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Hey I want to describe an event A happening roughtly 5-7 after event B. I'd like to describe it as the 'A happened after the sound of B echo'd off X', where X is recognisable space thingy...

(I'm talking mildly poetically, so I'm aware there is no sound in space, but I'm interested in what X might be, if such a thing was possible - if it makes you feel better you can use light and I'll describe 'A happened before X saw B') :)

Joe
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    I feel like I had a cerebral event happen to me whilst reading your question. I think it happened before A, about 5-7, and it might have been echoed off X. Not sure, but I think it was B. – TildalWave Jan 05 '14 at 20:08
  • This sort of thing does happen with light. People do research on things called light echos (http://en.wikipedia.org/wiki/Light_echo), which occur when light from supernovae reflect off of nearby dust and gas. They can use the timing of the echo to map out distances to these reflective objects. – astromax Jan 05 '14 at 21:42
  • I would recommend asking a more clear and concise question, though. I'm not really understanding what you mean here. – astromax Jan 05 '14 at 21:43

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If there would be air at room temperature between Earth and Moon, sound would take about 28 days from Earth to Moon and echoed back, with helium instead of air 9 days, and with hydrogen about 7 days.

Calculated from http://www.phy.mtu.edu/~suits/SpeedofSoundOther.html and http://en.wikipedia.org/wiki/Moon

You could also reflect Earth light from Pluto back to Earth, 11 times this double Earth - Pluto distance to get about 5 days of travel of light, much depending of the orbital position of Pluto.

Gerald
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