CLOSING YEARS.
Soon after its publication Kepler's "Epitome" was placed along with the
book of Copernicus, on the list of books prohibited by the Congregation
of the Index at Rome, and he feared that this might prevent the
publication or sale of his books in Austria also, but was told that
though Galileo's violence was getting him into trouble, there would be
no difficulty in obtaining permission for learned men to read any
prohibited books, and that he (Kepler) need fear nothing so long as he
remained quiet.
In his various works on Comets, he adhered to the opinion that they
travelled in straight lines with varying velocity. He suggested that
comets come from the remotest parts of ether, as whales and monsters
from the depth of the sea, and that perhaps they are something of the
nature of silkworms, and are wasted and consumed in spinning their own
tails. Napier's invention of logarithms at once attracted Kepler's
attention. He must have regretted that the discovery was not made early
enough to save him a vast amount of labour in computations, but he
managed to find time to compute some logarithm tables for himself,
though he does not seem to have understood quite what Napier had done,
and though with his usual honesty he gave full credit to the Scottish
baron for his invention.
Though Eugenists may find a difficulty in reconciling Napier's
brilliancy with the extreme youth of his parents, they may at any rate
attribute Kepler's occasional fits of bad temper to heredity. His
cantankerous mother, Catherine Kepler, had for some years been carrying
on an action for slander against a woman who had accused her of
administering a poisonous potion. Dame Kepler employed a young advocate
who for reasons of his own "nursed" the case so long that after five
years had elapsed without any conclusion being reached another judge was
appointed, who had himself suffered from the caustic tongue of the
prosecutrix, and so was already prejudiced against her. The defendant,
knowing this, turned the tables on her opponent by bringing an
accusation of witchcraft against her, and Catherine Kepler was
imprisoned and condemned to the torture in July, 1620. Kepler, hearing
of the sentence, hurried back from Linz, and succeeded in stopping the
completion of the sentence, securing his mother's release the following
year, as it was made clear that the only support for the case against
her was her own intemperate language. Kepler returned to Linz, and his
mother at once brought another action for costs and damages against her
late opponent, but died before the case could be tried.
A few months before this Sir Henry Wotton, English Ambassador to Venice,
visited Kepler, and finding him as usual, almost penniless, urged him to
go to England, promising him a warm welcome there. Kepler, however,
would not at that time leave Germany, giving several reasons, one of
which was that he dreaded the confinement of an island. Later on he
expressed his willingness to go as soon as his Rudolphine Tables were
published, and lecture on them, even in England, if he could not do it
in Germany, and if a good enough salary were forthcoming.
In 1624 he went to Vienna, and managed to extract from the Treasury 6000
florins on account of expenses connected with the Tables, but, instead
of a further grant, was given letters to the States of Swabia, which
owed money to the Imperial treasury. Some of this he succeeded in
collecting, but the Tables were still further delayed by the religious
disturbances then becoming violent. The Jesuits contrived to have
Kepler's library sealed up, and, but for the Imperial protection, would
have imprisoned him also; moreover the peasants revolted and blockaded
Linz. In 1627, however, the long promised Tables, the first to discard
the conventional circular motion, were at last published at Ulm in four
parts. Two of these parts consisted of subsidiary Tables, of logarithms
and other computing devices, another contained Tables of the elements of
the sun, moon, and planets, and the fourth gave the places of a thousand
stars as determined by Tycho, with Tycho's refraction Tables, which had
the peculiarity of using different values for the refraction of the sun,
moon, and stars. From a map prefixed to some copies of the Tables, we
may infer that Kepler was one of the first, if not actually the first,
to suggest the method of determining differences of longitude by
occultations of stars at the moon's limb. In an Appendix, he showed how
his Tables could be used by astrologers for their predictions, saying
"Astronomy is the daughter of Astrology, and this modern Astrology again
is the daughter of Astronomy, bearing something of the lineaments of her
grandmother; and, as I have already said, this foolish daughter,
Astrology, supports her wise but needy mother, Astronomy, from the
profits of a profession not generally considered creditable". There is
no doubt that Kepler strongly resented having to depend so much for his
income on such methods which he certainly did not consider creditable.
It was probably Galileo whose praise of the new Tables induced the Grand
Duke of Tuscany to send Kepler a gold chain soon after their
publication, and we may perhaps regard it as a mark of favour from the
Emperor Ferdinand that he permitted Kepler to attach himself to the
great Wallenstein, now Duke of Friedland, and a firm believer in
Astrology. The Duke was a better paymaster than either of the three
successive Emperors. He furnished Kepler with an assistant and a
printing press; and obtained for him the Professorship of Astronomy at
the University of Rostock in Mecklenburg. Apparently, however, the
Emperor could not induce Wallenstein to take over the responsibility of
the 8000 crowns, still owing from the Imperial treasury on account of
the Rudolphine Tables. Kepler made a last attempt to secure payment at
Ratisbon, but his journey thither brought disappointment and fatigue and
left him in such a condition that he rapidly succumbed to an attack of
fever, dying in November, 1630, in his fifty-ninth year. His body was
buried at Ratisbon, but the tombstone was destroyed during the war then
raging. His daughter, Susanna, the wife of Jacob Bartsch, a physician
who had helped Kepler with his Ephemeris, lost her husband soon after
her father's death, and succeeded in obtaining part of Kepler's arrears
of salary by threatening to keep Tycho's manuscripts, but her
stepmother was left almost penniless with five young children. For their
benefit Louis Kepler printed a "Dream of Lunar Astronomy," which first
his father and then his brother-in-law had been preparing for
publication at the time of their respective deaths. It is a curious
mixture of saga and fairy tale with a little science in the way of
astronomy studied from the moon, and cast in the form of a dream to
overcome the practical difficulties of the hypothesis of visiting the
moon. Other writings in large numbers were left unpublished. No attempt
at a complete edition of Kepler's works was made for a long time. One
was projected in 1714 by his biographer, Hantsch, but all that appeared
was one volume of letters. After various learned bodies had declined to
move in the matter the manuscripts were purchased for the Imperial
Russian library. An edition was at length brought out at Frankfort by C.
Frisch, in eight volumes, appearing at intervals from 1858-1870.
Kepler's fame does not rest upon his voluminous works. With his peculiar
method of approaching problems there was bound to be an inordinate
amount of chaff mixed with the grain, and he used no winnowing machine.
His simplicity and transparent honesty induced him to include
everything, in fact he seemed to glory in the number of false trails he
laboriously followed. He was one who might be expected to find the
proverbial "needle in a haystack," but unfortunately the needle was not
always there. Delambre says, "Ardent, restless, burning to distinguish
himself by his discoveries he attempted everything, and having once
obtained a glimpse of one, no labour was too hard for him in following
or verifying it. All his attempts had not the same success, and in fact
that was impossible. Those which have failed seem to us only fanciful;
those which have been more fortunate appear sublime. When in search of
that which really existed, he has sometimes found it; when he devoted
himself to the pursuit of a chimera, he could not but fail, but even
then he unfolded the same qualities, and that obstinate perseverance
that must triumph over all difficulties but those which are
insurmountable." Berry, in his "Short History of Astronomy," says "as
one reads chapter after chapter without a lucid, still less a correct
idea, it is impossible to refrain from regrets that the intelligence of
Kepler should have been so wasted, and it is difficult not to suspect at
times that some of the valuable results which lie embedded in this great
mass of tedious speculation were arrived at by a mere accident. On the
other hand it must not be forgotten that such accidents have a habit of
happening only to great men, and that if Kepler loved to give reins to
his imagination he was equally impressed with the necessity of
scrupulously comparing speculative results with observed facts, and of
surrendering without demur the most beloved of his fancies if it was
unable to stand this test. If Kepler had burnt three-quarters of what he
printed, we should in all probability have formed a higher opinion of
his intellectual grasp and sobriety of judgment, but we should have lost
to a great extent the impression of extraordinary enthusiasm and
industry, and of almost unequalled intellectual honesty which we now get
from a study of his works."
Professor Forbes is more enthusiastic. In his "History of Astronomy," he
refers to Kepler as "the man whose place, as is generally agreed, would
have been the most difficult to fill among all those who have
contributed to the advance of astronomical knowledge," and again _à
propos_ of Kepler's great book, "it must be obvious that he had at that
time some inkling of the meaning of his laws--universal gravitation.
From that moment the idea of universal gravitation was in the air, and
hints and guesses were thrown out by many; and in time the law of
gravitation would doubtless have been discovered, though probably not by
the work of one man, even if Newton had not lived. But, if Kepler had
not lived, who else could have discovered his Laws?"
APPENDIX I.
LIST OF DATES.
Johann Kepler, born 1571; school at Maulbronn, 1586; University of
Tübingen, 1589; M.A. of Tübingen, 1591; Professor at Gratz, 1594;
"Prodromus," with "Mysterium Cosmographicum," published 1596; first
marriage, 1597; joins Tycho Brahe at Prague, 1600; death of Tycho, 1601;
Kepler's optics, 1603; Nova, 1604; on Comets, 1607; Commentary on Mars,
including First and Second Laws, 1609; Professor at Linz, 1612; second
marriage, 1613; Third Law discovered, 1618; Epitome of Copernican
Astronomy, 1618-1621; Rudolphine Tables published, 1627; died, 1630.
APPENDIX II.
BIBLIOGRAPHY.
For a full account of the various systems of Kepler and his predecessors
the reader cannot do better than consult the "History of the Planetary
Systems, from Thales to Kepler," by Dr. J.L.E. Dreyer (Cambridge Univ.
Press, 1906). The same author's "Tycho Brahe" gives a wealth of detail
about that "Phoenix of Astronomers," as Kepler styles him. A great
proportion of the literature relating to Kepler is German, but he has
his place in the histories of astronomy, from Delambre and the more
modern R. Wolfs "Geschichte" to those of A. Berry, "History of
Astronomy" (University Extension Manuals, Murray, 1898), and Professor
G. Forbes, "History of Astronomy" (History of Science Series, Watts,
1909).
GLOSSARY.
Apogee: The point in the orbit of a celestial body when it is furthest
from the earth.
Apse: An extremity of the major axis of the orbit of a body; a body is
at its greatest and least distances from the body about which it
revolves, when at one or other apse.
Conjunction: When a plane containing the earth's axis and passing
through the centre of the sun also passes through that of the moon
or a planet, at the same side of the earth, the moon or planet is in
conjunction, or if on opposite sides of the earth, the moon or
planet is in opposition. Mercury and Venus cannot be in opposition,
but are in inferior or superior conjunction according as they are
nearer or further than the sun.
Deferent: In the epicyclic theory, uneven motion is represented by
motion round a circle whose centre travels round another circle, the
latter is called the deferent.
Ecliptic: The plane of the earth's orbital motion about the sun, which
cuts the heavens in a great circle. It is so called because
obviously eclipses can only occur when the moon is also
approximately in this plane, besides being in conjunction or
opposition with the sun.
Epicycle: A point moving on the circumference of a circle whose centre
describes another circle, traces an epicycle with reference to the
centre of the second circle.
Equant: In Ptolemy's excentric theory, when a planet is describing a
circle about a centre which is not the earth, in order to satisfy
the convention that the motion must be uniform, a point was found
about which the motion was apparently uniform,[4] and this point was
called the equant.
[Footnote 4: I.e. the _angular_ motion about the equant was uniform.]
Equinox: When the sun is in the plane of the earth's equator the lengths
of day and night are equal. This happens twice a year, and the times
when the sun passes the equator are called the vernal or spring
equinox and the autumnal equinox respectively.
Evection: The second inequality of the moon, which vanishes at new and
full moon and is a maximum at first and last quarter.
Excentric: As an alternative to epicycles, planets whose motion round
the earth was not uniform could be represented as moving round a
point some distance from the earth called the excentric.
Geocentric: Referred to the centre of the earth; e.g. Ptolemy's theory.
Heliocentric: Referred to the centre of the sun; e.g. the theory
commonly called Copernican.
Inequality: The difference between the actual position of a planet and
its theoretical position on the hypothesis of uniform circular
motion.
Node: The points where the orbit of the moon or a planet intersect the
plane of the ecliptic. The ascending node is the one when the planet
is moving northwards, and the line of intersection of the orbital
plane with the ecliptic is the line of nodes.
Occultation: Usually means when a planet or star is hidden by the moon,
but it also includes "occultation" of a star by a planet or of a
satellite by a planet or of one planet by another.
Opposition v. Conjunction.
Parallax: The error introduced by observing from some point other than
that required in theory, e.g. in geocentric places because the
observations are made from the surface of the earth instead of the
centre, or in heliocentric places because observations are made from
the earth and not from the sun.
Perigee: The point in the orbit of a celestial body when it is nearest
to the earth.
Precession: Owing to the slow motion of the earth's pole around the pole
of the ecliptic, the equator cuts the ecliptic a little earlier
every year, so that the equinox each year slightly precedes, with
reference to the stars, that of the previous year.