Why stars twinkle and planets do not

Look at a clear night and the brightest points are steady while the fainter ones flicker. That is not a property of the stars. It is a property of how much of the sky each one’s light passes through — and how big the source is.

Point sources versus discs

A star is far too small to resolve. Even the nearest, Proxima Centauri, has an angular diameter of about 0.001 arcseconds; the largest stars as seen from Earth are under 0.05 arcseconds. Both are thousands of times smaller than the atmosphere’s own blurring, which is typically 1 to 2 arcseconds.

So a star’s light arrives through a single narrow column of air. If the whole column is pushed one way by a temperature gradient, all of the light is pushed with it — sometimes into your pupil, sometimes partly out. That is the flicker.

A planet is different. Jupiter is about 40 arcseconds across at opposition, Venus up to 60. Those are tens of times larger than a turbulence cell, so the light reaching your eye has passed through hundreds of independent cells at once. Each cell pushes its own patch of the disc, and the eye integrates over the whole thing, which averages the motion out.

The rule is therefore about angular size, not about whether the object is a star. A planet low on the horizon — where you are looking through five or ten times as much air — will twinkle like a star. Sirius, which is bright enough to almost resolve at times, twinkles spectacularly and flashes colors when low.

Where the colors come from

The coloured flashes near the horizon have a second cause: refraction depends on wavelength. Blue light is bent more than red, so different colors from the same star are deflected by slightly different amounts by the same turbulent cell. As the cell moves, the star appears to change color rather than just brightness.

That is the same physics as the green flash at sunset, at a much smaller scale.

Seeing is not transparency

Two independent qualities decide whether a night is good:

• Transparency — how much light gets through. Affected by haze, dust, water vapour and smoke. Good transparency means faint objects are visible.
• Seeing — how steady the image is. Affected by turbulence, wind shear and temperature gradients. Good seeing means sharp stars and detailed planetary views.

They often oppose each other. A hazy, humid night can have excellent seeing because the air is stable; a crystal-clear night after a cold front often has terrible seeing because the air is churning.

For planets you want seeing. For galaxies and nebulae you want transparency and darkness. This is why observers check two forecasts, and why “clear” in a weather app tells you almost nothing.

Why telescopes end up on mountains

Turbulence is worst near the ground and near temperature gradients. A mountain site is above the worst of the boundary layer, and a site with laminar airflow — where the wind comes off the sea or a smooth plateau — gives steadier images than one downwind of rough terrain.

Adaptive optics takes the next step: measure the distortion with a reference star and deform a mirror to correct it, thousands of times a second. That is how ground-based telescopes now match or beat space telescopes on some measurements, at the cost of enormous complexity.

And this is why the atmosphere is worth understanding even if you only look up with your eyes: the twinkling you see is a direct readout of how much air has been stirred between you and everything above it.

Angular diameters are the standard published values. Seeing figures are typical rather than site-specific.