# Plane Angle

The plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.

- The secondary SI unit for the plane angle is the radian.

**Secondary SI Unit**

Conversions | ||||||

1 degree (°) | = | π/180 radian | 1 radian (rad) | = | 57.2958 degree (°) | |

1 minute (′) | = | 1/60 degree (°) | 1 second (") | = | 1/60 minute (′) | |

1 grad (or grade) | = | 1/100 right angle | right angle | = | π/2 radian (rad) |

The size of an object in the sky can be measured by the angle that it covers when viewed from Earth. The diameter of the full Moon is about one-half of a degree or 30 arcmin.

**See also: **Angles, Complementary Angles, Degree, Goniometer, Grade of a Slope, Inscribed Angle, Minute, pi, Radian, Rotation, Second, Sexagesimal.

**Subjects: ** Geometry