Robert I. Karafin, age 12, of Gary, Indiana, for his question:
Who figured out the speed of light?
The great Galileo pondered the speed of light, way back in the 1600s. He clocked lantern signals between two Italian hilltops. But the scale of his experiment was too small. He concluded that light is instantaneous and travels from place to place in no time at all. About 75 years later, a Danish astronomer used a much larger scale test to recheck the problem.. He proved that Galileo's conclusion was wrong and that indeed light does require time to travel from here to there.
The Danish astronomer Roemer made what most scientists take to be the first successful attempt to clock the speed of light. This was about two centuries ago. He turned his telescope on the moons of Jupiter and carefully clocked their motions across the face of the giant planet. He made detailed measurements when Jupiter and the earth were on the same side of the sun. Six months later he repeated them, when the two planets are on opposite sides of the sun and so much further apart.
If images reflected from these moons require time to travel across space, they would take longer to arrive when the two planets are farther apart. And Roemer's double set of measurements proved this to be true. The light images of Jupiter's moons arrived 16 minutes later when they had to travel the longer distance.
Roemer's tests were far from precise and a century later two French researchers decided that lab experiments on earth could give a more accurate estimate of the speed of light. They used distant mirrors to reflect back light beams to focus on rotating mirrors and figured the speed of light to be somewhere between 100,000 and 200,000 miles per second. The great American physicist Albert Michelson devoted 50 patient years to reducing these estimates to a more precise figure.
His first race track was between the tops of Mt. Wilson in California and Mt. San Antonio, 22 1/2 miles away. His racer was a powerful arc beam, focused on a distant mirror and reflected back to strike an eight sided mirror. When the mirror¬ wheel was adjusted to rotate at 30,000 times per second, it split the beam returning from its 45 mile trip. One thread of light could be focused into a telescope. From this test, Michelson figured the speed of light to be 186,000 miles per second. Later he did more precise experiments and more refinements can be expected in the future.
But the world of science awards Albert Michelson the credit for the first really good job of figuring the speed of light.
One thing that concerned Michelson was the fact that the speed of light is affected by its passage through the atmosphere. In a later experiment, he reflected his beam back and forth through an airless tube. He used the results to refine his value of light velocity and to recheck his figures. This painstaking scientist died before his work was completed to his satisfaction. It was finished and even improved upon by later researchers.