Why do stars twinkle but not planets?Posted on June 23rd, 2010 No comments
First correct answer was from @mlv: Planets don’t twinkle because of their larger apparent size.
The twinkle phenomenon, technically “atmospheric scintillation,” is caused by shifting layers of atmosphere distorting light from a star. A star has infinitesimal cross-sectional area as seen from Earth. Therefore, even a slight change in atmosphere causes noticeable distortion. A planet has a finite cross-sectional area as seen from Earth. Atmospheric distortions within this small area tend to average out.
Picture the photons traveling from a star, through the Earth’s atmosphere to your eye. The beam of light that actually reaches your retina is so small in diameter that even a few extra molecules of air shifting in or out of it will make a slight difference in its position and brighness. And in the miles of atmosphere you’re looking through, there are uncountable billions of molecules of air even along that infinitesimally narrow beam of light. There are plenty of opportunities for that small cylinder of air to fluctuate by a few billions of molcules here and there. The result is that the star appears to twinkle.
In the case of a planet, the beam of light between the planet and your eye is of finite diamter. It’s pretty small, but not infinitesimal. Within each infinitesimal cross-section of that beam, the same sort of shifting of molecules and “twinkling” phenomenon occurs. However, there are enough of these tiny “twinkling” beams of light that, overall, they average out to a fairly steady position and brightness of the planet as seen from Earth.
The actual effect of the unsteadiness of the atmosphere on the light from a planet is that the details become fuzzy. There is a limit to how much detail you can see on a planet’s surface, no matter how powerful your telescope, as long as that telescope is inside the Earth’s atmosphere. Better to put your telescope into orbit. (Actually, most telescopes in Earth orbit are not used to observe planets, but more distant objects. But when they do point at planets, the images can be quite spectacular.)
Some sources say it is a myth that planets don’t twinkle. I say they’re splitting hairs. Yes, under extremely unstable seeing conditions but absent clouds, planets do twinkle. Yes, under extremely good seeing conditions, stars do not twinkle. But most of the time that the atmosphere is unstable enough to make planets twinkle, the sky is obscured by clouds. In the most common viewing conditions, if the atmosphere is clear enough to see stars, the stars twinkle and the planets don’t. In the most common viewing conditions, an otherwise untutored observer can tell stars from planets fairly reliably by their twinkling or not.
More about twinkling stars: http://en.wikipedia.org/wiki/Scintillation_(astronomy)
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