Why Do Stars Twinkle?

Why do stars twinkle?

Tom Callen, Cosmonova.

Why stars twinkle Tom Callen / Cosmonova

Though it wouldn’t work so well in the nursery rhyme, a there is actually a technical term for when stars twinkle: astronomical scintillation, an effect due to our planet’s atmosphere. As light travels through the blanket of air around our planet, it is diffracted (bounced around) causing a quick apparent dimming and brightening — a star's signature "twinkle".

While some stars do physically change in brightness over time, they typically do so on long timescales — amateur astronomers monitor these changes sometimes over hours, but more often over days, weeks, or years. These variable stars are well studied and often signal complex physical changes happening to the stars in question.

The more rapid changes of scintillation, on the other hand, come about long after the light has left the star. Light waves traveling through Earth’s atmosphere diffract as they pass through pockets of air at different temperatures. Because the light waves come from a single point, this effect can make the star’s brightness and/or position appear to change.

Why Don't Planets Twinkle Too?

Unlike stars, planets don't twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Because all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).

The much closer planets appear instead as tiny disks in the sky (a distinction more easily discerned with a telescope than with the naked eye). Their apparent sizes are usually larger than the pockets of air that would distort their light, so the diffractions cancel out and the effects of astronomical scintillation are negligible.


Star wheels will help you find your way among the twinkling constellations, and you can trace the appearance of the planets along the ecliptic with a Skygazer's Almanac.

One thought on “Why Do Stars Twinkle?

  1. Astrodomical

    I had a random thought, maybe someone out there has a better idea. Another reason stars twinkle might be because of objects traveling in the lights path. With this sort of occurance happening randomly every minute, every second. We could possibly looking at a very busy space, instead of the predictable space we know now. Anyone?

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