Minute and second of arc

masarcsecondarc secondarcminutearcsecondsMOAarcminutesminutesmilliarcsecond
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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Parsec

Mpcpckpc
an object of diameter one astronomical unit (149,597,871 km) at a distance of one parsec.
A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 1⁄648000 astronomical units.

Prime (symbol)

prime symbolprimeprime mark
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), arcminutes (am), and minutes (min).

Angular unit

angular measurementsangular
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°}.

Hubble Space Telescope

HubbleHSTNASA/ESA HST
Hubble Space Telescope has calculational resolution of 0.05 arcseconds and actual resolution of almost 0.1 arcseconds, which is close to the diffraction limit.
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.

Light-year

light yearlight yearsly
an object of diameter 45,866,916 km at one light-year,
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).

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 supergiant with a diameter of 0.05 arcsecond.
Since these angular diameters are typically small, it is common to present them in arcseconds .

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.

Eiffel Tower

Tour Eiffelthe Eiffel TowerJules Verne
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 0.1 mm and angles worked out to one second of arc.

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 one nautical mile (1852 meters, or ≈1.15078 statute miles).
The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator.

Nautical mile

nmnautical milesnmi.
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 one nautical mile (1852 meters, or ≈1.15078 statute miles).
A nautical mile (also known as an air mile) is a unit of measurement defined as exactly 1852 m. Historically, it was defined as one minute of latitude, which is one sixtieth of a degree of latitude.

Degree symbol

°degreedegree sign
In the case of degrees of angular arc, the degree symbol follows the number without any intervening space (e.g., 30°). 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″).

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.

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.

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.

Parallax

solar parallaxmotion parallaxtrigonometric 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.

Astronomical unit

AUastronomical unitsAUs
an object of diameter 725.27 km at a distance of one astronomical unit,
The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of 1 arcsecond.

Latitude

Southlatitudeslatitudinal
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.

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.

Astrometry

astrometricastrometricalastrometrist
The ESA astrometric space probe 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.

Gaia (spacecraft)

GaiaGaia spacecraftGaia'' spacecraft
The ESA astrometric space probe Gaia, launched in 2013, can approximate star positions to 7 microarcseconds (µas).
Determine the position, parallax, and annual proper motion of 1 billion stars with an accuracy of about 20 microarcseconds (µas) at 15 mag, and 200 µas at 20 mag.

Adaptive optics

AOadaptive optics (AO)adaptive 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.

Mile

mimilesstatute mile
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 one nautical mile (1852 meters, or ≈1.15078 statute miles).
It extended the Roman mile to fit an astronomical approximation of 1 arcminute of latitude measured directly north-and-south along a meridian.

R Doradus

R Dor
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 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.

Equator

equatorial planeequatorial
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 one nautical mile (1852 meters, or ≈1.15078 statute miles).
The geographical mile is defined as one arc-minute of the Equator, so it has different values depending on which radius is assumed.

M24 Sniper Weapon System

M24M24 Sniper RifleM24 SWS
For example, Remington's M24 Sniper Weapon System is required to shoot 0.8 MOA or better, or be rejected.
The radial distance from the calculated center of impact of the first target compared to the calculated center of impacts of the subsequent targets shall be less or equal to 1.086 MOA (3.3 inches @ 300 yards, 2.2 inches @ 200 yards, 2.4 inches @ 200 meters) on an average basis.