Explain why the planets do not twinkle.

solution:
The apparent size of stars is very small as compared to apparent size of planets. Thus, the star may be considered as a ‘point sized’ source of light and the planet as an ‘extended source’ of light. So, the planet can be considered as a collection of large number of ‘point sized’ sources of light, such that the dimming effect of some ‘point sources’ is nullified by the brighter effect of the other ‘point sources’. The variable atmospheric conditions are unable to create variations in light flux from the planet entering our eye and thus, planets do not twinkle.