Old 01-27-2017, 08:47 PM #1
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Default Just for the fun of it.

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.

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

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-d...n-to-see-a-dot


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Old 01-27-2017, 09:12 PM #2
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Default Re: Just for the fun of it.

But,,, can you actually get the beam to bounce back?
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Old 01-27-2017, 09:19 PM #3
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Default Re: Just for the fun of it.

Actually I believe it takes light about 1.3 seconds to come back to Earth, not 2.3 seconds.

https://www.google.com/webhp?sourcei...to+reach+earth

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Old 01-27-2017, 09:30 PM #4
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Default Re: Just for the fun of it.

Quote:
Originally Posted by Hap View Post
Actually I believe it takes light about 1.3 seconds to come back to Earth, not 2.3 seconds.

https://www.google.com/webhp?sourcei...to+reach+earth

-Alex
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.
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Old 01-27-2017, 09:42 PM #5
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Default Re: Just for the fun of it.

Quote:
Originally Posted by Crazlaser View Post
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.
Darn it, forgot it was to the Moon & back. I apologize.

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Old 01-27-2017, 09:52 PM #6
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Default Re: Just for the fun of it.

Quote:
Originally Posted by ElectricPlasma View Post
But,,, can you actually get the beam to bounce back?
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!
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Old 01-27-2017, 09:54 PM #7
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Default Re: Just for the fun of it.

Quote:
Originally Posted by diachi View Post
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!
Apparently 5-10 photons enter the telescope as the articles says. Sheesh, that's hardly anything. Wow.
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Old 01-27-2017, 09:59 PM #8
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Default Re: Just for the fun of it.

Quote:
Originally Posted by Crazlaser View Post
Apparently 5-10 photons enter the telescope as the articles says. Sheesh, that's hardly anything. Wow.

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



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.
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Old 01-28-2017, 12:40 AM #9
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Default Re: Just for the fun of it.

I recall from Sam's Laser FAQ a fellow using a 25W yag way back in the early 70's shining it at one of the retroreflectors and was able to see naked eye winkling. I'm sure the story is still there.

Along the same line. https://science.nasa.gov/science-new...03jun_oldrover

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Old 01-28-2017, 12:44 AM #10
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Default Re: Just for the fun of it.

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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