# What is the Speed of Light in Kilometers Per Second?

#### February 15, 2024

The speed of light in km hr is 299,792,458 m/s (or 670,616,629 mph). If you were to travel at this incredible rate, you could circle the planet more than 400,000 times in one minute. It's the fastest thing that can happen in our universe, which is why it is often used to set limits for space travel.

For much of human history, it was believed that light moved instantaneously. Galileo tried to use lanterns to measure it, but was unable to come up with a number for its speed. In the late 1600s, Ole Roemer made a more precise determination by measuring the time it took for light to reach Jupiter's moons.

It wasn't until the 19th century that a number was truly established. Maxwell's electromagnetic equations posited that the speed of light was constant in any inertial reference frame, regardless of the position or motion of the observer. It was then possible to calculate c using techniques like spectroscopy and laser interferometers. In 1972, a group at the US National Bureau of Standards used this technique to obtain our currently-recognized value of 299,792,458 m/s.

While it is a constant in space, the speed of light can be slowed down in a material like glass or air. This is due to the fact that light bends when it passes through material, a phenomenon called refraction. When this occurs, it takes a little longer to get from point A to point B.

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