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steve001

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For how long do I have to aim a laser at the moon to see a dot?

Robert Frost, Aerospace engineer
Updated Jul 25, 2014
There are two possible questions here. The first is simply how long would it take for a laser to travel to the moon and back.
main-qimg-e5ccd263d9f26e3f988ed1ea3b04f6db

The moon is around 384,000 km away. The speed of light is 299,792,458 m/s. The journey is round trip, so:
main-qimg-8b9c4df3738e66cee3379a3a8f1f1df9

However, if the question really is about seeing the dot - you won't. Ever.

Don't go yet, there's more. Read on. https://www.quora.com/For-how-long-do-I-have-to-aim-a-laser-at-the-moon-to-see-a-dot
 





Last edited:
Crazlaser said:
No that says it takes 1.3 seconds for light from earths surface to reach us. Not for light to go to the moon from earth and then come back. So that's why it's 2.56, round to 2.6. :thinking:
Click to expand...

Darn it, forgot it was to the Moon & back. I apologize. :p

-Alex
 
diachi said:
If you hit the retro-mirror the Apollo astronauts left when they were there with enough power and a good enough detector you'll get a few photons back!
Click to expand...
Apparently 5-10 photons enter the telescope as the articles says. Sheesh, that's hardly anything. Wow.
 
Crazlaser said:
Apparently 5-10 photons enter the telescope as the articles says. Sheesh, that's hardly anything. Wow.
Click to expand...


Yep, it's amazing they can detect it at all actually.

Apollo14Reflectors.jpg


That's the retro-mirror with footprints for scale... So the beam has to travel ~384,000km there, and then ~384,000km back after being reflected off that tiny mirror.
 
Yeah, using those tiny retroreflectors is quite difficult.

But this does not mean the amateur scientist cannot use the moon as a reflector.

Radio amateurs have done so for decades, moonbouncing signals that do take the 2+ second delay to return to earth. Path losses are large, but with basically the entire moon acting as a reflector instead of a small artefact left there, the signals are usable for communication.

Intestingly this does not have to be hyper-expensive as optical experiments would be.

Equipment to get a reliable moonbouce would be under $10.000 a station (including both transmitter and antenna system, assemby required on the latter!). You could do it with a single station just listening for your own reflection a few seconds later, though that wouldn't make interesting conversation ;)
 
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