In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit (such as to form a right angle at S). Thus the distance ES is one astronomical unit (au). The angle SDE is one arcsecond (⁠ 1 / 3600 ⁠ of a degree) so by definition D is a point in space at a distance of one parsec from the Sun. Through trigonometry, the distance SD is calculated as follows: