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Q&A: General Astronomy and Space Science

Q:
If light travels to us from the Sun at 186,000 miles per second and takes 8 minutes, does that mean if I stood 100,000 miles from someone and then switched on a torch they would see it just after I switched it on (if we stood in the same place long enough)?

A:
Yes, they would see it 100,000/186,000 = 0.53 seconds after you switched the light on. Galileo is reputed to have tried to use this method about 350 years ago to determine the speed of light, by having one observer uncover a lantern and another observer about a mile away uncover his lantern as soon as he saw the light. The first observer would record when he saw the light from the second lantern, and could in principle compute the time it took light to make the round trip, and hence the speed of light, taking into account the time it took to uncover the lanterns. However, since the round trip would take only a few-thousandths of a second, far less than the accuracy of the measurement, this experiment was doomed to failure.

Years later, in 1926, Albert Michelson used a variation of this technique to make the a precise measurement of the speed of light. He bounced a light beam off a rotating eight-side mirror on Mt. Wilson near Los Angeles, which was directed toward another mirror on Mt. San Antonio 35 kilometer (about 21 miles) away, where it was reflected back to Mt. Wilson. In this way he was able to measure the speed of light to an accuracy of 0.02 percent. In preparation for this measurement, Michelson arranged to have the distance between the two peaks surveyed to a precision of better than one centimeter (0.4 inch) !