The plane angle XO, Figure 1-1, is formed by
the two rays OX and OY. The point O is called the vertex and the half lines are
called the sides of the angle.
An angle is called positive if the direction of
rotation is counter clockwise and negative if the direction of motion is
clockwise. The angle is positive in Figure 1-2(a) and (e). Negative in Figure
Measure of Angles
A degree is defined as the measure of the
central angle subtended by an arc of a circle equal to 1/360 of the
circumference of the circle.
A minute is 1/60 of a degree; a second is 1/60
of a minute, or 1/3600 of a degree.
When changing angles in decimals to minutes and
seconds, the general rule is that angles in tenths will be changed to the
nearest minute and all other angles will be rounded to the nearest hundredth and
then changed to the nearest second. When changing angles in minutes and seconds
to decimals, the results in minutes are rounded to tenths, and angles in seconds
have the results to hundredths.