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I was watching Venus with the naked eye yesterday at about 7 pm and I noticed that it was flickering, almost like a star.

I have always learned that planets don't flicker to the naked eye, only stars (indeed, every other time I've seen a planet it wasn't flickering), so I was rather confused. I even checked Stellarium to make sure that what I was seeing was, in fact, Venus.

Does anybody have an explanation for that behaviour?

Thank you all.

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  • $\begingroup$ Very poor seeing would do it. en.wikipedia.org/wiki/Astronomical_seeing $\endgroup$ – asawyer Apr 13 '15 at 19:58
  • $\begingroup$ Personally I've seen Mars scintillate almost every time I've watched it, so I can tell you that planets are capable of flickering. In Mars' case it's more of a change in color than a change in brightness; I'm not sure why. I've never seen Venus scintillate, but then I don't go looking for Venus very much at all. I've never seen Jupiter or Saturn scintillate though, probably because I only notice them when they're high in the sky at night. $\endgroup$ – DrZ214 Feb 3 '16 at 0:55
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Because planets actually do twinkle. Most people were told that the major difference between stars and planets is that only the former twinkle - but that's an oversimplification. Given the right conditions, planets will twinkle too, it just happens more rarely.

Several factors that contribute to it:

  • lots of air turbulence; or, as astronomers call it, "bad seeing"

  • closeness to horizon; if the planets are high in the sky, the air column is shorter so there's less chance they will twinkle; but when they are low, their light goes through more air and so it is perturbed to a larger degree

The observation you've made, Venus twinkling, is not very unusual. Many stargazers are used to seeing that once in a while. I've seen Venus scintillate several times in the past, always at sunset when it was about to drop below horizon; I would presume you could see the same behavior very early in the morning as Venus has just risen.

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It's true that planets usually don't flicker, or twinkle (or scincillate, as astronomers like to call it). The reason is that they are close enough that they are actually seen as a disk with a larger diameter than the atmosphere can "wash out" (the seeing disk). Stars, on the other hand, are point sources, and this point jumps back and forth on the sky, as the light takes slightly different paths through the atmosphere. This is what we see as twinkling.

I can think of three reasons that may make a planet twinkle nevertheless:

  1. If it stands very close to the horizon, so that the total column of air that the light passees (the "airmass") is very high. If this is the case, it should also look a bit redder than usual.

  2. If observed close to some building that emits heat, like an exhaust, so that the light passes through parcels of hot and cold air on small scales.

  3. If an airplane passed by, emitting hot air.

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  • $\begingroup$ The planet at an elevation of about 20-25 degrees; would that be enough? It wasn't redder than usual, though. Items 2 or 3 are for sure not true in this case. $\endgroup$ – TomCho Apr 13 '15 at 18:56
  • $\begingroup$ Hmm… at that elevation, the air mass is 2-3. Pretty high for astronomical standards, but the angular size of Venus should still be larger than the seeing disk. The only other reason I can think of is small patches of clouds. This could make its brightness vary, which could perhaps be perceived as flickering. $\endgroup$ – pela Apr 13 '15 at 20:46

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