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Kepler and his laws of planetary motion

Portrait of Johannes Kepler.   | Photo Credit: Wikimedia Commons

When we are talking about astronomy, it is hard to not mention Johannes Kepler. A German astronomer and mathematician, Kepler was one of the central figures in the scientific revolution of the 17th Century. For the thought that our solar system had planets travelling in circles around the Earth was forever debunked by Kepler and his laws of planetary motion.

Born into an economically weak family in Germany in December 1571, Kepler’s intelligence was apparent from an early age and he won a scholarship to the University of Tubingen. Studying to become a Lutheran minister, it was here that Kepler was introduced to the ideas of heliocentrism by Polish polymath Nicolaus Copernicus.

Backs heliocentrism

Kepler found work teaching mathematics at Graz, Austria and spent his spare time studying astronomy. It was during this time, in 1596, that Kepler wrote his first outspoken defence of the Copernican system with the sun as the centre of the solar system.

This was a dangerous stance at those times when popular belief and religious thoughts placed Earth in the centre of the solar system. The political and the religious difficulties of the era meant that Kepler was banished from Graz.

Brahe's data

Fortunately for Kepler, he found an opportunity to work as the assistant of renowned Danish astronomer Tycho Brahe and so moved with his family to Prague (now in Czech Republic). Having built an observatory and used it to track the motion of planets, Brahe possessed the most accurate astronomical observations of the time.

Even though Brahe was impressed with Kepler’s studies during an earlier meeting and had therefore invited Kepler to work with him, he also mistrusted Kepler, fearing that he might supersede him as the best astronomer of the time.

Mars mystery

This meant that Brahe shared with Kepler only a small part of all the data that he had at his disposal. Brahe tasked Kepler with solving the mystery surrounding Mars, whose movements were among the most puzzling problems in astronomy at the time. Some believe that Brahe did this assuming that the difficulty of the problem would keep Kepler occupied long enough, during which time Brahe was hoping to perfect his own geocentric model with Earth as the centre of the solar system. If this were indeed the case, then Brahe might have unwillingly given the part of his data that led Kepler to his correct model of the solar system.

Kepler is believed to have said that he would solve the Martian problem in eight days, but it took more like eight years. Brahe died in 1601 and Kepler managed to gain possession of Brahe’s vast observations as well.

Kepler published what are now known as the first two laws of planetary motion in 1609 in Astronomia Nova, considered to be among his most important work. It took him almost another decade to conceive what is now known as the third law of planetary motion. For, he first arrived at the idea on March 8, 1618 and discarded it, only to recall it months later when he realised that it was the fruit of his “labor of seventeen years on Brahe’s observations”.

The three laws

In short, the three laws of planetary motion can be given as follows: (1) Planets move in ellipses with the sun at one focus; (2) The radius vector describes equal areas in equal times; and (3) The squares of the periodic times are to each other as the cubes of the mean distances.

Even though Kepler never called these as the first, second and third laws of planetary motion, that is how they are now known. Apart from placing the sun at its rightful place in the solar system and allowing us to better appreciate planetary motion, these laws also led English polymath Isaac Newton to his law of gravitation. Our understanding of the universe was never the same again.

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Printable version | Sep 21, 2021 9:04:51 AM |

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