# Green laser as sky pointer - parallax issues? **URL:** \ **Category:** Factual Questions\ **Created:** [September 6, 2007, 9:52pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296 "2007-09-06T21:52:40Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [September 6, 2007, 9:52pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/1 "2007-09-06T21:52:40Z")
If I’m using a green laser pointer to indicate stars and planets for the benefit of others, do I have to worry about parallax for observers a few yards away from me? Might I try to point at Castor, but someone 20 feet away see the line go to Pollux? I can’t really do the math, because I don’t know how far away the apparent “end” of the visible beam is. And that’s a good question all its own: how long is the visible beam from, say, a 15 mW green pointer, for someone viewing from very near the source (maximum back scattering), and how might this drop off as the vieing angle increases? I realize that the first question could be answered with a simple back yard experiment, but I think it’s going to be cloudy for a couple days here. ---
**Author:** ![sturmhauke](https://avatars.discourse-cdn.com/v4/letter/s/e47c2d/32.png) [@sturmhauke](https://boards.straightdope.com/u/sturmhauke)\ **Post date:** [September 6, 2007, 11:05pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/2 "2007-09-06T23:05:56Z")
Laser beams are only visible when something scatters the light into your eye, such as dust. As such, using a laser pointer to point to things in the sky (as opposed to on a screen or something) isn’t going to be much more effective than using your finger. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 6, 2007, 11:10pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/3 "2007-09-06T23:10:30Z")
You can see the dot on the crystal dome holding the stars… Actually, the green lasers are bright enough to see the beam itself. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [September 6, 2007, 11:16pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/4 "2007-09-06T23:16:04Z")
[QUOTE=sturmhauke] Laser beams are only visible when something scatters the light into your eye, such as dust. [/QUOTE] I’m aware. I’m also aware that my green laser pointer _does_ produce a visible beam in dark skies, not unlike a _really_ long light saber. I saw it…I was there. Here’s a [picture](http://www.kk.org/cooltools/archives/green_laser.png) of the effect. It may be photoshopped for all I know, but it’s definitely representative of the effect. The reason the beam can be visible midair is because it is scattered by dust, and even air itself. Since green laser light is near the peak sensitivity frequency for the human eye, the scattered light is easily visible in the dark. Green also scatters in clear air much better than red, so that helps. I imagine those new blue lasers must scatter really well. ---
**Author:** ![postcards](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@postcards](https://boards.straightdope.com/u/postcards)\ **Post date:** [September 6, 2007, 11:37pm UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/5 "2007-09-06T23:37:22Z")
I don’t know about parallax issues, but if you’re near an airport, you may find you’ll have [FAA issues](http://www.skyandtelescope.com/observing/3309906.html?page=1&c=y) that I’m sure you’re aware of. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [September 7, 2007, 12:00am UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/6 "2007-09-07T00:00:02Z")
That’s a good point in the context of laser safety. I do take care when I’m using it outside (seriously or just goofing off) to never point it near an airplane. I don’t imagine it could even reach an airliner at cruising altitude with any appreciable power, but I still treat it as if it will slice a wing right off, even for the high ones. Soo…any ideas about the parallax question? ---
**Author:** ![sturmhauke](https://avatars.discourse-cdn.com/v4/letter/s/e47c2d/32.png) [@sturmhauke](https://boards.straightdope.com/u/sturmhauke)\ **Post date:** [September 7, 2007, 12:14am UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/7 "2007-09-07T00:14:14Z")
OK, so green lasers are more visible than I thought. The link says that the visible length of the beam depends on sky conditions, so let’s say it’s 500 meters. 20 feet is about 6 meters. This works out to a parallax of about 0.7 degrees, although this will vary depending on the elevation of the star you are pointing at and where the other person is standing relative to you. ---
**Author:** ![exastris](https://avatars.discourse-cdn.com/v4/letter/e/b38774/32.png) [@exastris](https://boards.straightdope.com/u/exastris)\ **Post date:** [September 7, 2007, 6:46am UTC](https://boards.straightdope.com/t/green-laser-as-sky-pointer-parallax-issues/418296/8 "2007-09-07T06:46:43Z")
Andecdotal data here: I used one for stargazing nights many times. I usually needed to gather people around me or make my way through the crowd, pointing every so often, so they could see exactly which star I was talking about. They are waaaaay cool, by the way. If you have tall children or irritating adults around, they’re going to want to put their hands in front of it while it’s on. When your eyes are dark adjusted, that laser pointer reflecting off of bare skin is BRIGHT!!