Why Stars Twinkle More Than Planets

The story

Twinkle, twinkle, little star. You have sung it. You have seen it: stars really do twinkle, trembling and sparkling like tiny lights that cannot hold still.

But here is a stargazer's secret. A few of the brightest lights in the sky usually twinkle much less than the rest. They shine calm and steady, like tiny lanterns. And those steady ones are often not stars at all. They are planets, our neighbours, like Venus and Jupiter. Once you know this clue, you can stand outside and make a very good guess about which bright lights are stars and which are planets, with nothing but your own two eyes.

So why does a star tremble while a planet stays calmer? The answer is not up there with the stars. It is right above your head. It is the AIR.

Between your eyes and space sits the atmosphere, the thick blanket of air wrapped around our whole planet. It looks empty, but it is always moving: warm air rising, cool air sinking, winds sliding over winds, layer upon restless layer. And when light passes through moving air, the air bends it, a tiny bit this way, a tiny bit that way, wobbling it around.

You have already seen this happen with water. Drop a coin into a pool and look down at it. The coin seems to wriggle and dance, because the moving water bends its light on the way up to your eyes. The air above you does the same thing to starlight. Every restless layer gives the light another tiny nudge, so the star seems to jitter and flash. Twinkling is not something stars do. It is something our air does to their light, on the very last stretch of its journey.

Then why do planets twinkle so much less? Here is the beautiful part. Stars are unimaginably far away, so far that even a mighty star shrinks to a single pinpoint of light, and a pinpoint is easy to nudge. Wobble it, and the whole thing jumps. But planets are our near neighbours. A planet is really a tiny circle of light in the sky, made of many pinpoints packed together, though your eyes see them blended into one. The air nudges each of those points differently, one flickering up while another flickers down, and the little wobbles largely cancel each other out. Steady, calm, lantern light. On a very unsettled sky, even a planet can shimmer a little, especially low near the horizon, but most of the time the calm ones give themselves away.

Astronomers, the scientists who study the sky, find the twinkling beautiful but bothersome, because wobbling light makes blurry pictures. So they build great telescopes on high mountaintops, above the thickest air, and they have sent powerful telescopes into space, where there is no air at all and the stars finally hold still.

So keep the clue in your pocket for the next clear evening sky. A trembling light is probably a faraway sun. And a bright, steady one may well be a neighbour, worth a quiet hello.