AFTER THE GREAT MASTERS
Following Kepler and Galileo was a half century of great astronomical
progress along many lines laid out by the work of the great masters. The
telescope seemed only a toy, but its improvement in size and quality
showed almost inconceivable possibilities of celestial discoveries.
Hevelius of Danzig took up the study of the moon, and his
"Selenographia" was finely illustrated by plates which he not only drew
but engraved himself. Lunar names of mountains, plains, and craters we
owe very largely to him. Also he published among other works two on
comets, the second of which was published in 1668 and called the
"Cometographia," the first detailed account of all the comets observed
and recorded to date.
Many were the telescopes turned on the planet Saturn, and every variety
of guess was made as to the actual shape and physical nature of the
weird appendages discovered by Galileo. The true solution was finally
reached by Huygens, whose mechanical genius had enabled him to grind and
polish larger and better lenses than his contemporaries; in 1659 he
published the "Systema Saturnium" interpreting the ring and the cause of
its various configurations, and the first discovery of a Saturnian
satellite is due to him.
Gascoigne in England about 1640 was the first to make the important
application of the micrometer to enhance the accuracy of measurement of
small angles in the telescopic field; an invention made and applied
independently many years later by Huygens in Holland and Auzout and
Picard in France, where the instrument was first regularly employed as
an accessory in the work of an observatory.
Another Englishman, Jeremiah Horrocks, was the first observer of a
transit of Venus over the disk of the sun, in 1639. Horrocks was
possessed of great ability in calculational astronomy also. This was
about the time of the invention of the pendulum clock by Huygens, which
in conjunction with the later invention of the transit instrument by
Roemer wrought a revolution in the exacting art of practical astronomy.
This was because it enabled the time to be carried along continuously,
and the revolution of the earth could be utilized in making precise
measures of the position of sun, moon, and stars. Louis XIV had just
founded the new Observatory at Paris in 1668, and Picard was the first
to establish regular time-observations there.
Huygens followed up the motion of the pendulum in theory as well as
practice in his "Horologium Oscillatorium" (1673), showing the way to
measure the force of gravity, and his study of circular motion showed
the fundamental necessity of some force directed toward the center in
planetary motions.
The doctrine of the sphericity of the earth being no longer in doubt,
the great advance in accuracy of astronomical observation indicated to
Willebrord Snell in Holland the best way to measure an arc of meridian
by triangulation. Picard repeated the measurements near Paris with even
greater accuracy, and his results were of the utmost significance to
Newton in establishing his law of gravitation.
Domenico Cassini, an industrious observer, voluminous writer, and a
strong personality, devised telescopes of great size, discovered four
Saturnian satellites and the main division in the ring of Saturn,
determined the rotation periods of Mars and Jupiter, and prepared tables
of the eclipses of Jupiter's satellites. At his suggestion Richer
undertook an expedition to Cayenne in latitude 5 degrees north, where it
was found that the intensity of gravity was less than at Paris, and his
clock therefore lost time, thus indicating that the earth was not a
perfect sphere as had been thought, but a spheroid instead.
The planet Mars passed a near opposition, and Richer's observations of
it from Cayenne, when combined with those of Cassini and others in
France, gave a new value of the sun's parallax and distance, really the
first actual measurement worth the name in the history of astronomy.
To close this era of signal advance in astronomy we may cite a discovery
by Roemer of the first order: no less than that of the velocity of
transmission of light through space. At the instigation of Picard,
Roemer in studying the motions of Jupiter's satellites found that the
intervals between eclipses grew less and less as Jupiter and the earth
approached each other, and greater and greater than the average as the
two planets separated farther and farther. Roemer correctly attributed
this difference to the progressive motion of light and a rough value of
its velocity was calculated, though not accepted by astronomers
generally for more than a century.
Why the laws of Kepler should be true, Kepler himself was unable to say.
Nor could anyone else in that day answer these questions: (1) The
planets move in orbits that are elliptical not circular--why should they
move in an imperfect curve, rather than the perfect one in which it had
always been taught that they moved? (2) Why should our planet vary its
velocity at all, and travel now fast, now slow; especially why should
the speed so vary that the line of varying length, joining the planet to
the sun, always passes over areas proportional to the time of describing
them? And (3) Why should there be any definite relation of the distances
of planets from the sun to their times of revolution about him? Why
should it be exactly as the cube of one to the square of the other?
We must remember that the Copernican system itself was not yet, in the
beginning of the seventeenth century, accepted universally; and the
great minds of that period were most concerned in overturning the
erroneous theory of Ptolemy.
The next step in logical order was to find a basic explanation of the
planetary motions, and Descartes and his theory of vortices are worthy
of mention, among many unsuccessful attempts in this direction.
Descartes was a brilliant French philosopher and mathematician, but his
hypothesis of a multitude of whirlpools in the ether, while ingenious in
theory, was too vague and indefinite to account for the planetary
motions with any approach to the precision with which the laws of Kepler
represented them.
Another great astronomer whose labors helped immensely in preparing the
way for the signal discoveries that were soon to come was Huygens, a man
of versatility as natural philosopher, mechanician, and astronomical
observer. Huygens was born thirteen years before the death of Galileo,
and to the discovery of the laws of motion by the latter Huygens added
researches on the laws of action of centrifugal forces. Neither of them,
however, appeared to see the immediate bearing on the great general
problem of celestial motions in its true light, and it was reserved for
another generation, and an astronomer of another country, to make the
one fundamental discovery that should explain the whole by a single
simple law.