In 1609, Astronomer Johannes Kepler published Astronomia Nova, a book containing 10 years of work to understand the orbit of Mars planet using his revolutionary observations from his mentor and employer Tycho Brahe who was famous for generating huge quantity of high quality data and he needed to find the best explanations for the motion of Mars.
There were three models of planetary motions at that time but not a single of them worked well. First one was the Ptolemy System (presented bt Claudius Ptolemy) that puts the Earth at the center and all the other planets of our solar system and the Sun orbiting around the Earth in perfect circular paths so the Ptolemy system was an earth-centric system which suggested that the seemingly improper paths of the Sun, Moon, and planets were actually a combination of some proper circular motions seen in perspective from a stationary Earth. The second model was the Copernicus's heliocentric model. According to this model, Earth was placed among other planets in revolving around the Sun. Later Galileo confirmed Copernicus's theory with evidence. Galileo discovered evidence to support Copernicus' heliocentric theory when he observed four moons in orbit around Jupiter. 3rd and the last one model was the Tycho Brahe's model. He puts the Earth at the center with Sun and Earth's moon orbiting around it but other planets orbiting around the Sun.
All these three models were based on circular orbitary systems as the circle was considered as an ideal shape for an orbit. Ptolemy, Copernicus, Galileo and Tycho all believed that planets should travel around circular paths but facts/ data didn't fit. Instead, Kepler found just another shape (i.e Ellipse) that works a lot better. An ellipse is just like a flattened circle with some special properties. Area between some random two points on the orbit and the focus and area between any other two points of the orbit and focus is always the same in elliptical orbit. We can draw an ellipse by taking a loose string attached from both sides with the paper and using the pencil to keep the string tight while moving. The shape drawn will be an ellipse. The two attaching points are focii of the ellipse. Whereas, in the circle, both focii combines at the center.
In Anstronomia Nova, Kepler states that the Mars travels in the elliptical orbit around the Sun which is at one of the focii of the elliptical orbit of the Sun. Later on, he used the same concept for all the other planets. The farther apart the two focii are, the longer and the skinnier the ellipse will be. This skinniness perimeter is called the 'Eccentricity'. It was not easy but with his first law, Kepler rejected the idea of perfect circular orbits and showed that the ellipse could better describe the motion of Mars and he generalized the same idea for all other planets.
In short, '' Kepler's first law means that planets move around the sun in elliptical orbits. An ellipse is a shape that resembles a flattened circle. How much the circle is flattened is expressed by its eccentricity"
Great information
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