James Frazer, of Dunedin, asks :-

Why do stars twinkle but planets dont?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

It isn't an infallible rule but is true most of the time.

Stars twinkle because of the mixing of warm and cold air just above us. Cold air bends (refracts) light more than warm air. So when the two mix light rays passing through get bent to and fro. The effect is obvious when one looks across a beach or road on a hot day.

To understand how stars twinkle, think of light patterns on the bottom of a bath or pond caused by ripples on the surface. Water bends light much more than air, so ripples on water greatly effect light passing through them. If the light source is tiny then the ripples are very sharply defined. A fish or underwater swimmer sees the light twinkle.

If the light source is bigger, like the sun, then the ripples are much blurrier. The light still brightens and fades as ripples distort it, but much less so than the tiny light source. Seen from under the water the big light doesn't twinkle as much.

So it is with the air. Stars, though they are big fiery objects like the sun, are so far away that they appear as very tiny lights. So they are greatly affected by the warm and cold `ripples' in the air and they twinkle.

To the eye planets look just like stars but in binoculars or telescopes they show a small disk. The disk blurs the effect of the air currents, so planets twinkle less than stars.