For Chilean astrophotographer Osvaldo Castillo, the night sky above the European Southern Observatory's (ESO) Paranal Observatory held a breathtaking sight.

"I couldn't believe I was photographing a circumpolar startrail in Paranal; without a doubt, one of the most incredible experiences I've had as a photographer," said Castillo in a recent ESO article.

The photograph reveals a hypnotic circular ballet of starlight turning slowly across the night sky.

What is it?

Star trails are the visible imprint of Earth's rotation. As our planet spins around its axis, the stars appear to sweep across the sky in long, circular arcs. When a camera takes repeated long-exposure images over hours and the frames are later stacked together, these arcs become luminous trails revealing the circular (appare

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