A measure of atmospheric turbulence affecting astronomical observations, typically expressed in arcseconds as the apparent size of a point source's disk due to atmospheric blurring. Better seeing means sharper, steadier images.
From Old English 'seon' meaning 'to see', adapted in the mid-20th century by astronomers to quantify atmospheric image quality. The technical usage evolved from the general concept of visibility to a precise measurement of optical performance.
Excellent astronomical seeing is rarer than perfect weather! Even on crystal-clear nights, atmospheric turbulence can make stars appear to dance and blur, which is why the world's best observatories are built on remote mountaintops where the air flows smoothly.
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