<p>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.</p>

<p>Calculated from <a href="http://www.phy.mtu.edu/~suits/SpeedofSoundOther.html" rel="nofollow">http://www.phy.mtu.edu/~suits/SpeedofSoundOther.html</a> and <a href="http://en.wikipedia.org/wiki/Moon" rel="nofollow">http://en.wikipedia.org/wiki/Moon</a></p>

<p>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.</p>

CC BY-SA 3.0
2014-01-06T06:10:39.253Z
2014-01-06T06:10:39.253Z
Gerald
<p>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... </p>

<p>(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') :) </p>

CC BY-SA 3.0
2014-01-05T20:08:41.280Z
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
CC BY-SA 3.0
2014-01-05T21:42:28.247Z
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
CC BY-SA 3.0
2014-01-05T21:43:29.890Z
I would recommend asking a more clear and concise question, though. I'm not really understanding what you mean here.
astromax
<space>
(Sound) Echo from X takes 5-7 days,
Joe
# (Sound) Echo from X takes 5-7 days,

(Asked by: username_0 on 2014-01-05)

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') :)

**Tags:** space
> **Comments:**
> **username_1**: 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.  
> **username_2**: 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.  
> **username_2**: I would recommend asking a more clear and concise question, though. I'm not really understanding what you mean here.  

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## 1 answers

### Answer by username_3 on 2014-01-06. Score: 2 | Accepted answer

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.


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