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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') :)

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closed as unclear what you're asking by TildalWave, MBR, Moriarty, Donald.McLean Jan 6 '14 at 12:28

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

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 5 '14 at 20:08
This sort of thing does happen with light. People do research on things called light echos (, 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 5 '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 5 '14 at 21:43
up vote 2 down vote accepted

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 and

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.

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