Sunday, December 27, 2015

Tonight's Sky for December 27: Johannes Kepler Born (1571)

It was on this date that Johannes Kepler was born. Initially sent to a seminary to become a member of the Lutheran Clergy, Kepler instead found a love for mathematics, which became his first profession when he took a job as a math teacher. While a teacher, Kepler wrote a book on astronomy, specifically planetary orbits. This work attracted the attention of many astronomers, including Tycho Brahe, the greatest of the pre-telescopic astronomers, who invited Kepler to join his staff.

The relationship between the two men was awkward. Brahe was an astronomer, not a mathemitician, and he knew that he needed Kepler's mathematical genius to make sense of his volumes upon volumes of data. At the same time, Kepler knew he needed Brahe's data if he were to make any discoveries about planetary orbits. Unfortunately for Kepler, Brahe regarded his observational data as his life's work and wasn't about to start giving it out freely. Between the time he joined Brahe's staff in 1599 to the time Brae died in 1601, Kepler almost quit his position several times over frustration about not being granted access to the volumes of data Brahe had compiled in over 30 years of observations.
After Brahe died, though, Kepler would be appointed to Brahe's position of Imperial Mathemitician, which thus granted Kepler access to all of Brahe's data.

Eagerly plunging into his work, Kepler immediately came across problems, specifically that the observational data could not be reconciled with planets having a circular orbit, as proposed by Copernicus. In the time that followed, Kepler tried various models for planetary movement in the hope of finding one that would fit the observations. Try as he might, Kepler just couldn't reconcile the models to the observations, especially in regards to the planet Mars, which exhibited the greatest irregularity in its orbit.. Finally, in desperation, Kepler, a deeply religious man who sought to find proof of a divine blueprint for the solar system, gave up the perfect circles that had so dominated astronomy for centuries. Upon calculating the orbits of the planets as ellipses (slightly elongated circles) the observation and theory finally agreed. It was this discovery that planets' orbits ere elliptical that inspired Kepler's 3 laws of planetary motion.
Law 1: All planets move in elliptical orbits with the Sun at one of the foci.
Law 2: All planets move through equal areas of space in equal times ()basically, planets move faster when closer to the Sun and slower when farther away).

Law 3: A planet's period of orbit is proportional to its distance (orbital period (in years) of the planet squared equals the semimajor axis (in AU) cubed). This discovery showed that there was some common force governing planetary motion (this force is gravity, but Kepler didn't know this yet).

Unfortunately, try as he might, Kepler, while he could explain “hows” of planetary motion, he couldn't explain the “whys.” This task would occupy him for the rest of his life until 1630.

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