"Is That a Star, or a Planet?"

Occasionally, you'll notice a passerby spotting a particularly bright star and asking, "Is that a star, or a planet?"

Here's how to tell the difference--and why there's a difference.

In real short form, the difference is that stars twinkle, while planets shine steadily.

Now, why is that?

The answer lies in the apparent diameter of a planetary disc, versus the apparent diameter of a 'turbulence cell' in the upper atmosphere.

Let's go back to the beginning. In any amateur telescope, a star is an infinitesimal point of light. You can magnify it to the limits imposed by the physical parameters of your telescope; but unless you're using a huge, research-grade instrument, you'll never get a measurable disc from a star. What you see in your telescope is a 'diffraction disk,' whose size is a direct measure of the perfection of your instrument; the smaller, the better.

A planet, however, being much smaller than any star, but also much, much, much nearer, displays a measurable disc. Some--Jupiter, Venus, Saturn, for example--show a measurable disc in binoculars. Some people with very sharp eyesight can actually see for themselves. The point is that a planetary disc is much larger than a star's.

Now we must shift our attention to the upper atmosphere, where cells of air are shifting up and down and otherwise roiling with motion. On most nights, the cells tend to be of on the order of a metre or two in diameter. From the ground, that makes their apparent diameter larger than a star's, but much smaller than a planet's.

So, a star--that point source of light--is affected by every turbulence cell between you and it, and it will twinkle in all but the most exceptionally stable sky. In a telescope, you'll find that it will even change apparent position minutely, from instant to instant, the result of refraction effects.

With a planet, it's very much different. The entire planetary disc is covered by dozens, even hundreds, of individual turbulence cells; their combined effect is negligible.

That's not to say that a particularly unstable sky won't still ruin planetary images. In 'bad seeing,' the planetary disc will appear 'smeared,' with lower definition, at times misshapen and shimmering, because of all those turbulence cells roiling away.

So now you know; and now you know why. Another Useless Fact for your exploitation. You're welcome again. ;-)

-Bill