Born: 27 Dec 1571
in Weil der Stadt, Württemberg, Holy Roman Empire (now
Germany)

Died: 15 Nov 1630
in Regensburg (now in Germany)

Johannes Kepler is now chiefly remembered for discovering the
three laws of planetary motion that bear his name published in 1609 and 1619).
He also did important work in optics (1604, 1611), discovered two new regular
polyhedra (1619), gave the first mathematical treatment of close packing of
equal spheres (leading to an explanation of the shape of the cells of a
honeycomb, 1611), gave the first proof of how logarithms worked (1624), and
devised a method of finding the volumes of solids of revolution that (with
hindsight!) can be seen as contributing to the development of calculus (1615,
1616). Moreover, he calculated the most exact astronomical tables hitherto
known, whose continued accuracy did much to establish the truth of
heliocentric astronomy (Rudolphine Tables, Ulm, 1627).

A large quantity of Kepler's correspondence survives. Many of
his letters are almost the equivalent of a scientific paper (there were as yet
no scientific journals), and correspondents seem to have kept them because they
were interesting. In consequence, we know rather a lot about Kepler's life, and
indeed about his character. It is partly because of this that Kepler has had
something of a career as a more or less fictional character (see
historiographic note).

Childhood

Kepler was born in the small town of Weil der Stadt in Swabia
and moved to
nearby Leonberg with his parents in 1576. His father was a mercenary soldier
and his mother the daughter of an innkeeper. Johannes was their first child.
His father left home for the last time when Johannes was five, and is believed
to have died in the war in the Netherlands. As a child, Kepler lived with his
mother in his grandfather's inn. He tells us that he used to help by serving in
the inn. One imagines customers were sometimes bemused by the child's unusual
competence at arithmetic.

Kepler's early education was in a local school and then at a
nearby seminary, from which, intending to be ordained, he went on to enrol at
the University of Tübingen, then (as now) a bastion of Lutheran orthodoxy.

Kepler's
opinions

Throughout his life, Kepler was a profoundly religious man. All
his writings contain numerous references to God, and he saw his work as a
fulfilment of his Christian duty to understand the works of God. Man being, as
Kepler believed, made in the image of God, was clearly capable of understanding
the Universe that He had created. Moreover, Kepler was convinced that God had
made the Universe according to a mathematical plan (a belief found in the works
of Plato and associated with Pythagoras). Since it was generally
accepted at the time that mathematics provided a secure method of arriving at
truths about the world ( Euclid's common notions and postulates being regarded
as actually true), we have here a strategy for understanding the Universe.
Since some authors have given Kepler a name for irrationality, it is worth
noting that this rather hopeful epistemology is very far indeed from the
mystic's conviction that things can only be understood in an imprecise way that
relies upon insights that are not subject to reason. Kepler does indeed
repeatedly thank God for granting him insights, but the insights are presented
as rational.

University
education

At this time, it was usual for all students at a university to
attend courses on "mathematics". In principle this included the four
mathematical sciences: arithmetic, geometry, astronomy and music. It seems,
however, that what was taught depended on the particular university. At
Tübingen Kepler was taught astronomy by one of the leading astronomers of the
day, Michael Maestlin (1550 - 1631). The astronomy of the curriculum was, of
course, geocentric astronomy, that is the current version of the Ptolemaic
system, in which all seven planets - Moon, Mercury, Venus, Sun, Mars, Jupiter
and Saturn - moved round the Earth, their positions against the fixed stars
being calculated by combining circular motions. This system was more or less in
accord with current (Aristotelian) notions of physics, though there were
certain difficulties, such as whether one might consider as 'uniform' (and
therefore acceptable as obviously eternal) a circular motion that was not
uniform about its own centre but about another point (called an 'equant').
However, it seems that on the whole astronomers (who saw themselves as
'mathematicians') were content to carry on calculating positions of planets and
leave it to natural philosophers to worry about whether the mathematical models
corresponded to physical mechanisms. Kepler did not take this attitude. His
earliest published work (1596) proposes to consider the actual paths of the
planets, not the circles used to construct them.

At Tübingen, Kepler studied not only mathematics but also Greek
and Hebrew (both necessary for reading the scriptures in their original
languages). Teaching was in Latin. At the end of his first year Kepler got 'A's
for everything except mathematics. Probably Maestlin was trying to tell him he
could do better, because Kepler was in fact one of the select pupils to whom he
chose to teach more advanced astronomy by introducing them to the new,
heliocentric cosmological system of Copernicus. It was from Maestlin that
Kepler learned that the preface to On the revolutions, explaining that this was
'only mathematics', was not by Copernicus. Kepler seems to have accepted
almost instantly that the Copernican system was physically true; his reasons
for accepting it will be discussed in connection with his first cosmological
model (see below).

It seems that even in Kepler's student days there were
indications that his religious beliefs were not entirely in accord with the
orthodox Lutheranism current in Tübingen and formulated in the 'Augsburg
Confession' (Confessio Augustana). Kepler's problems with this Protestant
orthodoxy concerned the supposed relation between matter and 'spirit' (a
non-material entity) in the doctrine of the Eucharist. This ties up with Kepler's
astronomy to the extent that he apparently found somewhat similar intellectual
difficulties in explaining how 'force' from the Sun could affect the planets.
In his writings, Kepler is given to laying his opinions on the line - which is
very convenient for historians. In real life, it seems likely that a similar
tendency to openness led the authorities at Tübingen to entertain well-founded
doubts about his religious orthodoxy. These may explain why Maestlin persuaded
Kepler to abandon plans for ordination and instead take up a post teaching
mathematics in Graz. Religious intolerance sharpened in the following years.
Kepler was excommunicated in 1612. This caused him much pain, but despite his
(by then) relatively high social standing, as Imperial Mathematician, he never
succeeded in getting the ban lifted.

Kepler's
first cosmological model (1596)

Instead of the seven planets in standard geocentric astronomy
the Copernican system had only six, the Moon having become a body of kind
previously unknown to astronomy, which Kepler was later to call a 'satellite'
(a name he coined in 1610 to describe the moons that Galileo had
discovered were orbiting Jupiter, literally meaning 'attendant'). Why six
planets?

Moreover, in geocentric astronomy there was no way of using
observations to find the relative sizes of the planetary orbs; they were simply
assumed to be in contact. This seemed to require no explanation, since it
fitted nicely with natural philosophers' belief that the whole system was
turned from the movement of the outermost sphere, one (or maybe two) beyond the
sphere of the 'fixed' stars (the ones whose pattern made the constellations),
beyond the sphere of Saturn. In the Copernican system, the fact that the annual
component of each planetary motion was a reflection of the annual motion of the
Earth allowed one to use observations to calculate the size of each planet's
path, and it turned out that there were huge spaces between the planets. Why
these particular spaces?

Kepler's answer to these questions, described in his Mystery of
the Cosmos (Mysterium cosmographicum, Tübingen, 1596), looks bizarre to
twentieth-century readers (see the figure on the right). He suggested that if a
sphere were drawn to touch the inside of the path of Saturn, and a cube were
inscribed in the sphere, then the sphere inscribed in that cube would be the
sphere circumscribing the path of Jupiter. Then if a regular tetrahedron were
drawn in the sphere inscribing the path of Jupiter, the insphere of the
tetrahedron would be the sphere circumscribing the path of Mars, and so
inwards, putting the regular dodecahedron between Mars and Earth, the
regular icosahedron between Earth and Venus, and the regular
octahedron between Venus and Mercury. This explains the number of planets perfectly:
there are only five convex regular solids (as is proved in Euclid's
Elements , Book 13). It also gives a convincing fit with the sizes of the paths
as deduced by Copernicus, the greatest error being less than 10% (which
is spectacularly good for a cosmological model even now). Kepler did not
express himself in terms of percentage errors, and his is in fact the first
mathematical cosmological model, but it is easy to see why he believed that the
observational evidence supported his theory.

Kepler saw his cosmological theory as providing evidence for the
Copernican theory. Before presenting his own theory he gave arguments to
establish the plausibility of the Copernican theory itself. Kepler asserts that
its advantages over the geocentric theory are in its greater explanatory power.
For instance, the Copernican theory can explain why Venus and Mercury are never
seen very far from the Sun (they lie between Earth and the Sun) whereas in the
geocentric theory there is no explanation of this fact. Kepler lists nine such
questions in the first chapter of the Mysterium cosmographicum.

Kepler carried out this work while he was teaching in Graz, but
the book was seen through the press in Tübingen by Maestlin. The agreement with
values deduced from observation was not exact, and Kepler hoped that better
observations would improve the agreement, so he sent a copy of the Mysterium
cosmographicum to one of the foremost observational astronomers of the
time, Tycho Brahe (1546 - 1601). Tycho, then working in Prague, had
in fact already written to Maestlin in search of a mathematical assistant.
Kepler got the job.

The
'War with Mars'

Naturally enough, Tycho's priorities were not the same as
Kepler's, and Kepler soon found himself working on the intractable problem of
the orbit of Mars [[(See Appendix below)]]. He continued to work on this
after Tycho died (in 1601) and Kepler succeeded him as Imperial
Mathematician. Conventionally, orbits were compounded of circles, and rather
few observational values were required to fix the relative radii and positions
of the circles. Tycho had made a huge number of observations and Kepler
determined to make the best possible use of them. Essentially, he had so many
observations available that once he had constructed a possible orbit he was
able to check it against further observations until satisfactory agreement was
reached. Kepler concluded that the orbit of Mars was an ellipse with the
Sun in one of its foci (a result which when extended to all the planets is now
called "Kepler's First Law"), and that a line joining the planet to
the Sun swept out equal areas in equal times as the planet described its orbit
("Kepler's Second Law"), that is the area is used as a measure of
time. After this work was published in New Astronomy ... (Astronomia nova, ...,
Heidelberg, 1609), Kepler found orbits for the other planets, thus establishing
that the two laws held for them too. Both laws relate the motion of the planet
to the Sun; Kepler's Copernicanism was crucial to his reasoning and to his
deductions.

The actual process of calculation for Mars was immensely
laborious - there are nearly a thousand surviving folio sheets of arithmetic -
and Kepler himself refers to this work as 'my war with Mars', but the result
was an orbit which agrees with modern results so exactly that the comparison
has to make allowance for secular changes in the orbit since Kepler's time.

Observational
error

It was crucial to Kepler's method of checking possible orbits
against observations that he have an idea of what should be accepted as
adequate agreement. From this arises the first explicit use of the concept of
observational error. Kepler may have owed this notion at least partly to
Tycho, who made detailed checks on the performance of his instruments (see
the biography of Brahe).

Optics, and the New Star of 1604

The work on Mars was essentially completed by 1605, but there
were delays in getting the book published. Meanwhile, in response to concerns
about the different apparent diameter of the Moon when observed directly and
when observed using a camera obscura, Kepler did some work on optics, and came
up with the first correct mathematical theory of the camera obscura and the
first correct explanation of the working of the human eye, with an upside-down
picture formed on the retina. These results were published in Supplements to
Witelo, on the optical part of astronomy (Ad Vitellionem paralipomena, quibus
astronomiae pars optica traditur, Frankfurt, 1604). He also wrote about the New
Star of 1604, now usually called 'Kepler's supernova', rejecting numerous
explanations, and remarking at one point that of course this star could just be
a special creation 'but before we come to [that] I think we should try
everything else' (On the New Star, De stella nova, Prague, 1606, Chapter 22,
KGW 1, p. 257, line 23).

Following Galileo's use of the telescope in discovering
the moons of Jupiter, published in his Sidereal Messenger (Venice, 1610), to
which Kepler had written an enthusiastic reply (1610), Kepler wrote a study of
the properties of lenses (the first such work on optics) in which he presented
a new design of telescope, using two convex lenses (Dioptrice, Prague, 1611).
This design, in which the final image is inverted, was so successful that it is
now usually known not as a Keplerian telescope but simply as the astronomical
telescope.

Leaving
Prague for Linz

Kepler's years in Prague were relatively peaceful, and
scientifically extremely productive. In fact, even when things went badly, he
seems never to have allowed external circumstances to prevent him from getting
on with his work. Things began to go very badly in late 1611. First, his seven
year old son died. Kepler wrote to a friend that this death was particularly
hard to bear because the child reminded him so much of himself at that age.
Then Kepler's wife died. Then the Emperor Rudolf, whose health was failing, was
forced to abdicate in favour of his brother Matthias, who, like Rudolf, was a
Catholic but (unlike Rudolf) did not believe in tolerance of Protestants.
Kepler had to leave Prague. Before he departed he had his wife's body moved
into the son's grave, and wrote a Latin epitaph for them. He and his remaining
children moved to Linz (now in Austria).

Marriage
and wine barrels

Kepler seems to have married his first wife, Barbara, for love
(though the marriage was arranged through a broker). The second marriage, in
1613, was a matter of practical necessity; he needed someone to look after the
children. Kepler's new wife, Susanna, had a crash course in Kepler's character:
the dedicatory letter to the resultant book explains that at the wedding
celebrations he noticed that the volumes of wine barrels were estimated by
means of a rod slipped in diagonally through the bung-hole, and he began to
wonder how that could work. The result was a study of the volumes of solids of
revolution (New Stereometry of wine barrels ..., Nova stereometria doliorum
..., Linz, 1615) in which Kepler, basing himself on the work of
Archimedes, used a resolution into 'indivisibles'. This method was later
developed by Bonaventura Cavalieri (c. 1598 - 1547) and is part of the
ancestry of the infinitesimal calculus.

The
Harmony of the World

Kepler's main task as Imperial Mathematician was to write
astronomical tables, based on Tycho's observations, but what he really
wanted to do was write The Harmony of the World, planned since 1599 as a
development of his Mystery of the Cosmos. This second work on cosmology
(Harmonices mundi libri V, Linz, 1619) presents a more elaborate mathematical
model than the earlier one, though the polyhedra are still there. The
mathematics in this work includes the first systematic treatment of
tessellations, a proof that there are only thirteen convex uniform polyhedra
(the Archimedean solids) and the first account of two non-convex regular
polyhedra (all in Book 2). The Harmony of the World also contains what is now
known as 'Kepler's Third Law', that for any two planets the ratio of the
squares of their periods will be the same as the ratio of the cubes of the mean
radii of their orbits. From the first, Kepler had sought a rule relating the
sizes of the orbits to the periods, but there was no slow series of steps
towards this law as there had been towards the other two. In fact, although the
Third Law plays an important part in some of the final sections of the printed
version of the Harmony of the World, it was not actually discovered until the
work was in press. Kepler made last-minute revisions. He himself tells the
story of the eventual success:

...and if you want the exact moment in time, it was conceived
mentally on 8th March in this year one thousand six hundred and eighteen, but
submitted to calculation in an unlucky way, and therefore rejected as false,
and finally returning on the 15th of May and adopting a new line of attack,
stormed the darkness of my mind. So strong was the support from the combination
of my labour of seventeen years on the observations of Brahe and the
present study, which conspired together, that at first I believed I was
dreaming, and assuming my conclusion among my basic premises. But it is
absolutely certain and exact that "the proportion between the periodic
times of any two planets is precisely the sesquialterate proportion of their
mean distances ..."
(Harmonice mundi Book 5, Chapter 3, trans. Aiton, Duncan and Field, p. 411).

Witchcraft
trial

While Kepler was working on his Harmony of the World, his mother
was charged with witchcraft. He enlisted the help of the legal faculty at
Tübingen. Katharina Kepler was eventually released, at least partly as a result
of technical objections arising from the authorities' failure to follow the
correct legal procedures in the use of torture. The surviving documents are
chilling. However, Kepler continued to work. In the coach, on his journey to
Württemberg to defend his mother, he read a work on music theory by Vincenzo
Galilei (c.1520 - 1591, Galileo's father), to which there are numerous
references in The Harmony of the World.

Astronomical
Tables

Calculating tables, the normal business for an astronomer,
always involved heavy arithmetic. Kepler was accordingly delighted when in 1616
he came across Napier's work on logarithms (published in 1614). However,
Maestlin promptly told him first that it was unseemly for a serious
mathematician to rejoice over a mere aid to calculation and second that it was
unwise to trust logarithms because no-one understood how they worked. (Similar
comments were made about computers in the early 1960s.) Kepler's answer to the
second objection was to publish a proof of how logarithms worked, based on an
impeccably respectable source: Euclid's Elements Book 5. Kepler
calculated tables of eight-figure logarithms, which were published with the
Rudolphine Tables (Ulm, 1628). The astronomical tables used not only
Tycho's observations, but also Kepler's first two laws. All astronomical tables
that made use of new observations were accurate for the first few years after
publication. What was remarkable about the Rudolphine Tables was that they
proved to be accurate over decades. And as the years mounted up, the continued
accuracy of the tables was, naturally, seen as an argument for the correctness
of Kepler's laws, and thus for the correctness of the heliocentric astronomy.
Kepler's fulfilment of his dull official task as Imperial Mathematician led to
the fulfilment of his dearest wish, to help establish Copernicanism.

Wallenstein

By the time the Rudolphine Tables were published Kepler was, in
fact, no longer working for the Emperor (he had left Linz in 1626), but for
Albrecht von Wallenstein (1583 - 1632), one of the few successful military
leaders in the Thirty Years' War (1618 - 1648).

Wallenstein, like the emperor Rudolf, expected Kepler to give
him advice based on astrology. Kepler naturally had to obey, but repeatedly
points out that he does not believe precise predictions can be made. Like most
people of the time, Kepler accepted the principle of astrology, that heavenly
bodies could influence what happened on Earth (the clearest examples being the
Sun causing the seasons and the Moon the tides) but as a Copernican he did not
believe in the physical reality of the constellations. His astrology was based
only on the angles between the positions of heavenly bodies ('astrological
aspects'). He expresses utter contempt for the complicated systems of
conventional astrology.

Death

Kepler died in Regensburg, after a short illness. He was staying
in the city on his way to collect some money owing to him in connection with
the Rudolphine Tables. He was buried in the local church, but this was
destroyed in the course of the Thirty Years' War and nothing remains of the
tomb.

Historiographic
note

Much has sometimes been made of supposedly non-rational elements
in Kepler's scientific activity. Believing astrologers frequently claim his
work provides a scientifically respectable antecedent to their own. In his
influential Sleepwalkers the late Arthur Koestler made Kepler's battle with
Mars into an argument for the inherent irrationality of modern science. There
have been many tacit followers of these two persuasions. Both are, however,
based on very partial reading of Kepler's work. In particular, Koestler seems
not to have had the mathematical expertise to understand Kepler's procedures.
Closer study shows Koestler was simply mistaken in his assessment.

The truly important non-rational element in Kepler's work is his
Christianity. Kepler's extensive and successful use of mathematics makes his
work look 'modern', but we are in fact dealing with a Christian Natural
Philosopher, for whom understanding the nature of the Universe included
understanding the nature of its Creator.