Sunday, September 23, 2007

Johannes Kepler. The man, the myth, the enigma!

And now a quick word about Johannes Kepler - the rennaissance scientist I borrowed my moniker from. Even though he was a recluse and a bit of a mystic, I've always been a big fan.

Kepler was born in 1571 and died in 1630 and hung out mostly in Germany, Austria, and Denmark. He was a contemporary of Galileo (though they didn't have much more than a passing familiarity with each other, probably mostly due to religious differences (Catholic vs. Protestant)). You may read a detailed bio here:

He is on the *very* short list of scientists who, if he didn't do what he did, it wouldn't have been done for a long time. Einstein is probably also on the list... Euler... Chandreshekar, a few others. (But not Newton. Not Darwin. Not Hawking. Not almost anyone you've heard of.) But he developed his three laws of orbital motion almost a century before they 'should' have been discovered.(*)

Kepler was positively obsessed with planetary orbits - working on them for years, obtaining crucial orbital elements from Tycho Brahe (Kep's eyesight was poor while Brahe's was excellent and he was a skilled observational astronomer). In Kepler's time, no one was all that sure what shape the planetary orbits made. - they just knew that whatever it was, it wasn't a circle. He cranked on the observational data until he discovered what is now known as Kepler's First Law:

The shape of a planetary orbit is an ellipse with the Sun at one focus.

He then tried to quantify orbital speed for a planet, which led to his 2nd law:
A line drawn from the Sun to a Planet sweeps out equal areas in equal time. [This is conservation-of-angular-momemtum, but no one knew it then.]

Then he related the orbital speed of one planet to another via his 3rd law:

The cube of the semimajor axis of the orbit is proportional to the square of the period.

An amazing accomplishment. No one else was wrestling with this stuff during Kepler's time.
Kepler even predicted a transit of Venus (where, from Earth's point of view, Venus moves across the face of the Sun) in 1639 or so, but unfortunately for him he died years before it happened(*2). For you, however, you'll have the chance to see one in 2012! Which is cool, they are rare. Many people never have the chance to see one during their lifetimes. After 2012, it'll happen again in 2117 when we're all well into the dirtnap.

I'll close with a quote about Kepler from Bruce Stephenson (which I believe is from his book Kepler's_Physical_Astronomy),

"It can be said of Kepler, as of few great scientists, that what he accomplished would never have been done had he himself not done it. The discovery from the examination of naked-eye observational reports that planets move on ellipses, and according to the area law, is so exceedingly improbable - and Kepler's manner of arriving at it was so decidedly personal - that it lies outside the course of any inevitable development."

Johannes Kepler. Rock star.

(*) 2 of Kep's 3 laws could have been figured out by Newton by the time Newton published the Principia Mathematica in 1687. But even the 3rd Law would IMHO have eluded him... perhaps Euler would have later caught it. It's now an undergrad problem in Physics to derive Kep's 3 laws given Newton's law of gravitation, and it is very cumbersome to derive Newton's law given Kep's three (I don't know how to do it without some simplifying assumptions). You can argue Newton's Laws 'should' have been discovered first then Kepler's - but that's not how it happened.
(*2) Kepler died in 1630. Word of Kepler's calculations reached a young Astronomy student in England named Horrocks, who was almost definitely the first person ever on the planet to knowingly observe it. Kep actually predicted a transit in 1631 and a near miss in 1639, but Horrocks found the error in Kep's calculations, and he and friend William Crabtree made the observation. It's a cool story:

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