by Nitpicker » Mon Jun 09, 2014 10:35 pm
Chris Peterson wrote:Nitpicker wrote:I was taught as a child that planets don't twinkle. It seems to be a common belief, almost a superstition. Since taking up astronomy as a hobby, I see twinkling planets (and stars) all the time. I prefer it when they don't twinkle.
It's still a good rule of thumb. Even under terrible seeing conditions (like I have under the jetstream here in Colorado) it is rare for a planet above 20 or 30 degrees altitude to twinkle anywhere near as much as a star. It only takes a few inch wide ray path through the atmosphere to significantly reduce or eliminate scintillation (which is why stars rarely twinkle when seen through binoculars).
When I've got a group outside under the stars, I explain where the scintillation comes from, and show them (if the right objects are present) how different planets and stars always appear if you look at them critically.
(Check out Jupiter in the evening sky right now- a brilliant evening "star", which even very low looks nothing like any of the bright stars around it, such as Procyon or Capella. They may all be twinkling, but there's an obvious qualitative difference between the planet and the stars. Obvious if you make the effort to actually observe, that is. Most people don't- most people outside forums like this, anyway. BTW, Mercury is visible now, as well. It's not bright enough, or contrasty enough against the light sky, to normally show twinkling, even right on the horizon.)
In my (more humid) sky, high jetstream velocities do not seem to correlate with twinkling stars and planets. I can observe almost no twinkling with my eyes, then set up my scope and observe planets that have a smeared (rather than wobbly) appearance, which does correlate with high jetstream velocities. Then, I can pick another time with low jetstream velocities and the stars are twinkling like crazy. Twinkling seems to be caused more by turbulence at lower altitudes. I have not studied this topic exhaustively, but I have read explanations which match my own observations.
I can generally pick the planets because I know where they are roughly in the sky at most times and I know their hues and brightness. But I don't think I'm very good at quantifying scintillation. Right at the moment, Jupiter has the same brightness as Sirius, both low in the West after sunset for me, and (if I'm not in my backyard) it is easy to confuse them in a humid sky, or through patchy cloud, or when constrained by trees such that only a small portion of the sky is visible.
[quote="Chris Peterson"][quote="Nitpicker"]I was taught as a child that planets don't twinkle. It seems to be a common belief, almost a superstition. Since taking up astronomy as a hobby, I see twinkling planets (and stars) all the time. I prefer it when they don't twinkle.[/quote]
It's still a good rule of thumb. Even under terrible seeing conditions (like I have under the jetstream here in Colorado) it is rare for a planet above 20 or 30 degrees altitude to twinkle anywhere near as much as a star. It only takes a few inch wide ray path through the atmosphere to significantly reduce or eliminate scintillation (which is why stars rarely twinkle when seen through binoculars).
When I've got a group outside under the stars, I explain where the scintillation comes from, and show them (if the right objects are present) how different planets and stars always appear if you look at them critically.
(Check out Jupiter in the evening sky right now- a brilliant evening "star", which even very low looks nothing like any of the bright stars around it, such as Procyon or Capella. They may all be twinkling, but there's an obvious qualitative difference between the planet and the stars. Obvious if you make the effort to actually observe, that is. Most people don't- most people outside forums like this, anyway. BTW, Mercury is visible now, as well. It's not bright enough, or contrasty enough against the light sky, to normally show twinkling, even right on the horizon.)[/quote]
In my (more humid) sky, high jetstream velocities do not seem to correlate with twinkling stars and planets. I can observe almost no twinkling with my eyes, then set up my scope and observe planets that have a smeared (rather than wobbly) appearance, which does correlate with high jetstream velocities. Then, I can pick another time with low jetstream velocities and the stars are twinkling like crazy. Twinkling seems to be caused more by turbulence at lower altitudes. I have not studied this topic exhaustively, but I have read explanations which match my own observations.
I can generally pick the planets because I know where they are roughly in the sky at most times and I know their hues and brightness. But I don't think I'm very good at quantifying scintillation. Right at the moment, Jupiter has the same brightness as Sirius, both low in the West after sunset for me, and (if I'm not in my backyard) it is easy to confuse them in a humid sky, or through patchy cloud, or when constrained by trees such that only a small portion of the sky is visible.