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=== Astronomy ===
In [[astronomy]], the [[angular size]] or angle subtended by the image of a distant object is often only a few [[arcsecond]]s (denoted by the symbol ″), so it is well suited to the small angle approximation.<ref name=Green1985/> The linear size ({{mvar|D}}) is related to the angular size ({{mvar|X}}) and the distance from the observer ({{mvar|d}}) by the simple formula:
:<math>D = X \frac{d}{206\,265{''}}</math>
where {{mvar|X}} is measured in arcseconds.
The quantity {{val|206265|u="}} is approximately equal to the number of arcseconds in 1 radian, which is the number of arcseconds in a [[circle]] ({{val|1296000|u="}})
The exact formula is
:<math>D = d \tan \left( X \frac{2\pi}{1\,296\,000{''}} \right)</math>
and the above approximation follows when {{math|tan ''X''}} is replaced by {{mvar|X}}.
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