Why do stars twinkle, but planets do not?

Stars twinkle: Top half of Earth with a cloud covering it that represents the atmosphere. There is a straight line to the top and a line doing zigzag to the left.
The more atmosphere you are peering through, the more stars (or planets) appear to twinkle. Read below why stars twinkle, but planets do not. Chart by AstroBob. Used with permission.

Stars twinkle, while planets (usually) shine steadily. Why?

Stars twinkle because they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star. As a star’s light pierces our atmosphere, it’s refracted – causing it to change direction slightly – by the various temperature and density layers in Earth’s atmosphere. You might think of it as the light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didn’t have an atmosphere.

Astronomers use the term “scintillation” to describe the twinkling of stars.

On the other hand, planets are a more steady light

Planets shine more steadily because they’re closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. You can see planets as disks if you looked through a telescope, while stars remain pinpoints. That’s because Earth’s atmosphere refracts the light from these little disks as it travels toward our eyes. But – while the light from one edge of a planet’s disk might “zig” one way – light from the opposite edge of the disk might be “zagging” in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and that’s why planets appear to shine steadily.

You might see planets twinkling if you spot them low in the sky. That’s because, in the direction of any horizon, you’re looking through more atmosphere than when you look overhead.

If you could see stars and planets from outer space, both would shine steadily. There’d be no atmosphere to disturb the steady streaming of their light.

Can you figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers? Experienced observers often can, but, at first, if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star.

Bottom line: Explanation of why stars twinkle in the night sky but planets do not.

Read more: Flashing star? Here are 3 candidates

June 28, 2023

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