Question

How long does it take for moonlight to reach the earth?

Asked by: Cara

Answer

Light travels through space at just over 186,000 miles per second. The moon is just under 250,000 miles from Earth, so light from the Moon's surface has to travel more than one second (about 1.3 seconds) to reach us.

Among other things, the astronauts left reflecting mirrors on the moon. By shining a laser beam on those mirrors from Earth, and measuring the time (less than 3 seconds) it takes to see its reflection, the length of the round trip can be easily calculated to within a few inches!

If you've watched any of the videos of the moon landings, you might have noticed that the radio responses from the moon walking astronauts sometimes included a delayed echo of the questions. That also was a result of the 3 seconds it takes for a radio signal to travel to and from the moon, since radio waves are another form of light waves.

Since moonlight is reflected sunlight, it originates at the Sun's surface 93 million miles from the Earth/Moon system. It takes about 8 minutes for that trip, so the short, final hop from the Moon to the Earth is trivial by comparison.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

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