In 1676, Ole Roemer calculated the speed of light; however, he didn’t have any incentive to measure it. Roemer carefully observed the fourth moon of the Jupiter i.e. Lo, which disappeared during Jupiter’s gigantic shadow while orbiting it and was later illuminated by the light of the sun. He predicted that there was a difference in the predicted time of eclipse and the real-time they were observing. He told the reason for light reaching our telescope stretched and contracted both, is because the Earth and Jupiter moved closer or father towards each other in their orbits and this made the light to travel an elongated distance.
Later, Roemer approximated the speed of light to be 214,000 km/s, which was 80000 Km/s slower to what is actual. However, he reached a result which was quite near the original speed of light 300 years ago of the time when the true value was calculated.
The astronomers constantly worked upon to minimize the error and in the year 1728, James Bradley observed an aberration and made an estimation. An aberration is a small change in the position of any heavenly bodies by the connivance of the motion of light and the observer. James observed a star in a constellation called Draco in the northern sky and got to know that there is a change in the position throughout the year as earth revolved around the sun.
By using the angle of deviation of the light coming from stars, along with the velocity of earth moving around the sun, he calculated the speed of light to be 301,000 km/s.