When we look up at the night sky, one of the most familiar sights is the twinkling of stars. At first, it might seem as though the stars themselves are flickering, but in reality, the effect comes from here on Earth.

The light from a star travels across space in a perfectly straight path, but once it enters Earth’s atmosphere, things change. Our atmosphere is full of turbulent layers of air, each with varying temperatures and densities. These variations cause the air to bend, or refract, the incoming starlight in constantly shifting ways. As the beams of light are redirected ever so slightly before reaching our eyes, we perceive the star as flickering. This phenomenon is known scientifically as stellar scintillation.

We can observe that stars twinkle more when they are closer to the horizon. That’s because their light passes through a thicker slice of atmosphere compared to stars directly overhead. With more air to travel through, there are more chances for the light to be disturbed, which exaggerates the twinkling effect.

Interestingly, planets behave differently. Even though they shine by reflecting sunlight, they usually appear steady rather than twinkling. The reason lies in how large they look to us. Planets are much closer to Earth than stars, so instead of being seen as a single point of light, they appear as tiny disks in the sky. The atmospheric distortions affect different parts of that disk, but when averaged together, the changes cancel out, making planets shine with a steadier glow.

So when we watch stars twinkling above us, what we are really seeing is not a change in the stars themselves, but the restless movement of our own atmosphere. It’s a reminder that even something as distant as a star can reveal the dynamics of the air surrounding our planet.