Johannes Kepler was born in 1571
at Weil der Stadt in Wurttemberg. His mother Frau Kepler had occult leanings
and must have looked exactly like a witch. In fact, at a later stage she was actually
accused of witchcraft, and Johannes had a great deal of trouble in securing her
acquittal.

However, Frau Kepler did one good
deed. In 1577 she took her son outdoors to show him a bright comet (the one
which Tycho Brahe saw), and from that moment on the boy was enthralled by
astronomy.

In 1589 he arrived at the
University of Tubingen to study theology. It was here that he became a
convinced Copernican, and he was quite open about his views. In 1596 he wrote a
book which contained some good ideas, plus a great deal of fantasy; for example
he believed that each planet must have a "sphere" or band in space in
which it was always to be found, and that these must touch each other, so that
they could be linked with the five regular solids of geometry!

*Please click on the images to enlarge*

Fortunately Tycho, in Prague,
read the book and liked it. He invited Kepler to join him, and Kepler did so.
On Tycho's death Kepler replaced him as Imperial Mathematician, and inherited
all the Hven observations, though not without opposition from some of Tycho's
family.

Kepler set out to give a final
solution of the problem of the planetary motions. He had total faith in Tycho's
observations, but for a long time he could not make the positions of the
planets fit in with any theory. Finally he realized the truth. The planets do
indeed move around the Sun—but they do so not in circles, but in ellipses. From
this he was led on to draw up his three Laws of Planetary Motion, which are as
follows:

1. A planet moves round the Sun
in an elliptical orbit. The Sun occupies one focus of the ellipse while the
other focus is empty.

2. The radius vector—the
imaginary line joining the centre of the planet' to the centre of the Sun—sweeps
out equal areas in equal times. (This means that a planet. or for that matter a
comet, moves fastest when closest to the Sun, and slowest when furthest away.)

3. The cubes of the mean distances of the
planets of the Sun are proportional to the squares of their revolution periods
(so that if you know the period, which is easy to find by observation. you can
also find the distance compared with that of the Earth. A complete model of the
Solar System can be drawn up, and if one absolute value is known all the rest
will follow.)

Law 3 is rather more complex.
Essentially, it means that there is a definite relationship between a planet's
sidereal period - that is to say, the time taken for it to go once round the
Sun - and its mean distance from the Sun. This has proved to be very important
in measuring the length of the astronomical unit, or Earth- Sun distance.

What was done, in the 1960s, was
to make an actual measure of the real distance of the planet Venus. Of course,
the sidereal periods of both Venus and the Earth were already known very
accurately, and in Kepler's relationship the only remaining unknown was the
distance between the Earth and the Sun. We now know it to have a mean value of
92,957,000 miles. The measures were made by means of the latest radar
techniques; radar pulses were bounced off Venus, and the time-lag between the
transmission of the pulse and the reception of the "echo" provided
all the information for working out the distance of the planet. Radar is
strictly a twentieth-century development, but the method was based on Kepler's
Laws of 350 years earlier!

The first two Laws were published
in 1609, and the third in 1618 in a book called

*Harmony the World,*which was the unusual Mixture of brilliant science and outdated mysticism. Kepler also prepared a new set of planetary tables, which appeared in 1628—long after the Emperor Rudolphine had been deposed, but which Kepler called the Rudolphine Tables.
He died in 1630 while
on a journey to try to collect some of the overdue wages owed to him.

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