Units Of Angle
The distances between objects in the night sky are measured as angles. An object on the horizon is said to lie at a distance of 90° from an object vertically above the observer, at the zenith.
A crude estimate of the distance between two objects may be found visually, by holding a fist at arm's length: for a typical adult the fist will measure around 10° across. The disk of the Moon measures almost exactly half a degree across.
For smaller distances, for example between the individual members of the Pleiades star cluster, it is convenient to use a measure of angle which is smaller than the degree: the arcminute is used, sometimes denoted ', which equals one sixtieth of a degree. The angular resolution of an unaided human eye, with near perfect eyesight, is around one to two arcminutes.
Still smaller angles are measured in arcseconds, sometimes denoted ", which equal one sixtieth of an arcminute, or one 3,600th of a degree. For example, the angular diameter of Jupiter varies between 30 and 50" depending upon its position in the night sky.
The smallest structures which can be seen through a conventional ground-based telescope, however large, are around one arcsecond across, as smaller structures are blurred by turbulence in the Earth's atmosphere. Some modern telescopes, however, can get around this limitation by the use of adaptive optics systems, or by being launched into space.