Twinkling

In simple terms, twinkling of stars is caused by the passing of light through different layers of a turbulent atmosphere. Most scintillation effects are caused by anomalous atmospheric refraction caused by small-scale fluctuations in air density usually related to temperature gradients. Scintillation effects are always much more pronounced near the horizon than near the zenith (directly overhead), since light rays near the horizon must have longer paths through the atmosphere before reaching the observer. Atmospheric twinkling is measured quantitatively using a scintillometer.

The effects of twinkling are reduced by using a larger receiver aperture; this effect is known as aperture averaging. Many modern large telescopes also use adaptive optical systems which precisely deform the figure of a mirror in order to compensate for scintillation.

While light from stars and other astronomical objects is likely to twinkle, twinkling usually does not cause images of planets to flicker appreciably. Stars twinkle because they are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence, which acts like lenses and prisms diverting the light's path. Large astronomical objects closer to Earth, like the Moon and other planets, can be resolved as objects with observable diameters. With multiple observed points of light traversing the atmosphere, their light's deviations average out and the viewer perceives less variation in light coming from them.