page_title: Angular diameter
section_title: Use in astronomy
hierarchical_section_title: Angular diameter / Use in astronomy
caption_attribution_description: Deutsch: Hochkontrastbild der Jupiterscheibe mit den vier Galileischen Monden Io, Ganymed, Europa und Kallisto (rechts oben bei maximaler Elongation) im Verhältnis zum Vollmond bei der Begegnung am 10. April 2017 im Sternbild Jungfrau. Der Winkeldurchmesser der Mondscheibe beträgt zirka 30 Bogenminuten, der Winkeldurchmesser der Umlaufbahn von Kallisto ungefähr 20 Bogenminuten, und der Winkeldurchmesser der Jupiterscheibe etwa 3/4 Bogenminuten. Das originale Bild im Bildseitenverhältnis 4:3 (5184 x 3888 Bildpunkte) wurde quadratisch auf 3200 x 3200 Bildpunkte beschnitten, die Bilddigonale des Ausschnitts beträgt 3,3°. Die Auflösungsgrenze des menschlichen Auges liegt bei etwa einer Bogenminute (= 60 Bogensekunden), und die horizontale respektive die vertikale Winkelgröße eines Bildpunktes beträgt 2,6 Bogensekunden. English: This HDR image shows the apparent size of Jupiter with its four Galilean moons – Io, Ganymede, Europa, and Callisto (near maximum elongation), respectively – compared to that of the full Moon. The photo was taken when Jupiter and the Moon were around conjunction on 10 April 2017, in the constellation Virgo. The angular diameter of the Moon's disk is about 30' (arcminutes), the angular radius of Callisto's orbit is about 10', and the angular diameter of Jupiter's disk is about 3/4'. The original image with an aspect ratio of 4:3 was quadratically cropped to 3200 × 3200 pixels, and the diagonal angle of view of the image is 3.3°. The resolution limit of the human eye is at about 1' (or 60"), the horizontal respectively vertical angular size of the quadratic pixels is 2.6'.
context_page_description: 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. In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side. Angular radius equals half of the angular diameter.
context_section_description: 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. Since these angular diameters are typically small, it is common to present them in arcseconds (″). An arcsecond is 1/3600th of one degree (1°) and a radian is 180/ degrees. So one radian equals 3,600*180/ arcseconds, which is about 206,265 arcseconds (1 rad ≈ 206,264.806247"). Therefore, the angular diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by:
  .
  These objects have an angular diameter of 1″:
  an object of diameter 1 cm at a distance of 2.06 km
  an object of diameter 725.27 km at a distance of 1 astronomical unit (AU)
  an object of diameter 45 866 916 km at 1 light-year
  an object of diameter 1 AU (149 597 871 km) at a distance of 1 parsec (pc)
  Thus, the angular diameter of Earth's orbit around the Sun as viewed from a distance of 1 pc is 2″, as 1 AU is the mean radius of Earth's orbit.
  The angular diameter of the Sun, from a distance of one light-year, is 0.03″, and that of Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is approximately the same as that of a human body at a distance of the diameter of Earth.
  This table shows the angular sizes of noteworthy celestial bodies as seen from Earth:
  The table shows that the angular diameter of Sun, when seen from Earth is approximately 32′ (1920″ or 0.53°), as illustrated above.
  Thus the angular diameter of the Sun is about 250,000 times that of Sirius. (Sirius has twice the diameter and its distance is 500,000 times as much; the Sun is 10¹⁰ times as bright, corresponding to an angular diameter ratio of 10⁵, so Sirius is roughly 6 times as bright per unit solid angle.)
  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 4×10¹⁰ 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 Sun's diameter is 400 times as large and its 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 to 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.
  Angular sizes measured in degrees are useful for larger patches of sky. (For example, the three stars of the Belt cover about 4.5° of angular size.) However, much finer units are needed to measure the angular sizes 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 as viewed from Earth is about ​¹⁄₂°, or 30′ (or 1800″). The Moon's motion across the sky can be measured in angular size: approximately 15° every hour, or 15″ per second. A one-mile-long line painted on the face of the Moon would appear from Earth to be about 1″ in length.
  In astronomy, it is typically difficult to directly measure the distance to an object, yet 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).
caption: This photo compares the apparent sizes of Jupiter and its four Galilean moons (Callisto at maximum elongation) with the apparent diameter of the full Moon during their conjunction on 10 April 2017.
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