The small-angle formula is a mathematical approximation, used in astronomy. It relates the linear size (D) of an object to its angular size (α) and its distance from the observer (d) and is a valid approximation if α is small.
The approximate formula is:
- D = α d / 206,265
The number 206,265 is approximately equal to the number of arcseconds in a circle (1,296,600), divided by 2&pi.
The exact formula is
- D = 2 d tan(απ/1,296,600)