Why do planets not twinkle? (Intermediate)

I am trying to figure out exactly why planets do not twinkle or twinkle as much as stars. I read the response that was given to an earlier question about why stars twinkle and planets do not, which stated:

The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, presenting a relatively stable image to the eye.

Can you explain "averages out" a little further. And do planets ever twinkle.

I can be more specific, sure. Remember that just like a computer screen, your eye is composed of a certain number of "pixels," each represented by a single light-receptor cell in your retina. If two (or more) points of light are close enough such that they are focused on the same receptor cell in your eye, you will experience them as a single point of light. This is referred to as the "resolution" of your eye, or any telescope for that matter.

Now, a star on the sky is in a true sense a single point of light. All the light comes through the atmosphere in exactly the same direction, through exactly the same atmospheric turbulence, and thus is bent in exactly the same way. So when it gets to your eye, the amount of light you see coherently varies. It also strikes (primarily) only a single receptor in your eye.

On the other hand, light from a planet is different. Each of a few receptors in your eye sees a large number of light rays coming from the planet, each of which has been bent differently be the atmosphere (since the planet has size on the sky, they are arriving in slightly different directions). Some of these rays will become brighter, some dimmer. But because they all illuminate the same receptor in your eye, that receptor only sees the total amount of light hitting it. There will be about the same number of enhanced rays as dimmed rays, so you experience a steady light, not a twinkle.

No, planets never twinkle to the naked eye for exactly this reason. If you look at one through a magnifying telescope, though, the telescope can have better resolution than the coherently refracting length of the atmosphere. In this case, you can see the edges of the planet "wiggling."

Note added July 2003 by Dave Rothstein: Strictly speaking, it is possible for planets to twinkle to the naked eye, but only under rare conditions when the Earth's atmosphere is extremely turbulent. See this page from Bad Astronomy for more information.

This page was last updated on July 18, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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