Very few of the asteroids are of measurable
size. Among these are the four that were the first to be
discovered--Ceres, Pallas, Juno, and Vesta. Their diameters, according
to the measurements of Prof. E.E. Barnard, of the Yerkes Observatory,
are as follows: Ceres, 477 miles; Pallas, 304 miles; Juno, 120 miles;
Vesta, 239 miles.
It is only necessary to mention these diameters in order to indicate how
wide is the difference between the asteroids and such planets as the
earth, Venus, or Mars. The entire surface of the largest asteroid,
Ceres, does not equal the republic of Mexico in area. But Ceres itself
is gigantic in comparison with the vast majority of the asteroids, many
of which, it is believed, do not exceed twenty miles in diameter, while
there may be hundreds or thousands of others still smaller--ten miles,
five miles, or perhaps only a few rods, in diameter!
Curiously enough, the asteroid which appears brightest, and which it
would naturally be inferred is the largest, really stands third in the
order of measured size. This is Vesta, whose diameter, according to
Barnard, is only 239 miles. It is estimated that the surface of Vesta
possesses about four times greater light-reflecting power than the
surface of Ceres. Some observations have also shown a variation in the
intensity of the light from Vesta, a most interesting fact, which
becomes still more significant when considered in connection with the
great variability of another most extraordinary member of the asteroidal
family, Eros, which is to be described presently.
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