s executed at Harvard in
1877 and 1879, Professor Pickering concluded their diameters to be
respectively six and seven miles.[1011] This is on the assumption that
they reflect the same proportion of the light incident upon them that
their primary does. But it may very well be that they are less
reflective, in which case they would be more extensive. The albedo of
Mars is put by Mueller at 0.27; his surface, in other words, returns 27
per cent. of the rays striking it. If we put the albedo of his
satellites equal to that of our moon, 0.17, their diameters will be
increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one,
being the larger. Mr. Lowell, however, formed a considerably larger
estimate of their dimensions.[1012] It is interesting to note that
Deimos, according to Professor Pickering's very distinct perception,
does not share the reddish tint of Mars.

Deimos completes its nearly circular revolutions in thirty hours
eighteen minutes, at a distance from the surface of its ruling body of
12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours
thirty-nine minutes twenty-two seconds, at a distance of only 3,760
miles. This is the only known instance of a satellite circulating faster
than its primary rotates, and is a circumstance of some importance as
regards theories of planetary development. To a Martian spectator the
curious effect would ensue of a celestial object, seemingly exempt from
the general motion of the sphere, rising in the west, setting in the
east, and culminating twice, or even thrice a day; which, moreover, in
latitudes above 69 deg. north or south, would be permanently and
altogether hidden by the intervening curvature of the globe.

* * * * *

The detection of new members of the solar system has come to be one of
the most ordinary of astronomical events. Since 1846 no single year has
passed without bringing its tribute of asteroidal discovery. In the last
of the seventies alone, a full score of miniature planets were
distinguished from the thronging stars amid which they seem to move;
1875 brought seventeen such recognitions; their number touched a minimum
of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped
to six in 1889, and sprang up with the aid of photography to
twenty-seven in 1892. That high level has since, on an average, been
maintained; and on January 1, 1902, nearly 500 asteroids were recognised
as revolving