Mars in the same time that
Mars can turn round once. This circumstance is unparalleled in the solar
system; indeed, as far as we know, it is unparalleled in the universe.
In the case of our own planet, the earth rotates twenty-seven times for
one revolution of the moon. To some extent the same may be said of
Jupiter and of Saturn; while in the great system of the sun himself and
the planets, the sun rotates on his axis several times for each
revolution of even the most rapidly moving of the planets. There is no
o