APOD: Red Moon, Green Beam (2014 Apr 18)

Comments and questions about the APOD on the main view screen.
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Chris Peterson
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Chris Peterson » Wed Apr 23, 2014 4:30 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.
You are only seeing a few thousand meters of illuminated air (mainly dust).
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Anthony Barreiro
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Anthony Barreiro » Wed Apr 23, 2014 5:14 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.
It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Chris Peterson » Wed Apr 23, 2014 5:53 pm

Anthony Barreiro wrote:It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!
Indeed, for all the stars ever pointed out with laser pointers- thousands and thousands- the beams are still on their way towards those stars. If we could somehow see them, the night would be permanently crossed with green lines.
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Anthony Barreiro
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Anthony Barreiro » Wed Apr 23, 2014 6:01 pm

Chris Peterson wrote:
Anthony Barreiro wrote:It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!
Indeed, for all the stars ever pointed out with laser pointers- thousands and thousands- the beams are still on their way towards those stars. If we could somehow see them, the night would be permanently crossed with green lines.
And if there are planets orbiting those stars, and if some of those planets have appropriately reflective surfaces at exactly the right angle, maybe someday we'll see a photon or two of 532 nm (plus redshift or minus blueshift) light reflected back at us!
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Nitpicker » Wed Apr 23, 2014 9:21 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.
It isn't. The beam becomes invisible a little before the Apollo 15 landing site. It looks visible for almost the whole distance, simply due to the perspective.

(BTW, I wrote earlier that the gratifying thing was it appeared the beam was pointing almost directly toward the landing site. I said this because I could imagine an image taken from a different angle, where the beam might appear less accurate.)

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Chris Peterson
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Chris Peterson » Wed Apr 23, 2014 10:27 pm

Nitpicker wrote:It isn't. The beam becomes invisible a little before the Apollo 15 landing site.
If we're picking nits, I'd say the beam becomes invisible about 385,000 km before the Apollo 15 landing site.
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Re: APOD: Red Moon, Green Beam (2014 Apr 18)

Post by Nitpicker » Wed Apr 23, 2014 11:01 pm

Chris Peterson wrote:
Nitpicker wrote:It isn't. The beam becomes invisible a little before the Apollo 15 landing site.
If we're picking nits, I'd say the beam becomes invisible about 385,000 km before the Apollo 15 landing site.
I was referring to how the beam appears in the APOD, but by all means, pick away. If you had not clipped out the rest of my comment, my meaning would have been more clear, I think.

(I also failed to see the answers by you and Anthony, to Claudio's question, before I responded.)