Kepler’s laws are stated as follows: . Law of orbits: Each planet moves around the Sun in an elliptical orbit with the Sun at one of the foci. AN ELLIPTICAL ORBIT OF A PLANET Perihelion Aphelion Planet Minor axis Focus Focus Sun b a Major axis Figure . An ellipse traced out by a planet around the Sun.
The closest point of approach of the planet to the Sun ‘P’ is called perihelion and the farthest point ‘A’ is called aphelion (Figure . ). The semi-major axis is ‘a’ and semi-minor axis is ‘b’. In fact, both Copernicus and Ptolemy considered planetary orbits to be circular, but Kepler discovered that the actual orbits of the planets are elliptical.
Perihelion Semi-major axis Aphelion Planet Minor axis Focus Focus Sun Major axis An Elliptical Path developed a theory to explain the motion of celestial objects like the Sun, the Moon, Mars, Jupiter etc. This theory was called the geocentric model. According to the geocentric model, the Earth is at the center of the universe and all celestial objects including the Sun, the Moon, and other planets orbit the Earth. Ptolemy’s model closely matched with the observations of the sky with our naked eye.
But later, astronomers found that even though Ptolemy’s model successfully explained the motion of the Sun and the Moon up to a certain level, the motion of Mars and Jupiter could not be explained effectively. Heliocentric Model of Nicholas Copernicus In the th century, a Polish astronomer, Nicholas Copernicus ( - ) proposed a new model called the ‘Heliocentric model’ in which the Sun was considered to be at the center of the solar system and all planets including the Earth orbited the Sun in circular orbits. This model successfully explained the motion of all celestial objects. Around the same time, Galileo, a famous Italian physicist discovered that all objects close to Earth were accelerated towards the Earth at the same rate.
Meanwhile, a noble man called Tycho Brahe ( - ) spent his entire lifetime in recording the observations of the stellar and planetary positions with his naked eye. The