The angle between two lines (or line segments) that have a point in common is the amount of rotation that separates the two lines from their vertex. Note that the lines must have a point in common in order to define an angle between them. Angles were described by Euclid as "the inclination to one another of two lines which meet each other in the same plane, and do not lie straight with respect to each other"
There are several units used to measure angles, the most common are:
Degree: Most commonly used in everyday life. It is defined as 1/360 of a full circle, therefore a circle has 360 degrees.
Radian: Most commonly used in science and engineering, is the angle subtended by an arc of a circle which have the same length as the circle radius. One full circle is equal to 2 radians, and one radian is 180/ degrees.
Arcminute: Commonly used in astronomy, 1 arcminute is 1/60 of a degree or 1/21600 of a circle. It is useful due to the small angles often encountered.
Arcsecond: Commonly used in astronomy, 1 arcsecond is 1/60 of an arcminute or 1/1296000 of a circle.
A simple way to classify angles is the following:
- acute (less than 90 degrees)
- right (90 degrees exactly)
- obtuse (greater than 90 degrees but less than 180 degrees)
- reflex (over 180 degrees)