Johannes Kepler's seminal work, The New Astronomy or Astronomia Nova was introduced in the year 1609. In today’s world, Kepler is best known for his three laws of planetary motion. Two of his laws were published in this book volume, which made sense for a number of reasons. The book in itself offers an interesting insight to Kepler’s mind, as he has honestly recorded the trials and errors in this effort to finally come to state these two laws on planetary motion. While he made a number of mistakes in his calculations and reasoning as he went along, they always seemed to nullify each other.
The first law of Kepler stated that the planets travel around the sun in elliptical orbits, with the sun located at one of the ellipse foci. This initially went against the long known theory of over two thousand years, which believed that, planets traveled with uniform motion in their circular orbits (also the belief of Copernicus who put forth the heliocentric theory). His second law, which was actually discovered first, stated that the speed of the planets varied as they circled the sun; they went faster when they were at a point nearer to the sun that they were at a point when they were farther away from it. However, he also added that, the planets swept out equal areas of their orbit in equal times. That is, the area of the elliptical orbit that was covered in a certain amount of time always remained constant. Thus, Kepler discarded the long accepted idea of circular planetary orbits and the ancient belief that the planets traveled in their orbits with a consistent speed.
These laws made a great impact and the astronomers could eventually dispose the notion of epicycles and the equant (which were only there to preserve the appearance of uniform circular motion), that helped the construction of a more simplified version of the Copernican universe. In this sense, astronomy could for the first time describe the physical reality of the universe. Kepler also reiterated his belief (earlier stated by him in Mystery of Cosmos, 1957) that a force emanating from the sun regulates the motion of the planets. Thus, he could fully address the cause of celestial motion rather than deriving a mere mathematical calculation of it.
To mention a couple of Kepler’s problems in finally deducing these laws, it was the first law that gave him most of the difficulties. Once Kepler was convinced that the planetary orbits were oval shaped, rather than circular, he strived to find a mathematical formula that would describe the shape of the ovals.
He tried but failed. He persisted on this problem for over an year and finally came up with an equation that seemed to exactly describe the orbit. Although, it remained elusive to Kepler for long, the equation that he eventually derived was that of an ellipse. Also, initially, with his own minor mistake, he concluded that his equation was incorrect. Almost in the phase of giving it up, Kepler decided to see what if he treated the orbit as if it was an ellipse… This led him to a point where he had started; and he realized that he had the answer for a long time with him.
Kepler, in his book, comments that had he not been assigned to work on the shape of Mars’s orbit, he would have never figured out the planetary orbits. In his eyes, only Mars’s orbit was irregular enough to provide the relevant data. Kepler mentioned it as an act of “Divine Providence” that this problem had just fallen into his lap. Thus, the Mars’s orbit represented Kepler’s greatest challenge and obsession just yet.
A nice way to end my abstract on this book would be to restate Kepler’s theory, with high regard, for his great contribution on the three planetary laws; which were largely instrumental in defining the Newton’s laws of gravitation later.
1. Planets travel around the sun in elliptical orbits with the sun located at one focal point.
2. As the planets travel around their orbits, they sweep out the same amount of area per unit of time, no matter wheree they are on the orbit.
3. The distance a planet’s orbit is from the sun, cubed, is directly proportional to the time it takes for the planet to travel around the orbit, squared. Mathematically, this could be stated as: a3/p2 = K, where, “a” stands for the distance a planet’s orbit is from the sun, “p” is the period, the time it takes for a planet to revolve round the sun once and “K” is a constant.