Minute and second of arc Minute and second of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn – it is for this reason that the Earth's circumference is almost exactly 21,600 nautical miles. A minute of arc is of a radian. A second of arc, arcsecond (arcsec), or arc second is of an arcminute, of a degree, of a turn, and (about ) of a radian. These units originated in Babylonian astronomy
History of longitude A decade later, Nevil Maskelyne, who as the newly appointed Astronomer Royal was on the Board of Longitude, armed with Mayer's tables and after his own experiments at sea trying out the lunar distance method, proposed annual publication of pre-calculated lunar distance predictions in an official nautical almanac for the purpose of finding longitude at sea. Being very enthusiastic for the lunar distance method, Maskelyne and his team of computers worked feverishly through the year 1766, preparing tables for the new Nautical Almanac and Astronomical Ephemeris. Published first with data for the year 1767, it included daily tables of the