Angular diameter measures the size an object appears to be, proportional to its actual size divided by distance. Astronomers use arcseconds and radians to measure objects in space, and telescopes can view objects as narrow as 0.1 arcsecond. The human hand can also estimate angular diameter, with the little finger covering 1° and a fist covering 10°.
The angular diameter of an object refers to the size it appears to be. Usually defined as an angle, it is usually proportional to the actual size of an object divided by its distance. The angular distance from one side of something to the other is often used to measure the size of objects in the night sky. A circle is 360°, while the distance across the sky is half a circle, or 180°. Objects in space are typically measured using one arcsecond, which is equal to 1/3,600 of 1°.
Another measurement often used with angular diameter is a radian, which is 180° divided by Pi, or 3.14 times the diameter of a circle. The angular diameter, therefore, can be determined by taking the answer to that calculation, which is 206.265, and dividing this by the actual distance. The apparent size of one object may be similar to another which may be much larger, but it is often the comparative distances from the Earth that make them appear the same.
The Sun and Moon are generally equal in angular diameter when viewed from Earth, even though one is about 400 times wider than the other. Each appears to be about ½° away from ground observers. While 1/3.600 of 1° is one second of arc, one minute of arc is 1/60 of 1°. People with normal visual acuity can see something one minute of arc in diameter; this is generally similar to viewing a 0.01 US dollar (USD) coin from 226 feet (about 70 meters) away.
Astronomers often use telescopes capable of illuminating objects 1 arc second wide. The most powerful telescopes can be used to view objects as narrow as 0.1 arcsecond. This apparent diameter can be used in a calculation, together with its known distance, to calculate its true width. Similar calculations have often been used to compare the size of the Sun with other stars and to determine the size of other galaxies, nebulae and other objects that can be seen in space.
The angular diameter can be estimated with the human hand. With the arm outstretched, the little finger typically covers 1° of the night sky. Generally, the fist is about 10° across objects such as stars and galaxies. Astronomers have calculated the angular diameters of the planets of the solar system and many other known objects in the sky.
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