How did Einstein know the speed of light. Who figured it out and how did they do it?

Answer 1

I think that danish astronomer Roemer back in the 1600s was one of the first to measure the speed of light with a good accuracy. He did so by observing that the times of the eclipses of Jupiter's moons where slightly behind (or slightly in advance) of the calculated/predicted times during the course of the year. This is because the relative distance between earth and Jupiter changes during the year and so light has to travel a longer (or shorter) path to reach us.
If I remember correctly a couple of centuries later another experiment was devised by Fizeau-Focault to get a more precise and practical measurement: a light source was directed between the teeth of a big gear which rotates at known great speed. The light is then reflected back. By adjusting the distance light travels and by rotating the gear at the right speed, light can be made to barely make the round trip only passing through one tooth of the gear.
Another similar method consists of rotating a mirror which reflects light which is shone on it. By measuring the deflecting angle of the beam, etc the speed is determined.

What Einstein did though was interpret the measurement results correctly so to promote the fact that light travels at c (in vacuum) regardless of the velocity of the source or of the observer (and that's the new thing) to a postulate. By doing this he got rid of the aether wind theory and absolute space and by combining this postulate with the principle of relativity (in the Galilean sense) he extended this principle (special principle of relativity) to all of physical phenomena and not just mechanics.

So his 2 postulates are:
1) the speed of light in vacuo is the same for every observer
2) the laws of physics are the same in every inertial frame of reference

Answer 2

The speed of light, c, is 299792458 m/s. Many people tried to find the exact value, but the earth is too small to see any delay in light travelling. The first rather accurate measurement was done by James Bradley, who made many observations of the night sky and made a calculation of 298000000m/s, impressively close as he did not have complex equipment. His observations were based on the "abberation of light"

Hippolyte Fizeau in 1849 tried to measure the speed of light with the following method: A beam of light was directed at a mirror several thousand metres away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second.

Later, Leon Foucault improved this measurement by using a rotating mirror rather than a cogwheel, giving him a measurement of 298,000,000 m/s.

Finally, Albert A. Michelson used a rotating prism to obtain precise measurements which brought us to the speed of 299,796 km/s

Answer 3

The first to come up with a relatively exact value for c was Ole Roemer back in 16something.
He used the time Jupiter's moon Io spent in the shadow (eclipse) seen from Earth's position close to and far from Jupiter. He estimated the travel time for light between these two points (this distance equals two astronomical units = diameter of earth's orbit) for 22 minutes. The exact value is around 16,6 minutes.

Answer 4

The speed of light was mesured by many scientist.
You shoul look for the experiments of Michelson and Morley

Answer 5

Eh?