How was the speed of light first measured historically, and how did different scientists contribute to determining its value?

-Karthik Gurumurthy

The first successful measurement of the speed of light on Earth was made by Fizeau in 1849. A beam of light was directed at a mirror ten miles away. The beam passed through a rotating cogwheel. By knowing the rate of rotation, the number of teeth, and the rate of light could be calculated. He got and the rate of light could be calculated.

Michelson, an American, Albert Michelson, measured the speed of light in 1926. He set up a rotating mirror system to measure the time for light to make a round trip from Mount Wilson to Mount San Antonio in southern California.

Michelson’s precise measurements yielded a value of 186,285 miles per second or 299.796 kilometers per second. He was awarded the Nobel Prize in 1907 for teaming up with Morley to try to measure the so-called ether.

Years earlier, Michelson was denied an appointment to the Naval Academy at Annapolis by his California congressman. So he waited on the White House lawn for President Grant to come out to take his daily walk. He asked Grant for an appointment and was granted the same (no pun intended).

Light is an electromagnetic wave and travels at the same speed in a vacuum or television waves. Light takes 1.2 seconds to go from the Earth to the moon. Sun to Earth time is 8.5 minutes. Sunlight takes one hundred years. The nearest star is 4.3 years. Light takes one hundred thousand years to cross our Milky Way Galaxy. We humans live in a very big universe!

Ole Romer, a Danish astronomer, made the first estimate of the speed of light in 1676. Using a telescope, he observed Io, one of four large Galilean moons of Jupiter. Io would alternately move in front of Jupiter, and then go behind Jupiter.

He recorded that as Earth and Jupiter moved further apart, exit from the shadow of Jupiter would become later than predicted. He estimated that twenty-two minutes was needed for light to cross the diameter of the orbit of Earth.

His calculation for the speed of light came out to one hundred and thirty-six thousand miles per second. Not bad for a first measurement. The accepted value today is about one hundred and eighty-six thousand miles per second.