The first four are by far the
largest members of the group, but their actual sizes remained unknown
until less than twenty years ago. It was long supposed that Vesta was
the largest, because it shines more brightly than any of the others;
but finally, in 1895, Barnard, with the Lick telescope, definitely
measured their diameters, and proved to everybody's surprise that
Ceres is really the chief, and Vesta only the third in rank. His
measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta,
239 miles; and Juno, 120 miles. They differ greatly in the reflective
power of their surfaces, a fact of much significance in connection
with the question of their origin. Vesta is, surface for surface,
rather more than three times as brilliant as Ceres, whence the
original mistake about its magnitude.
Nowadays new asteroids are found frequently by photography, but
physically they are most insignificant bodies, their average diameter
probably not exceeding twenty miles, and some are believed not to
exceed ten. On a planet only ten miles in diameter, assuming the same
mean density as the earth's, which is undoubtedly too much, the force
of gravity would be so slight that an average man would not weigh more
than three ounces, and could jump off into space whenever he liked.
Pages:
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224