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Question
There was a major collision of an asteroid with the Moon in medieval times. It was described by monks at Canterbury Cathedral in England as a red glow on and around the Moon. How long after the asteroid hit the Moon, which is $3.84 \times 10^5 \textrm{ km}$ away, would the light first arrive on Earth?
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Final Answer
$1.28 \textrm{ s}$
Solution Video

OpenStax College Physics Solution, Chapter 25, Problem 9 (Problems & Exercises) (0:49)

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Video Transcript
This is College Physics Answers with Shaun Dychko.  We want to know how much time between an asteroid colliding with the moon would elapse before someone on earth would see that collision. And, we can say that the distance the light travels from the moon to the earth equals the speed of light in a vacuum multiplied by time. And so, we'll divide both sides by C to solve for T. So, T is distance over speed. So, that's 3.84 times ten to the five kilometers between the earth and the moon but then we multiply it by ten to the three due to the prefix kilo. So, this is really times ten to the eight. And then, we're dividing by the speed of light three times ten to the eight meters per second, giving a time of 1.28 seconds. So, if an asteroid hit the moon, we would see that collision, 1.28 seconds, after if occurred.