_Circular or Angular Measures_ are used in surveying, navigation,
astronomy, geography, reckoning latitude and longitude, and computing
differences in time.
A _Circle_ is a plane figure bounded by a curved line, every point of
which is equally distant from a point within, called the center.
The _Circumference_ is the bounding line of a circle.
The _Radius_ of a circle is a straight line drawn from the circumference
to the center.
The _Diameter_ is a straight line drawn through the center, with the
ends terminating in the circumference.
An _Arc_ of a circle is any portion of the circumference.
An _Angle_ is the difference in direction between two straight lines
which meet.
If two diameters divide a circle into four equal parts, these diameters
make _right angles_ with each other.
An angle less than a right angle is an _acute angle_.
The circumference of a circle may be divided into 360 equal parts,
called _degrees_. If the circle is large, the degree is large, and if
the circle is small, the degree is small, but the degree is always 1/360
part of the circumference, whatever the size of the circle.
An angle at the center of a circle is measured by the arc which bounds
it.
If the angle is a right angle, it is measured by 1/4 of 360 degrees, or
90 degrees; hence, any angle of 90 degrees is a right angle.
An acute angle is always less than 90 degrees.
An obtuse angle is always more than 90 degrees.