Green laser as sky pointer - parallax issues?

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.

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.

You can see the dot on the crystal dome holding the stars…

Actually, the green lasers are bright enough to see the beam itself.

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 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.

I don’t know about parallax issues, but if you’re near an airport, you may find you’ll have FAA issues that I’m sure you’re aware of.

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?

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.

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!!