This discovery, one of the first fruits of the invention of the
telescope, and of Galileo’s early and happy idea of directing its
newly-found powers to the examination of the heavens, forms one of
the most memorable epochs in the history of astronomy. The first
astronomical solution of the great problem of _the longitude_,
practically the most important for the interests of mankind which has
ever been brought under the dominion of strict scientific principles,
dates immediately from this discovery. The final and conclusive
establishment of the Copernican system of astronomy may also be
considered as referable to the discovery and study of this exquisite
miniature system, in which the laws of the planetary motions, as
ascertained by Kepler, and specially that which connects their periods
and distances, were specially traced, and found to be satisfactorily
maintained. And (as if to accumulate historical interest on this point)
it is to the observation of the eclipses of Jupiter’s satellites
that we owe the grand discovery of the aberration of light, and the
consequent determination of the enormous velocity of that wonderful
element--192,000 miles per second. Mr. Dawes, in 1849, first noticed
the existence of round, well-defined, bright spots on the belts of
Jupiter. They vary in situation and number, as many as ten having been
seen on one occasion. As the belts of Jupiter have been ascribed to the
existence of currents analogous to our trade-winds, causing the body of
Jupiter to be visible through his cloudy atmosphere, Sir John Herschel
conjectures that those bright spots may possibly be insulated masses
of clouds of local origin, similar to the cumuli which sometimes cap
ascending columns of vapour in our atmosphere.
It would require nearly 1300 globes of the size of our earth to make
one of the bulk of Jupiter. A railway-engine travelling at the rate of
thirty-three miles an hour would travel round the earth in a month,
but would require more than eleven months to perform a journey round
Jupiter.