Happy Birthday, Johannes Kepler!

I’ve been slacking off on blogging for a few days — celebrating Christmas with my family and working on some home renovation projects, mainly — but I’m back!  Today, I have just a short post commemorating the 445th birthday of Johannes Kepler.

On December 27, 1571, in the free city of Weil der Stadt (in what is now the Stuttgart region of Germany), Heinrich and Katharina Kepler became the proud parents of little Johannes.  All accounts say that he was a sickly child, but his parents encouraged his interest in academics and the natural sciences, allowing him to watch both a lunar eclipse and the Great Comet of 1577, and sending him to school, where he excelled.  The Great Comet of 1577, incidentally, was widely observed across the world, but has never returned to the inner solar system.  Based on Tycho Brahe’s meticulous observations, it is currently believed to be about 320 astronomical units from the Sun.  It wasn’t just spectacular; Brahe made the revolutionary discovery that its tail always pointed away from the Sun and that it was beyond the orbit of the Moon (a point which Galileo disputed, believing comets to be optical effects in the atmosphere), and Kepler was so inspired by it that when he later became an apprentice to Brahe, he believed the comet’s behavior would help overturn the geocentric model of the Universe.  He was wrong; it would take something else — it would take Kepler’s own revolutionary discovery.

Kepler graduated from the University of Tübingen in 1591.  This was a religious school, and he had a desire to go into ministry, but his exceptional gifts in mathematics lead him down another path.  His passion for astronomy (including a dissertation in which he defended the heliocentric model from both a scientific and a theological perspective) led him eventually into service under Tycho Brahe in 1600.  This left him perfectly positioned to succeed Brahe upon the older astronomer’s death in 1601, and armed with Brahe’s insanely detailed observations, Kepler was poised to do what so many before him had failed to do: convince the world of the heliocentric model.

The reason people had rejected the heliocentric model was not because of religious superstition; that certainly created a bias, but the real reason heliocentrism was rejected was because it didn’t work.  Scientists, then as now, will not accept a model which doesn’t fit the observations.  Heliocentrism had a lot going for it, and it did make logical sense, but it did not accurately predict the motion of the planets.  Neither did geocentrism, though; the problem was the epicycles, the backward loops in the sky that planets appear to make.  Brahe’s meticulous observations had been intended to refine epicycles to perfect the Ptolomaic universe; the idea was that the epicycles were due to a circular motion around the ring of a planet’s orbit around the Earth, but although they came closer to accurately predicting the motion of the planets, they didn’t work over long timescales either.

Kepler made two crucial realizations that finally made the Copernican universe work: first, the orbits of the planets are not circles but rather are ellipses, and second, planets move faster at the near point of their orbit than at the far point, and this change in velocity fits the sort of geometrical perfection mathematicians had been yearning for and which fit the theological quest for heavenly perfection, thereby satisfying both the scientific and the theological in a single swoop.  Specifically, although the orbits are elliptical, the body they orbit sits precisely at the focus of the ellipsis, and although the planets move faster at one end, if you were to subdivide the orbit in terms of equal degrees out from the primary body, you’d find that the planet sweeps through each subdividision in exactly the same amount of time — it was changing, but it was not random.  It had the elegance everyone believed a scientific theory had to have, and, more importantly, it was far more accurate in its predictions — in 1631, it achieved its greatest triumph by correctly predicting a transit of Mercury, the first ever observed.

This was Kepler’s biggest contribution, but by no means his only.  He also worked a great deal on optics, deducing why the sunsets as well as the lunar eclipse are red, and also being the first to determine that the lens of the eye inverts and flips the image projected onto the retina.  And he made extensive observations of Supernova 1604, proving that it was on the “sphere of fixed stars” and therefore that this sphere was not immutable after all.  (Of course, today we all know it isn’t even a sphere, but Kepler’s measurement abilities were limited.)  He collaborated with Galileo Galilei, confirming the latter’s observation of moons around Jupiter, and further exploring the telescope.  He explored geometry, defining the Keplerian Solids.  He also developed a reputation as an accurate and reliable astrologer; his advice was often sought on political matters, a thing that was entirely ordinary for astronomers of the day, for at the time it was still believed that messages from God were delivered by means of the planets.  This may have increased his exposure to the shifting tides of politics; he was a Protestant, but with Calvinistic leanings, and so he would from time to time come under fire both from Protestants and Catholics.  There was even a witchcraft trial against his mother, although Kepler’s spirited defense in court helped ultimately to prove that the charges were pure hearsay and get her released.

Today, Kepler is known as one of the giants of astronomy, and he is remembered in many ways.  But my favorite is the Kepler Space Telescope.  Originally built as a planet-hunter, mimicking his keen attention to detail by sweeping out the same tiny spot in the sky night after night so that miniscule variations in brightness could be recorded as planets passed in front of their stars, Kepler has lost a great deal of its precision due to the failure of most of its reaction wheels — it can no longer hold itself quite as steady.  But it’s still functioning, still soldiering on and doing valuable astronomy, just like its namesake did over four hundred years ago in the face of opposition.

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