To Twinkle or Not

Date: August 1-15
Time: 8:15 pm, one hour after sunset
Place: the western sky

You probably wouldn’t notice this unless you looked for it, but stars twinkle and planets don’t.

Low in the west you can see a prime example of each. Spica (a star) and Jupiter (a planet) are the same pale bluish color, about the same brightness, and about the same elevation above the horizon. But while Spica twinkles, Jupiter shines with a more steady light. If you look for this subtle visual effect you will easily see it. The explanation is really quite simple, but difficult to show without an animation.

The stars you can see are, on average, 10,000 times farther away than the planets. At this distance they are seen only as “points,” even when magnified by telescopes. Planets, being very much closer, are seen as “discs,” albeit tiny ones.

When light enters our atmosphere it will experience refraction, or bending, as it passes through different layers of air. Since the light rays from a point are thin lines, and the light rays from a disc are much thicker, the latter is less likely to miss your eye when it bends and shifts around. It’s like the difference between trying to aim a squirt gun and a garden hose on a windy day. Twinkling is what you see when the light rays are hitting and missing your eye.