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.