Space
Why Do Stars Twinkle? The Science of Atmospheric Seeing
Stars twinkle because Earth's turbulent atmosphere bends starlight in constantly shifting ways. Because stars are infinitely distant, they act as point sources, so even tiny bends in light rays cause dramatic brightness and color flickers. Planets, being much closer, show a small disk that averages out the jitter. Twinkling is worst near the horizon and is why the best observatories sit on mountaintops or in orbit.