Define: Minutes of Arc

A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60) of one degree. [1] Since one degree is defined as one three hundred sixtieth (1/360) of a circle, 1 MOA is 1/21600 of the amount of arc in a closed circle. It is used in those fields which require a unit for the expression of small angles, such as astronomy or marksmanship.

Cartography
Minutes of angle (and its subunit, seconds of angle or SOA—equal to a sixtieth of a MOA) are also used in cartography and navigation. At sea level, one minute of angle (around the equator or a meridian) equals about 1.86 km or 1.15 miles, approximately one nautical mile (approximately, because the Earth is slightly oblate); a second of angle is one sixtieth of this amount: about 30 meters or 100 feet.

Traditionally positions are given using degrees, minutes, and seconds of angles in two measurements: one for latitude, the angle north or south of the equator; and one for longitude, the angle east or west of the Prime Meridian. Using this method, any position on or above the Earth's reference ellipsoid can be precisely given. However, because of the somewhat clumsy base-60 nature of MOA and SOA, many people now prefer to give positions using degrees only, expressed in decimal form to an equal amount of precision. Degrees, given to three decimal places (1/1000 of a degree), have about 1/4 the precision as degrees-minutes-seconds (1/3600 of a degree), and so identify locations within about 120 meters or 400 feet.

Astronomy
The arcminute and arcsecond are also used in astronomy. Degrees (and therefore arcminutes) are used to measure declination, or angular distance north or south of the celestial equator. The arcsecond is also often used to describe parallax, due to very small parallax angles, and tiny angular diameters (e.g. Venus varies between 10″ and 60″). The parallax, proper motion and angular diameter of a star may also be written in milliarcseconds (mas), or thousandths of an arcsecond. The parsec gets its name from "parallax second", for those arcseconds.

Apart from the sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcseconds. Due to the effects of atmospheric seeing, ground-based telescopes will smear the image of a star to an angular diameter of about 0.5 arcsecond; in poor seeing conditions this increases to 1.5 arcseconds or even more.

Space telescopes are not affected by the Earth's atmosphere, but are diffraction limited; for example the Hubble space telescope can reach an angular size of stars down to about 0.1". Techniques exist for improving seeing on the ground, for example adaptive optics, which can give images around 0.05 arcsecond on a 10 m class telescope.

Human vision
In humans, 20/20 vision is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc.

Source: Wiki