Apparent sizes of astronomical objects are often expressed in angular measure. So, for example, the Moon and Sun both appear to be about a half degree, or 30 minutes (') of arc, in diameter. A minute of arc of latitude is by definition exactly equal to a nautical mile (6076 feet). So the angle subtended by the solar or lunar disc is about 30 NM at the Earth's surface, as the projection onto the map of metro London indicates.

Astronomical almanacs and catalogs usually tabulate the angular sizes of planetary discs and nebulae and the separations of binary stars in seconds of arc (") and the resolving power of astronomical telescopes is also given in the same units. The resolving power of the unaided human eye is about a 1', or 60". My 100mm aperture refractor has a maximum practical resolving power of about 1.16". Binary stars with a separation less than that cannot be "split" (resolved) by my scope. A second of arc at Earth's surface subtends a distance of about a hundred feet.

The astronomical rule of thumb for telescope resolving power is given by Dawes' Limit, or 116/D seconds of arc, where D is in mm of aperture. Atmospheric conditions usually restrict resolution to about 0.5", regardless of the telescope used.