E514  De mensura angulorum solidorum
(On the measure of solid angles)
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 (15951632), suggests 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 wedgeshaped region bounded by semicircles intersecting at the given angle: If
the angle in radians measures a, then the area is 2ar^{2}. If the radius is 1, as we will assume
from now on, then the area is 2a. 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 asks his General Problem, to give the area of the triangle in terms of the lengths of the sides
and finds a formula, which he follows by 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.
According to the records, it was presented to the St. Petersburg Academy on January 9, 1775.
Publication:

Originally published in Acta Academiae Scientarum Imperialis Petropolitinae 2, 1781, pp. 3154

Opera Omnia: Series 1, Volume 26, pp. 204  223
Documents Available:
 Original Publication: E514
 The Euler Archive attempts to monitor current scholarship for articles and books that may be of interest to Euler Scholars. Selected references we have found that discuss or cite E514 include:
 Almkvist G, Berndt B., “Gauss, Landen, Ramanujan, the arithmeticgeometric mean, ellipses, pi, and the ladiesdiary.” American Mathematical Monthly, 95 (7), pp. 585608 (AugSep 1988).
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