Why do stars twinkle and planets do not?
In fact, both stars and planets twinkle! The twinkling is due to the turbulent air in the Earth's atmosphere, blurring and distorting the image of the star. The twinkling therefore has more of an effect nearer to the horizon, where the light must travel through more of the densest parts of the Earth's atmosphere. You can see this for yourself! Compare the twinkling of a star near the horizon (such as Sirius), and one close to zenith (straight up). Objects such as the Sun, Moon and the planets are called extended sources, because the light is emitted from a disc. Objects such as distant stars are called point sources, because they appear to be a point as they are very far away. In fact, the light from extended sources can be thought of as many point sources spread over an area. The turbulent air in the atmosphere causes a point source to appear to move around on the sky ever so slightly. However if we spread many point sources over the face of the planet, all point sources move around, but we do not notice a change in the total light from the object.