One sixtieth of an arcminute, or 1/3,600 of a degree, written ″: the apparent size of a coin about 2.5 cm wide seen from about 5 km away. Jupiter spans 30 to 50″, Mars 25″ at most. Seeing in western Europe often spreads stars over 2 to 3″.
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Related terms
- Arcminute
One sixtieth of a degree, written ′. The Moon is about 31′ across; a good eye separates two points about 1′ apart, like Mizar and Alcor (12′). Nebulae and galaxies measure a few arcminutes.
- Resolving power
The smallest separation between two stars that an instrument can show as distinct, in arcseconds. It depends only on the aperture: about 1.2″ for 100 mm and 0.6″ for 200 mm. Seeing often prevents you from reaching it.
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- Sampling
The angle of sky covered by one pixel, in arcseconds per pixel: 206.265 × pixel size (µm) ÷ focal length (mm). 3.76 µm pixels behind a 500 mm focal length give 1.55″/px. For deep sky, aim for about a third to a half of the seeing (often 1 to 2″/px).
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