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.
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- Degree
The basic unit of angle on the sky: a full turn of the horizon is 360°, and there are 90° from horizon to zenith. The full Moon is about 0.5° across; the Plough about 25°. A degree divides into 60 arcminutes.
- Arcsecond
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″.
- Angular distance
The gap between two objects measured in degrees on the sky. Your hand at arm’s length is a handy ruler: the little finger covers about 1°, three fingers 5°, a fist 10°, an outstretched hand about 20°. The two front stars of the Plough are a little more than 5° apart.
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