Astronomy Fundamentals

Matter and Energy

How have scientists measured the speed of light?

In the late 1500s, Galileo Galilei documented an experiment in which he tried to measure the speed of light by using lanterns on two distant hilltops. He was only able to say that it was much faster than he could measure. In 1675 Danish astronomer Olaus Roemer (1644–1710) used eclipses of the moons of Jupiter to measure the speed of light to be 141,000 miles per second, or about 76 percent of the modern value. Roemer came fairly close, but more importantly he showed that the speed of light was not infinite. That discovery had important implications on all of physics and astronomy.

In the mid-1700s, English astronomer James Bradley (1693–1762) noticed that some stars appeared to be moving because Earth was actually moving toward or away from the starlight that was coming toward us. Using this phenomenon, called the aberration of starlight, Bradley was able to measure the speed of light to an accuracy of less than one percent error: 185,000 miles per second. In the 1800s, the French scientist Jean-Bernard León Foucault (1819–1868) used a laboratory setup of two mirrors, one rotating and one unmoving, to measure the speed of light. As the spinning mirror reflected a light beam back and forth from the stationary one, it reflected the beam back at different angles. By using geometry, Foucault determined the speed of light to be just over 186,000 miles per second.

In 1926, American physicist Albert Abraham Michelson (1852–1931) repeated Foucault’s experiment on a much larger scale. Using mirrors positioned twenty-two miles apart on two mountains in California, he calculated light speed to be 186,271 miles per second.


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