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angle
| My geometry teacher was sometimes acute, and sometimes obtuse, but he was always right. |
| —anonymous |
In plane geometry, an angle id the opening between two lines or two planes that meet; the word comes from the Latin angulus for a sharp bend. Plane angles are measured in degrees or radians.
A right angle has exactly 90° (or π/2 radians), an acute angle less than 90°, an obtuse angle has between 90° and 180°, and a straight angle has exactly 180°. Any angle that is at most 180° is said to be convex. An angle between 180° and 360° is called a reflex angle. An angle of exactly 360° is said to be a full angle. Complementary angles add to give 90°, supplementary angles make a total of 180°.
In spherical geometry, a spherical angle is that formed by intersecting arcs of two great circles; its magnitude is equal to that of the angle between the two planes of the great circles.
See also solid angle.
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