TABLE OF CONTENTS While for the measurement of length the adoption of an ultimate standard, either material or natural, is needed, angular dimensions can be established by the subdivision of the circle.
The angular magnitudes that are derived from the subdivision of the circle have for a basic unit the right angle, resulting from the intersection of two mutually perpendicular straight lines. The point of intersection is then considered to coincide with the center of the imaginary circle being subdivided.
There are two known systems for the subdivision of the right angle into angular units: the sexagesimal and the centesimal.
The sexagesimal is the generally employed system in this country for the technical measurement of angles. This system divides the right angle into 90 units, known as degrees ( ). Each degree is further divided into 60 minutes ( ?) and each minute into 60 seconds ( ?).
The centesimal system, which is rarely used, divides the right angle into 100 units, called grades ( g), each grade containing 100 minutes ( c), and each minute 100 seconds ( cc).
In many theoretical applications the angle is measured as part of a circle, that is, it is subtended by an arc of defined length and the magnitude of that angle is expressed as the ratio
Angles so defined are expressed in circular measure, having as a unit the radian. Since the circumference of a circle is ? times its diameter, 2 ?
