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A night-sky flicker

Why Stars Twinkle

Stars shine perfectly steady. Our wobbly air does the twinkling.

1 Steadystarlight2 Wobbly air3 Twinkle
The twinkle happens in the last few miles, not out in space.

  1. Step 1 of 5

    A star's light is rock steady.

    A star Above the airSeen from space, stars do not twinkle at all

    A star is an enormous ball of burning gas pouring out light without a flicker. Out in space, that light travels in a perfectly straight line.

  2. Step 2 of 5

    Then it hits our air.

    Spaceno air, nobendingCold airlight bends onewayWarm airlight bends theotherYoulookingup

    Earth's air is not one smooth blanket. It is layers of warm and cool air, sliding and churning all the time.

  3. Step 3 of 5

    Warm and cold air bend light differently.

    Still air Light arrives straightChurning air Light arrives from awobbling direction

    Light changes direction slightly when it passes from cool air into warm air. Because those pockets keep moving, the bending keeps changing.

  4. Step 4 of 5

    So the star seems to jump and flicker.

    1 One steadypoint2 Bent this wayand that3 Looks likeflickering

    The star's light lands a hair to the left, then a hair to the right, dozens of times a second. Your eye reads that dance as twinkling.

  5. Step 5 of 5

    Planets barely twinkle at all.

    A star One point — wobblesshowA planet A tiny disc — wobblesaverage out
    If it shines steadily, it is probably a planet.

    A star is so far away it is a single pinpoint, easy to jiggle. A planet is close enough to be a tiny disc — many points at once, and the wobbles cancel out.

    This is exactly why big telescopes sit on high mountains — less air to look through.

The short version

Stars do not twinkle. Our restless air makes their light wobble on the way to your eye.

Try it yourself

Look at a streetlight far down a road on a hot day. The shimmer above the tarmac is the same trick, close up.

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