The sky has spellbound the human brains from time immemorial. The Babylonians had studied the sky three thousand years before Christ. The Greek Thales and Hipparchus applied astronomy to geography. Aristarchus in the third century B.C. surprisingly theorized that the sun was at the centre of the universe.
Modern astronomy was founded by Nicholas Copernicus, a Polish astronomer who lived between 1473 and 1543 A.D. He believed correctly that the earth orbited the sun. But he also believed mistakenly that the sun was the center of the entire universe, that the planets were all the same size and they mover in perfect circles.
Kepler’s three laws explained the orbits of planets as elliptical speed of motion related to the sun and the distance between the planet and the sun. Then came Galileo, an Italian mathematician. He made the telescope and studied heavenly bodies. He saw that Copernicus was right.
Jeremiah Horrocks, born in 1619, calculated the transit of Venus across the sun by means of the telescope. Sir Isaac Newton (1642-1727) made the discovery of the law of gravitation which completely revolutionised the coursed of astronomy. 
The Doppler effect is also applicable to light. Every atom emits light of definite wavelengths which appear in a spectroscope as a series of coloured lines-a different series of each atom. If the atom is in a receding body all the lines have slightly longer wavelengths than usual, and the amount of the change depends uniquely on the speed Longer wavelengths mean that the light is redder than usual so that a light from a receding body shows what is called a “red shift.” The speed of recession can be calculated from the amount of red shift.
