astronomical seeing

Why do stars twinkle? Not because they flicker — they shine steadily — but because their light, after crossing light-years of empty space undisturbed, is wobbled in the last few kilometres by Earth's restless air. That same turbulence smears the image in a telescope, and astronomers call the resulting blur the seeing.

Air is never perfectly still; pockets of warmer and cooler air with slightly different densities bend light by tiny, ever-shifting amounts, like the shimmer above a hot road. A telescope therefore receives a star's light from a jittering, distorting wavefront, and a sharp point gets spread into a dancing blob. Seeing is measured as the width of that blob in arcseconds: a typical lowland site delivers seeing of 2 to 3 arcseconds, while the very best mountaintop observatories on dry, stable air enjoy seeing as fine as 0.4 arcseconds on rare nights.

Seeing is the great enemy of ground-based sharpness. A 10 m telescope has a diffraction limit near 0.01 arcseconds, but seeing of one arcsecond throws away a hundredfold of that potential. This is why observatories are built high on isolated peaks with smooth airflow, like Mauna Kea or the Chilean Andes; why we launch telescopes into space above the air entirely; and why adaptive optics was invented to measure and cancel the atmosphere's distortion in real time.