Scintillation

Scintillation is the sparkling or twinkling effect noticeable in stars, and, since it is much more apparent when the star is on the horizon than when it is near the zenith, the effect is attributed to the earth's atmosphere. It is commonly stated that this scintillation distinguishes stars from planets, but some of the planets have been observed to scintillate very slightly when near the horizon, though the phenomenon is not of frequent occurrence. All the stars are so far off that they have no sensible disc even when viewed through a powerful telescope; hence they may be considered as single points of light. The star will therefore send out very few rays of light to the eye compared to the number from the relatively larger planet, and so these few rays will exhibit the effect produced by a heterogeneous atmosphere, wheteas the average effect on the greater number of planet rays will be constant. The planet will therefore give a steady light, whereas the star varies every moment.