#### English Title

On the measure of solid angles

#### Enestrom Number

514

#### Fuss Index

334

#### Original Language

Latin

#### Published as

Journal article

#### Published Date

1781

#### Written Date

1775

#### Content Summary

Euler notes that plane angles may be measured by the circular arcs that they subtend. He also notes that a very clever geometer, Albert Girard (1595-1632), suggested measuring solid angles in the same way, by the part of a sphere that they subtend. He also defines the "area of an angle" of a sphere to be the area of the wedge-shaped region bounded by semicircles intersecting at the given angle: if the angle in radians measures *a*, then the area is 2*ar*^{2}. If the radius is 1, as we will assume from now on, then the area is 2*a*. Euler then states and proves (with attribution) Girard's theorem: the area of a spherical triangle is always equal to the angle by which the sum of all three angles of the triangle exceeds two right angles. This gives the area of the triangle in terms of the angles. He then states his general problem: to give the area of the triangle in terms of the lengths of the sides. Euler manages to find a formula, which he follows with lots of examples. Then he derives a couple of rules for finding the measure of solid angles and wraps it up by measuring the solid angles of the regular solids.

#### Original Source Citation

Acta Academiae Scientiarum Imperialis Petropolitanae, Volume 1778: II, pp. 31-54.

#### Opera Omnia Citation

Series 1, Volume 26, pp.204-223.

#### Record Created

2018-09-25