t 200,000,000
miles broad. The mean distance from the sun of the nearest asteroid,
Eros, is 135,000,000 miles, and that of the most distant, Thule,
400,000,000 miles. Wide gaps exist in the asteroidal zone where few or
no members of the group are to be found, and Prof. Daniel Kirkwood long
ago demonstrated the influence of Jupiter in producing these gaps.
Almost no asteroids, as he showed, revolve at such a distance from the
sun that their periods of revolution are exactly commensurable with that
of Jupiter. Originally there may have been many thus situated, but the
attraction of the great planet has, in the course of time, swept those
zones clean.

Many of the asteroids have very eccentric orbits, and their orbits are
curiously intermixed, varying widely among themselves, both in
ellipticity and in inclination to the common plane of the solar system.

Considered with reference to the shape and position of its orbit, the
most unique of these little worlds is Eros, which was discovered in 1898
by De Witt, at Berlin, and which, on account of its occasional near
approach to the earth, has lately been utilized in a fresh attempt to
obtain a closer approximation to the true distance of the sun from the
earth. The mean distance of Eros from the sun is 135,000,000 miles, its
greatest distance is 166,000,000 miles, and its least distance
105,000,000 miles. It will thus be seen that, although all the other
asteroids are situated beyond Mars, Eros, at its mean distance, is
nearer to the sun than Mars is. When in aphelion, or at its greatest
distance, Eros is outside of the orbit of Mars, but when in perihelion
it is so much inside of Mars's orbit that it comes surprisingly near the
earth.

Indeed, there are times when Eros is nearer to the earth than any other
celestial body ever gets except the moon--and, it might be added,
except meteors and, by chance, a comet, or a comet's tail. Its least
possible distance from the earth is less than 14,000,000 miles, and it
was nearly as close as that, without anybody knowing or suspecting the
fact, in 1894, four years in advance of its discovery. Yet the fact,
strange as the statement may seem, had been recorded without being
recognized. After De Witt's discovery of Eros in 1898, at a time when it
was by no means as near the earth as it had been some years before,
Prof. E.C. Pickering ascertained that it had several times imprinted its
image on the photographic plates of the Harvard Obse