km/sec301,000

1728 James Bradley

In 1728 James Bradley, an English physicist, estimated the speed of light in vacuum to be around 301,000 km/s. He used stellar aberration to calculate the speed of light. Stellar aberration causes the apparent position of stars to change due to the motion of Earth around the sun. Stellar aberration is approximately the ratio of the speed that the earth orbits the sun to the speed of light. He knew the speed of Earth around the sun and he could also measure this stellar aberration angle. These two facts enabled him to calculate the speed of light in vacuum.

Speed of Light Historical Measurements