George Albert Wentworth

Plane Trigonometry and Tables George Albert Wentworth

As lengths are measured in terms of various conventional units, the foot, the meter, etc., so different units for measuring angles are employed, or have been proposed. In the common or sexagesimal system the circumference of a circle is divided into 360 equal parts. The angle at the centre subtended by each of these pai-ts is taken as the imit angle and is called a degree. The degree is subdivided into 60 minutes, and the minute into 60 seconds. Degrees, minutes, and seconds are denoted by symbols. Thus, 6degrees 5minutes 7seconds is written 57 . N ote. The sexagesimal system was employed by the early Babylonian astronomers to conform with their year of 360 days. In the circular system an arc of a circle is laid off equal in length to the radius. The angle at the centre subtended by this,arc is taken as the unit angle and is called a radi n. The number of radians in 360 is equal to the number of times the length of the radius is contained in the length of the circumference.
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