Angular unit

Throughout history, angles have been measured in many different units. These are known as angular units, with the most contemporary units being the degree ( ° ), the radian (rad), and the gradian (grad), though many others have been used throughout history. The purpose of this page is to aggregate other concepts pertaining to the angular unit, where additional explanation can be provided.
Angle measurement in general.
The size of a geometric angle is usually characterized by the magnitude of the smallest rotation that maps one of the rays into the other. Angles that have the same size are said to be "equal", "congruent," or "equal in measure".
In some contexts, such as identifying a point on a circle or describing the "orientation" of an object in two dimensions relative to a reference orientation, angles that differ by an exact multiple of a full turn are effectively equivalent. In other contexts, such as identifying a point on a spiral curve, or describing the "cumulative rotation" of an object in two dimensions relative to a reference orientation, angles that differ by a non-zero multiple of a full turn are not equivalent.
In order to measure an angle θ, a circular arc centered at the vertex of the angle is drawn (e.g. with a pair of compasses). The ratio of the length s of the arc by the radius r of the circle is the measure of the angle in radians.
The measure of the angle in another angular unit is then obtained by multiplying its measure in radians by the scaling factor , where "k" is the measure of a complete turn in the chosen unit (for example, 360 for degrees or 400 for gradians):
The value of θ thus defined is independent of the size of the circle: if the length of the radius is changed, then the arc length changes in the same proportion, so the ratio "s"/"r" is unaltered. (Proof. The formula above can be rewritten as One turn, for which units, corresponds to an arc equal in length to the circle's circumference, which is , so . Substituting "n" for "θ" and for "s" in the formula, results in .) 
In particular, the measure of angle is radian can be also interpreted as the arc length of its corresponding unit circle:
Angle addition postulate.
The angle addition postulate states that if B is in the interior of angle AOC, then
formula_2
The measure of the angle AOC is the sum of the measure of angle AOB and the measure of angle BOC.
In this postulate, it does not matter in which unit the angle is measured—as long as each angle is measured in the same unit.
Units.
One "radian" is the angle subtended by an arc of a circle that has the same length as the circle's radius. The radian is the derived quantity of angular measurement in the SI system. By definition, it is dimensionless, though it may be specified as "rad" to avoid ambiguity. Angles measured in degrees, are shown with the symbol °. Subdivisions of the degree are minute (symbol ′, 1′ = 1/60°) and second {symbol ″, 1″ = 1/3600°}. An angle of 360° corresponds to the angle subtended by a full circle, and is equal to radians, or 400 gradians.
Other units used to represent angles are listed in the following table. These units are defined such that the number of turns is equivalent to a full circle.
Equivalent time descriptors.
In astronomy, right ascension and declination are usually measured in angular units, expressed in terms of time, based on a 24-hour day.
Positive and negative angles.
Although the definition of the measurement of an angle does not support the concept of a negative angle, it is frequently useful to impose a convention that allows positive and negative angular values to represent orientations, and rotations in opposite directions relative to some reference.
In a two-dimensional Cartesian coordinate system, an angle is typically defined by its two sides, with its vertex at the origin. The "initial side" is on the positive x-axis, while the other side (i.e., the "terminal side") is defined by the measure from the initial side in radians, degrees, or turns. Typically, "positive angles" represent rotations toward the positive y-axis, and "negative angles" represent rotations toward the negative y-axis. When Cartesian coordinates are represented by "standard position", defined by the x-axis rightward and the y-axis upward, positive rotations are anticlockwise, and negative rotations are clockwise.
In many contexts, an angle of −"θ" is effectively equivalent to an angle of "one full turn minus "θ"". For example, an orientation represented as  −45° is effectively equivalent to an orientation represented as 360° − 45° or 315°. However, a rotation of  −45° would not be the same as a rotation of 315°.
In three-dimensional geometry, "clockwise" and "anticlockwise" have no absolute meaning, so the direction of positive and negative angles must be defined relative to some reference, which is typically a vector passing through the angle's vertex, and which is perpendicular to the plane in which the rays of the angle lie.
In navigation, bearings are measured relative to north. By convention, viewed from above, bearing angle are positive clockwise, so a bearing of 45° corresponds to a north-east orientation. Negative bearings are not used in navigation, so a north-west orientation corresponds to a bearing of 315°.
Alternative ways of measuring the size of an angle.
There are several alternatives to measuring the size of an angle by the corresponding angle of rotation.
The "grade of a slope", or "gradient", is equal to the tangent of the angle, or sometimes (rarely) the sine. Gradients are often expressed as a percentage. For very small values (less than 5%), the grade of a slope is approximately the measure of an angle in radians.
In rational geometry, the "spread" between two lines is defined at the square of sine of the angle between the lines. Since the sine of an angle and the sine of its supplementary angle are the same, any angle of rotation that maps one of the lines into the other leads to the same value of the spread between the lines.
Astronomical approximations.
Astronomers measure angular separation of objects in degrees from their point of observation.
These measurements clearly depend on the individual subject, and the above should be treated as rough rule of thumb approximations only.
Measurements that are not angular units.
Not all angle measurements are angular units, for an angular measurement, it is definitional that the angle addition postulate holds.
Some angle measurements where the angle addition postulate does not hold include: