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• Norway 2012

Why stars twinkle

Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle." This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained.

Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. 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.

The change in colour seen when stars twinkle is because of scintillation ("Twinkling") as the light passes through the atmosphere of the Earth. As the air moves in and out, the starlight is refracted, often different colors in different directions. Because of this "chromatic abberation," stars can appear to change colors when they are twinkling strongly.

Why don't planets twinkle?

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 by 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 (usually) 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."

However, 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