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.
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.
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:
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.
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.
If an airplane passed by, emitting hot air.
The other answers try to explain based on atmospherics alone, without stopping to consider the following:
While the outer planets (Mars and Jupiter et al.) are always almost fully illuminated, the inner planets Mercury and Venus are often crescents and so at least in one direction -- parallel to the ecliptic -- they can have nearly stellar sizes of only a few arcsecs.
This means that at these times we should expect them to twinkle unless observing under incredibly good conditions. And of course they are thinnest when closest to the Sun and so will be low to the horizon after sunset when their light travels through an enormous amount of air and temperatures are changing rapidly, so sort-of maximum convection/turbulence.
For more on how Venus's dance changes distance from Earth and illuminated fraction in such a way that it's almost always between -3.8 and -4.8 magnitude, see answers to
above: Phases of Venus, originally from Wikimedia but annotated.
For three times in 2004 (Feb 27, May 7, and June 8) I've taken the angular diameters of Venus from JPL's Horizons and using the largest (June 8) as a calibration for the image shown the full diameter of Venus.
Then I've drawn a thin rectangle to eyeball the width of the bright part of the limb that would contribute most of the light. The annotated numbers (15/5, 40/3, 58/1) are the apparent diameters of the full disk and the width of the bright limb in arcseconds.
