Why Don't Planets Twinkle?
You've no doubt heard that you can differentiate a star from a planet because stars twinkle and planets don't. Actually, planets also twinkle. Since stars are point sources of light, the atmosphere causes their light to constantly bend this way and that, causing the twinkling. But think of a planet as a collection of many point sources that don't always "twinkle" in unison. As an extended source, some of a planet's light usually comes straight through to your eye. At one moment it might be the center of the planet, at another it might be one edge, and at another moment some other part of the planet will reach your eye. The larger the planet's angular diameter, the greater the likelihood that there will not be any twinkling. But if the air is sufficiently turbulent, a planet will be seen to twinkle. In this case, the angular displacement by the refraction of light from moment to moment exceeds the angular diameter of the planet.