Angular diameter

The angular diameter of an object as seen from a given position is the diameter measured as an angle. It satisfies the formula delta = rctan (diameter/ distance).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In astronomy the size of objects in the sky are often measured in terms of their angular diameter as seen from Earth, rather than their actual size.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2" (two arcseconds).

Related Topics:
Parsec - Arcsecond

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The angular diameter of the Sun, from a distance of one lightyear, is 0.03", of the Earth 0.0003".

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This table shows the angular sizes of the most important Solar System bodies.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Sun30'

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Moon29' - 33'

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Venus10" - 58"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Jupiter 32" - 49"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Saturn 16" - 20"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Mars 4" - 16"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Uranus 3" - 4"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Neptune 2"

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Alpha Centauri A: ca. 0.007"
• Sirius: ca. 0.007"

Thus the angular diameter of the Sun is ca. 250,000 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 ca. 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 200,000-500,000 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).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

 
Angular diameter: See also ►