Angular diameter, sometimes also called angular size, apparent size or apparent diameter is an angular measurement that describes how large a circle appears. In astronomy, it is usually used to describe the sizes of objects in the sky as seen from an observer on Earth and is typically used for the Moon, the planets and other extended objects such as for example galaxies and nebula.
Angular distance is used to describe the distance between two points in the sky, for example, the Sun and Mercury.
When using angular measurements the sky is divided into 360°, each degree into 60 arcminutes (') and each arc minute into 60 arcseconds ("). This means that one arcsecond is 1/3600th of one degree.
As you can see from the diagram above knowing just the angular diameter will not tell you anything about the actual size of the object (unless you also know the distance to it, in which case you can calculate the actual diameter). Two objects that have the same apparent size does not necessarily have the same actual size. One example of this is the Sun and the Moon, where the actual diameter of the Sun is 400 times larger than the Moon but have almost the exact same apparent size because the Moon is so much closer to us than the Sun.
In astronomy, the angular diameter is often measured in arcseconds because most objects tend to be quite small when viewed from Earth. Below is a list of objects and their angular diameter.
|Andromeda Galaxy||~ 3° × 1°|
|Sun||~ 31' - 32' (arcminutes)|
|Moon||~ 29' - 34' (arcminutes)|
|Venus||~ 10" - 65" (arcseconds)|
|Mars||~ 4" - 25" (arcseconds)|
|Jupiter||~ 30" - 51" (arcseconds)|
|Saturn||~ 15" - 21" (arcseconds)|
|Alphard (star)||0.00909″ (arcseconds)|
You can estimate angular diameter or distances using you outstretched hand:
The angular resolution of the naked eye is about 1 arcminute, 1/60th of one degree so you will need aid to perceive the planets as anything other then points. Using a telescope with even moderate magnification will resolve the planets into discs and you will easily be able to separate them from bright stars which will remain points of light even under high magnification.
The angular diameter of the planets will vary depending on where they are in their orbit in relationship to Earth. If you step forward you'll see that the distance angular diameter of the planets will grow and shrink.