What Am I Looking At – Part VI

“Tycho and Kepler”

     by Aaron Clevenson

 

This is the sixth in a series of interviews with famous astronomers of the ages.  Our ultimate goal is to trace our understandings of the physical universe.  This month we are going to talk to Tycho and Kepler.

 

NORTHSTAR:  Tycho Brahe, please tell us a little about yourself.

TYCHO:  I am very much an observational astronomer.  What has been most valuable to me in generating my theories is the study of comets and novae.

NORTHSTAR:  Please give us more detail.

TYCHO:  In 1572, on the evening of November 11, I observed a bright star that had not been there previously.  I could tell, by virtue of the fact that it did not move against the background stars, that it was not in the orbit of any of the planets.  I could also tell that it was not located beneath the orbit of the moon.  It had to be in the outer reaches among the stars.  Then in 1577 I had the good fortune to view another great unexpected sight in the night sky.  It was a star but it was moving.  I was able to triangulate my data with that of other great astronomers.  Our data showed that it was well beyond the orbit of the moon.   This means that Aristotle’s belief that everything beyond the moon was unchanging was incorrect.

NORTHSTAR:  Kepler, didn’t you also observe a supernova?

KEPLER:  Yes indeed.  In 1604 I observed a very bright one for almost a year.  In fact we were able to chart it very accurately, and also attempt to triangulate it.  Our efforts showed that this star was obviously part of the starry realm.

NORTHSTAR:  Tycho, how did you get started in astronomy?

TYCHO:  When I was just 13 years old I watched a solar eclipse.  This was predicted by the Ptolemaic tables.  I was amazed.  When I started making my own observations, I became aware of the inconsistencies of the tables.  On August 24, 1563 the conjunction of Saturn and Jupiter was days away from that predicted by the tables.  This convinced me that I needed to pursue more accurate predictors.

NORTHSTAR:  What does your model of the Universe look like?

TYCHO:  I believe that the planets orbit the sun, but the moon and the sun orbit the Earth.  Although we were able to get more and more accurate observations, even my great model had its shortcomings.  I have a beautiful observatory, Uraniburg, which I built on a small island between Elsinor Castle and Copenhagen.  We collected and installed the best instruments available in all of Europe.  Although I am a great observer, theory is not my strong suit. 

NORTHSTAR:  And that is where you come in, is it not Kepler?

KEPLER:  Yes indeed.  My problem was that in my early years I did not have the resources to use great equipment.  My observing was difficult and experienced many setbacks.  I have also had my follies in theory.  My first great theory was that the intervals between the orbits of the planets describe a nest of Platonic solids.  Unhappily this was not true.

NORTHSTAR:  How did you get your start in Astronomy?

KEPLER:  In 1577 my mom took me outside to see the great comet and in 1580 she took me out to see an eclipsed moon.  In 1608 I vowed to merge Astronomy with the Copernican model of the Universe.  To do this, I needed better data.  Tycho had that data and I believed that he needed me to help put it all together.

TYCHO:  I was not particularly interested.

KEPLER:  Tycho gave me the task of trying to determine the orbit of Mars’ orbit.  I spent eight years working on that problem.  I applied seventy different circular orbits to the data, and none of them fit.  I then tried to imagine what the orbit would look like from a person on Mars looking at the Earth.  After 900 pages of calculations, I still did not have the answer.

NORTHSTAR:  How did you finally resolve the problem?

KEPLER:  I then tried looking at it from the Sun’s perspective. Then it hit me.  Mars’ orbit was a perfect ellipse.  In fact, everything fell into place.  All of the planets’ orbits were ellipses.  I had the perfect Copernican model.  My friends thought it was ridiculous, but I knew I had solved the problem.  My findings were published in my book The Harmonies of the World.

NORTHSTAR:  What were your final conclusions?

KEPLER:  Well my first law was that planets orbit in perfect ellipses.  My second law was a bit more complicated, but it states that for any period of time in a planet’s orbit, it will always sweep out an equal amount of space.  My third law states that the cube of the mean distance to a planet is proportional to the square of the time it takes to complete one orbit.  This was the tough one it took me 10 more years to get the third law.  My crowning accomplishment.

NORTHSTAR:  Thank you Tycho and Kepler for this wonderful view of the Universe.