arcminute

The arcminute is the unit eyesight is graded in. The 20/20 line of a Snellen chart is constructed so that the strokes of its letters subtend one arcminute at the viewing distance, with the whole letter subtending five, which makes one arcminute the nominal resolving power of a healthy eye. Astronomy uses the same unit for things you can just see: the Moon and the Sun each span about 30 arcminutes, and their near-equality in the sky is the only reason total solar eclipses exist, with the Moon's apparent diameter swinging between roughly 29.4 and 33.5 arcminutes as its distance varies.

Shooters know the unit as MOA. Rifle-scope turrets click in quarters of a minute of angle, and the trade's rule that 1 MOA is one inch at 100 yards is conveniently close without being exact: the true figure is 1.047 inches. Navigation leans on the identity in the origin note, one arcminute of latitude to one nautical mile, which is what makes a chart's latitude scale double as its distance scale. In the site's angle conversions the arcminute is the middle link of the sexagesimal chain, sixty to the degree and sixty arcseconds to each arcminute, the Babylonian inheritance the degree page describes.