e
somewhat more to say about this special employment of photography when
we come to deal with those bodies later on.

The path of Eros around the sun is so very elliptical, or, to use the
exact technical term, so very "eccentric," that the planetoid does not
keep all the time entirely in the space between our orbit and that of
Mars, which latter happens to be the next body in the order of planetary
succession outwards. In portions of its journey Eros, indeed, actually
goes outside the Martian orbit. The paths of the planetoid and of Mars
are, however, _not upon the same plane_, so the bodies always pass clear
of each other, and there is thus as little chance of their dashing
together as there would be of trains which run across a bridge at an
upper level, colliding with those which pass beneath it at a lower
level.

When Eros is in opposition, it comes within about 13-1/2 million miles
of our earth, and, after the moon, is therefore by a long way our
nearest neighbour in space. It is, however, extremely small, not more,
perhaps, than twenty miles in diameter, and is subject to marked
variations in brightness, which do not appear up to the present to meet
with a satisfactory explanation. But, insignificant as is this little
body, it is of great importance to astronomy; for it happens to furnish
the best method known of calculating the sun's distance from our
earth--a method which Galle, in 1872, and Sir David Gill, in 1877,
suggested that asteroids might be employed for, and which has in
consequence supplanted the old one founded upon transits of Venus. The
sun's distance is now an ascertained fact to within 100,000 miles, or
less than half the distance of the moon.

THE PLANET MARS

We next come to the planet Mars. This body rotates in a period of
slightly more than twenty-four hours. The inclination, or slant, of its
axis is about the same as that of the earth, so that, putting aside its
greater distance from the sun, the variations of season which it
experiences ought to be very much like ours.

The first marking detected upon Mars was the notable one called the
Syrtis Major, also known, on account of its shape, as the Hour-Glass
Sea. This observation was made by the famous Huyghens in 1659; and, from
the movement of the marking in question across the disc, he inferred
that the planet rotated on its axis in a period of about twenty-four
hours.

There appears to be very little atmosphere upon Mars, the result