Angular diameter

apparent diameterangular sizeapparent sizeangular radiusapparent sizesdiameterangular distanceangular minuteappearsdiameter appears
The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view.wikipedia
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Visual angle

In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture (of a lens).
It also is called the object's angular size.

Angular aperture

aperture angle
In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture (of a lens).
The angular aperture of a lens is the angular size of the lens aperture as seen from the focal point:

Moon

lunarthe MoonLuna
The angular diameter of the Sun is about the same as that of the Moon.
The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter.

Milliradian

milangular milmrad
Estimates of angular diameter may be obtained by holding the hand at right angles to a fully extended arm, as shown in the figure.
Since a mil is an angular measurement, the subtension covered by a given angle (angular distance or angular diameter) increases with viewing distance to the target.

Angular diameter distance

angular scale versus redshift relation
In that case, the angular diameter formula can be inverted to yield the angular diameter distance to distant objects as
It is defined in terms of an object's physical size, x, and \theta the angular size of the object as viewed from earth.

Alpha Centauri

α CentauriAlphaα Centauri A
The angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A (it has about the same diameter and the distance is 250,000 times as much; the Sun is 4×10 10 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).
As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB, about four times the angular diameter of the Moon.

Andromeda Galaxy

AndromedaM31Andromeda Nebula
Applying trigonometry (angular diameter), this is equivalent to an apparent 4.96° angle in the sky.

Betelgeuse

α OriBetelg'''euseBetelgeuse mass loss
Subsequent studies have reported an angular diameter (apparent size) ranging from 0.042 to 0.056 arcseconds, with the differences ascribed to the non-sphericity, limb darkening, pulsations, and varying appearance at different wavelengths.

Earth

terrestrialworldGlobal
In astronomy, the sizes of celestial objects are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes.
The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.

Proxima Centauri

Alpha Proximaits host starProxima
Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly.

Uranus

Uranian34 TauriGeorgium Sidus
Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter.

Jupiter

Jovianplanet JupiterGiove
The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds.

R Doradus

R Dor
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.

Solid angle

angular areasolid angles4-π
(Sirius has twice the diameter and its distance is 500,000 times as much; the Sun is 10 10 times as bright, corresponding to an angular diameter ratio of 10 5, so Sirius is roughly 6 times as bright per unit solid angle.)
The Sun is seen from Earth at an average angular diameter of 0.5334 degrees or 9.310 radians.

Minute and second of arc

masarcsecondarc second
Since these angular diameters are typically small, it is common to present them in arcseconds .
The full moon's average apparent size is about 31 arcminutes (or 0.52°).

Mars

Martianplanet MarsRed Planet
Although Mars's moon Phobos appears about one-third the angular diameter of the full moon on Earth, Deimos appears more or less star-like, looking only slightly brighter than Venus does from Earth.

Deneb

Alpha Cygniα CygAlpha Cygni (Deneb)
Based on its temperature and luminosity, and also on direct measurements of its tiny angular diameter (a mere 0.002-second of arc), Deneb appears to have a diameter of about over 200 times that of the Sun; if placed at the center of the Solar System, Deneb would extend out to the orbit of the Earth.

Parsec

Mpcpckpc
an object of diameter 1 AU (149 597 871 km) at a distance of 1 parsec (pc)
A corollary states that a parsec is also the distance from which a disc one astronomical unit in diameter must be viewed for it to have an angular diameter of one arcsecond (by placing the observer at D and a diameter of the disc on ES).

Neptune

NeptunianNeptune-masseighth planet
Because of the distance of Neptune from Earth, its angular diameter only ranges from 2.2 to 2.4 arcseconds, the smallest of the Solar System planets.

Pluto

134340 Pluto(134340) Plutomass of Pluto
It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11".

Perceived visual angle

visual angle illusion
Visual Angle Illusion
Also, in astronomy, "apparent size" refers to the physical angle θ rather than to the subjective apparent visual angle θ′.

Visual acuity

20/20 visionvision20/20
Visual acuity
Angular diameter

Sphere

sphericalhemisphereglobose
The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view.

Circle

circularcircles360 degrees
The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view.