Astronomy

Arcsecond

Angular measurement unit (1/3600 of a degree)

Definition

An arcsecond (") is a unit of angular measurement equal to 1/3600 of a degree. Jupiter appears about 45" across at opposition, Saturn 20", Mars up to 25". Seeing is measured in arcseconds - 1" seeing is excellent, 4" is poor. Understanding arcseconds helps match telescope resolution to camera pixel size (sampling).

Related Terms

Seeing

Atmospheric turbulence affecting image sharpness

Opposition

When a planet is closest to Earth and brightest

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