The smallest angular separation between two point sources that a telescope can distinguish as separate objects. Measured in arcseconds, it determines how much fine detail an instrument can resolve and is fundamentally limited by the telescope's aperture size and the wavelength of light observed.
From Latin 'angulus' (corner, angle) and 'resolutio' (a loosening, solving). The concept emerged in the 19th century as telescope makers sought to quantify their instruments' ability to separate close double stars, leading to precise mathematical definitions of optical performance based on wave theory.
Angular resolution is why bigger telescopes see sharper - the Hubble Space Telescope's 2.4-meter mirror can resolve details as small as 0.05 arcseconds, equivalent to reading a book from 40 kilometers away! Interestingly, your eye has an angular resolution of about 60 arcseconds, which means even a modest 6-inch telescope can see details 60 times finer than human vision.
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