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The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its centre that is perpendicular to the viewing direction. Because of foreshortening, it may be quite different from the actual physical diameter for an object that is seen under an angle. For a spherical object at a large distance, the visual and actual diameters are the same.
The angular diameter of a flat circular object (disc) can be calculated using the formula:
in which is the angular diameter, and and are the actual diameter of and the distance to the object, expressed in the same units. When is much larger than , may be approximated by the formula , in which case the result is in radians.
For a round spherical object whose actual diameter equals , the angular diameter can be found with the formula:
The difference is due to when you look at a sphere, the edges are the tangent points, which are somewhat on your side of the facing hemisphere cross section. measures opposite/adjacent, whereas measures opposite/hypotenuse. For practical use, the distinction between the visual diameter and the actual diameter only makes a difference for spherical objects that are relatively close.
For very distant or stellar objects, the Small-angle approximation can also be used:
Which simplifies the above equations to:
- (for small )
Use in astronomy 
In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes. Since these angular diameters are typically small, it is common to present them in arcseconds. An arcsecond is 1/3600th of one degree, and a radian is 180/ degrees, so one radian equals 3600*180/ arcseconds, which is about 206265 arcseconds. Therefore, the angular diameter of an object with visual diameter d at a distance D, expressed in arcseconds, is given by:
- = 206265 d / D arcseconds.
The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2″ (two arcseconds).
The angular diameter of the Sun, from a distance of one light-year, is 0.03″, and that of the Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth.
This table shows the angular sizes of noteworthy celestial bodies as seen from the Earth:
|Celestial body||Angular diameter||Relative size (10 pixels per arcsecond)|
|Sun||31.6′ – 32.7′||28.7 – 29.7 times the maximum value for Venus (orange bar below) / 1896″ – 1962″|
|Moon||29.3′ – 34.1′||26.6 – 31.0 times the maximum value for Venus (orange bar below) / 1758″ – 2046″|
|Venus||9.565″ – 66.012″||
|Jupiter||29.800″ – 50.115″||
|Saturn||14.991″ – 20.790″||
|Mars||3.492″ – 25.113″||
|Mercury||4.535″ – 13.019″||
|Uranus||3.340″ – 4.084″||
|Neptune||2.179″ – 2.373″||
|Ceres||0.330″ – 0.840″||
|Vesta||0.20" – 0.64"||
|Pluto||0.063″ – 0.115″||
|R Doradus||0.052″ – 0.062″||
|Betelgeuse||0.049″ – 0.060″||
|Eris||0.034" – 0.089″||
|Alpha Centauri A||0.007″||
The table shows that the angular diameter of Sun, when seen from Earth is approximately 32 arcminutes (1920 arcseconds or 0.53 degrees), as illustrated above.
This meaning the angular diameter of the Sun is about 250,000 times that of Sirius (it has twice the diameter and the distance is 500,000 times as much; the Sun is 10,000,000,000 times as bright, corresponding to an angular diameter ratio of 100,000, so Sirius is roughly 6 times as bright per unit solid angle).
The angular diameter of the Sun is also about 250,000 that of Alpha Centauri A (it has the same diameter and the distance is 250,000 times as much; the Sun is 40,000,000,000 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).
The angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000 to 500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450–700, so a celestial body with a diameter of 2.5–4″ and the same brightness per unit solid angle would have the same brightness as the full Moon).
Even though Pluto is physically larger than Ceres, when viewed from Earth (e.g., through the Hubble Space Telescope) Ceres has a much larger apparent size.
While angular sizes measured in degrees are useful for larger patches of sky (in the constellation of Orion, for example, the three stars of the belt cover about 3 degrees of angular size), we need much finer units when talking about the angular size of galaxies, nebulae or other objects of the night sky.
Degrees, therefore, are subdivided as follows:
- 360 degrees (°) in a full circle
- 60 arc-minutes (′) in one degree
- 60 arc-seconds (″) in one arc-minute
To put this in perspective, the full moon viewed from earth is about 1⁄2 degree, or 30 arc minutes (or 1800 arc-seconds). The moon's motion across the sky can be measured in angular size: approximately 15 degrees every hour, or 15 arc-seconds per second. A one-mile-long line painted on the face of the moon would appear to us to be about one arc-second in length.
In astronomy, it is typically difficult to directly measure the distance to an object. But the object may have a known physical size (perhaps it is similar to a closer object with known distance) and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the Angular diameter distance to distant objects as
In non-Euclidean space, such as our expanding universe, the angular diameter distance is only one of several definitions of distance, so that there can be different "distances" to the same object. See Distance measures (cosmology).
Non-circular objects 
Many deep sky objects such as galaxies and nebulas are non-circular and are thus typically given two measures of diameter: Major Diameter and Minor Diameter. For example, the Small Magellanic Cloud has a visual apparent diameter of 5° 20′ × 3° 5′.
Defect of illumination 
Defect of illumination is the maximum angular width of the unilluminated part of a celestial body seen by a given observer. For example, if an object is 40 seconds of arc across and is 75 percent illuminated, the defect of illumination is 10 seconds of arc.
See also 
- Angular diameter distance
- Angular resolution
- Solid angle
- Visual acuity
- Visual angle
- Visual Angle Illusion