Why do stars twinkle but planets mostly don't?

Stars twinkle because they are so far away that their light reaches you as a single point, and Earth's atmosphere is full of moving pockets of warmer and cooler air that bend that light slightly, again and again. A point has nothing to average with, so it flickers. Planets are close enough to show a tiny disc, and the flickers from different parts of the disc cancel out. Astronomers call the brightness flicker scintillation.

What is the air doing to the light?

Light bends when it passes between air of different density. It is the same effect, refraction, that makes a straw look bent in a glass of water. The atmosphere above you isn't one smooth layer. It is stirred into cells of slightly different temperature, and so slightly different density, which drift and churn. NASA's Ask an Astrophysicist answer describes it directly. The density contrast between those cells causes refraction, so a point-like star seems to jump around, and your eye reads that as twinkling.

Much of the effect starts high up. A 2019 study of scintillation in astronomical measurements traces it to turbulence high in the air, and names the troposphere, the lowest layer of the atmosphere, which reaches up to about 10 kilometres, as the origin of most of it.

A good mental picture is a coin at the bottom of a swimming pool on a breezy day. The ripples on the surface keep refocusing the light, so the coin shimmers and wobbles even though it is lying perfectly still. You are the one at the bottom of the pool, looking up through about ten kilometres of rippling air.

The amount of air matters. NASA's StarChild page notes that stars close to the horizon twinkle more, because there is a lot more atmosphere between you and them. Look straight up and you look through the least.

How small does a star actually look?

For decades the star with the largest apparent size was Betelgeuse, measured in the 1920s at 0.044 seconds of arc. In 1997, ESO announced that R Doradus beats it at about 0.057 seconds of arc. A second of arc is 1/3600 of a degree. Those are the biggest-looking stars in the sky. Almost all the others are smaller still.

Now compare that with the blurring from the air. NASA's answer says the seeing, the atmosphere's smearing of a star's image, can be as good as about half a second of arc at the best observatory sites, and two seconds counts as bad. So even the largest-looking star is several times smaller than the blur at the very best sites. To the air, every star is a point.

Why do planets mostly shine steadily?

Planets are much nearer, so they look much bigger. NASA's Jupiter fact sheet gives its apparent diameter from Earth as between about 30.5 and 50.1 seconds of arc. That is dozens of times wider than typical seeing. You may not be able to make out the disc with your eyes, but the averaging happens anyway.

EarthSky explains it this way. Light from one edge of a planet's disc is bent one way while light from another part is bent another way, and the zigs and zags cancel each other out. Think of the disc as a crowd of points, each twinkling on its own schedule. One point flickers wildly. A thousand of them, flickering out of step, add up to a fairly steady glow.

Press Play, then switch from a point to a disc and watch the brightness bar. Drag sideways or use Turn to see how the paths cross different patches of air.

Big telescopes benefit from the same averaging. In the 2019 study's formula, scintillation weakens as the telescope's aperture grows. One way to see it is that a wider mirror takes in light that has crossed many patches of turbulent air at once.

Planets don't always shine steadily, though. EarthSky notes that planets low in the sky can twinkle too, since their light is crossing so much more air.

Scintillation and seeing are not quite the same

Astronomers split twinkling into two effects. Scintillation is the change in brightness, the flicker. Seeing is the blurring and jumping of the image, and NASA describes twinkling as closely related to it. For people measuring how bright a star is, scintillation can be the dominant source of noise from the ground, according to the 2019 study. For people trying to take sharp pictures, seeing is the enemy.

ESO says turbulence makes stars twinkle "in a way that delights poets but frustrates astronomers". There are two ways around it. One is to leave the atmosphere, which is why a star seen from space doesn't twinkle at all. NASA's Astronomy Picture of the Day once showed a twinkling Betelgeuse with the note that from space it would look steady. The other is adaptive optics, where a computer-controlled mirror changes shape in real time to undo the distortion, using a bright reference star or an artificial one made with a laser shone into the upper atmosphere.

Stars shine steadily, and the air adds the flicker

It isn't. NASA's StarChild page points out that most stars actually shine with a steady light. The twinkle is added in the last few tens of kilometres of a very long trip. The light crosses all that empty space unscrambled and gets jostled only at the very end, in our air.

A smaller mix-up is the rule "if it twinkles it's a star, if it doesn't it's a planet". It is a decent rule of thumb high in the sky. Near the horizon it fails, because planets twinkle there too.

What about the Moon and the space station?

The Moon is the extreme case. NASA's fact sheet gives its mean apparent diameter as about 1,896 seconds of arc, roughly half a degree. That is a huge disc by these standards, and as a whole it doesn't twinkle. The air still bends its light, but there are far too many points across the disc for any one flicker to show.

The International Space Station is a useful test of the idea, because it is close but small. NASA gives its length as 109 metres end to end and its average altitude as about 250 miles, roughly 400 kilometres. Work out the angle and, when it passes overhead, it spans roughly one minute of arc, about the size of Jupiter's disc at its biggest. The Planetary Society describes it as a bright, solid light gliding across the sky. Distance doesn't decide twinkling. Apparent size does, and the station's light still crosses the whole atmosphere on its way down. One caveat. The station's steadiness fits the disc explanation, but we haven't found a study that measured its scintillation directly.

Bendy mirrors and shimmering roads

The deeper thread is adaptive optics, a mirror that reshapes itself fast enough to take the twinkle back out. The sideways one is the shimmer over a hot road, which is the same bending of light by uneven air, much closer to the ground. Learnfold can start a course from either and add a linked page for each question you follow, much like following a curiosity rabbit hole, or what bees see that you can't if eyes interest you more than air. Figures like the arc-second sizes above are easy for an AI tutor to get wrong, so check them against a source.