Minute and second of arc

masarcsecondarc secondarcminuteminutesarcsecondsMOAarcminutesmilliarcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to 1⁄60 of one degree.wikipedia
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Nautical mile

nautical milesnmnmi.
The nautical mile was originally defined as a minute of latitude on a hypothetical spherical Earth so the actual Earth circumference is very near 21 600 nautical miles.
Historically, it was defined as one minute (1⁄60 of a degree) of latitude along any line of longitude.

Angular unit

angular measurementsangularmultiples of
A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to 1⁄60 of one degree.
Subdivisions of the degree are minute (symbol ', 1' = 1/60°) and second {symbol ", 1" = 1/3600°}.

Degree (angle)

°degreesdegree
A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to 1⁄60 of one degree.
Timocharis, Aristarchus, Aristillus, Archimedes, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minutes.

Prime (symbol)

prime symbolprimeDouble prime
The standard symbol for marking the arcminute is the prime (U+2032), though a single quote (U+0027) is commonly used where only ASCII characters are permitted.
The prime symbol is commonly used to represent feet (ft) and arcminutes (arcmin).

Parsec

Mpcpckpc
The unit of distance, the parsec, named from the parallax of one arc second, was developed for such parallax measurements.
A parsec is obtained by the use of parallax and trigonometry, and is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, ie.

Angular diameter

apparent diameterangular sizeapparent size
The full moon's average apparent size is about 31 arcminutes (or 0.52°). Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red giant with a diameter of 0.05 arcsecond.
Since these angular diameters are typically small, it is common to present them in arcseconds .

Degree symbol

°degree signdegree
The addition of minute and second of arc units follow the degree units, with intervening spaces between the units but no spaces between the numbers and arc symbols, e.g. 30° 12′ 5″.

Hubble Space Telescope

HubbleHSTHubble Telescope
For example, the Hubble Space Telescope can reach an angular size of stars down to about 0.1″.
At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m in diameter.

Celestial navigation

astronavigationnavigationcelestial
In celestial navigation, seconds of arc are rarely used in calculations, the preference usually being for degrees, minutes and decimals of a minute, for example, written as 42° 25.32′ or 42° 25.322′.
Navigators measure distance on the globe in degrees, arcminutes and arcseconds.

Light-year

light yearlight yearsMly
The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).

Eiffel Tower

Tour Eiffelthe Eiffel Towera tower
A milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City.
The task of drawing the components was complicated by the complex angles involved in the design and the degree of precision required: the position of rivet holes was specified to within 1 mm and angles worked out to one second of arc.

Sexagesimal

sexagesimal systembase 60base-60
These units originated in Babylonian astronomy as sexagesimal subdivisions of the degree; they are used in fields that involve very small angles, such as astronomy, optometry, ophthalmology, optics, navigation, land surveying, and marksmanship.
There are 60 minutes of arc in a degree, and 60 arcseconds in a minute.

Declination

DecDec.declinations
In the ecliptic coordinate system, latitude and longitude ; in the horizon system, altitude (Alt) and azimuth (Az); and in the equatorial coordinate system, declination, are all measured in degrees, arcminutes and arcseconds.
Any units of angular measure can be used for declination, but it is customarily measured in the degrees, minutes, and seconds of sexagesimal measure, with 90° equivalent to a quarter circle.

Proper motion

proper motionsproper-motionhigh proper motion star
The arcsecond is also often used to describe small astronomical angles such as the angular diameters of planets (e.g. the angular diameter of Venus which varies between 10″ and 60″), the proper motion of stars, the separation of components of binary star systems, and parallax, the small change of position of a star in the course of a year or of a solar system body as the Earth rotates.
It has dimensions of angle per time, typically arcseconds per year or milliarcseconds per year.

Astronomical unit

AUastronomical unitsAUs
The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of 1 arcsecond.

Parallax

trigonometric parallaxsolar parallaxmotion parallax
The arcsecond is also often used to describe small astronomical angles such as the angular diameters of planets (e.g. the angular diameter of Venus which varies between 10″ and 60″), the proper motion of stars, the separation of components of binary star systems, and parallax, the small change of position of a star in the course of a year or of a solar system body as the Earth rotates.
The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond.

Geographical mile

mile (geographical)miles
At sea level one minute of arc along the equator or a meridian (indeed, any great circle) equals exactly one geographical mile along the Earth's equator or approximately 1 nmi.
The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator.

Babylonian astronomy

Babylonian astronomersBabylonianastronomer
These units originated in Babylonian astronomy as sexagesimal subdivisions of the degree; they are used in fields that involve very small angles, such as astronomy, optometry, ophthalmology, optics, navigation, land surveying, and marksmanship.

Radian

radiansradmicroradian
A minute of arc is undefined⁄10,800 of a radian.
More common is arc second, which is undefined⁄648,000 rad (around 4.8481 microradians).

Astrometry

astrometricastrometristastrometrical
The ESA astrometric satellite Gaia, launched in 2013, can approximate star positions to 7 microarcseconds (µas).
Like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Beg's catalogue is estimated to have been precise to within approximately 20 minutes of arc.

Astronomical seeing

seeingatmospheric seeingatmospheric turbulence
Because of 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.
The best conditions give a seeing disk diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma.

Gaia (spacecraft)

GaiaGaia Data Release 2Gaia spacecraft
The ESA astrometric satellite Gaia, launched in 2013, can approximate star positions to 7 microarcseconds (µas).

Latitude

latitudesSouthlatitudinal
Positions are traditionally given using degrees, minutes, and seconds of arcs for latitude, the arc north or south of the equator, and for longitude, the arc east or west of the Prime Meridian.
It is measured in degrees, minutes and seconds or decimal degrees, north or south of the equator.

R Doradus

R Dor
Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red giant with a diameter of 0.05 arcsecond.
Having a uniform disk diameter of 0.057 ± 0.005 arcsec, it is currently believed to be the extrasolar star with the largest apparent size as viewed from Earth.

Adaptive optics

adaptive optics (AO)AOadaptive optic
Adaptive optics, for example, can produce images around 0.05 arcsecond on a 10 m class telescope.
For example, an 8–10 m telescope (like the VLT or Keck) can produce AO-corrected images with an angular resolution of 30–60 milliarcsecond (mas) resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.