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Why do stars twinkle… but not planets?

A star is so far away it's just a point of light.

As that light crosses a hundred kilometres of turbulent air, every tenth of a second the point gets nudged left, right, brighter, dimmer.

That's what twinkling is.

A planet, though, reaches us as a tiny disc.

The atmosphere shakes it too — but each point across that disc gets pushed in a different direction.

They cancel each other out.

The disc holds steady.

Astronomers call this *atmospheric seeing*.

To get around it, they climb to high altitude, correct in real time with deformable mirrors… or leave the atmosphere altogether.

The star hasn't moved.

It's the air between you and it… that shimmers.