Angle


In plane geometry,[] an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.
Angles formed by two rays lie in a plane, but this plane does not have to be a Euclidean plane. Angles are also formed by the intersection of two planes in Euclidean and other spaces. These are called dihedral angles. Angles formed by the intersection of two curves in a plane are defined as the angle determined by the tangent rays at the point of intersection. Similar statements hold in space, for example, the spherical angle formed by two great circles on a sphere is the dihedral angle between the planes determined by the great circles.
Angle is also used to designate the measure of an angle or of a rotation. This measure is the ratio of the length of a circular arc to its radius. In the case of a geometric angle, the arc is centered at the vertex and delimited by the sides. In the case of a rotation, the arc is centered at the center of the rotation and delimited by any other point and its image by the rotation.
The word angle comes from the Latin word angulus, meaning "corner"; cognate words are the Greek ἀγκύλος , meaning "crooked, curved," and the English word "ankle". Both are connected with the Proto-Indo-European root *ank-, meaning "to bend" or "bow".
Euclid defines a plane angle as the inclination to each other, in a plane, of two lines which meet each other, and do not lie straight with respect to each other. According to Proclus an angle must be either a quality or a quantity, or a relationship. The first concept was used by Eudemus, who regarded an angle as a deviation from a straight line; the second by Carpus of Antioch, who regarded it as the interval or space between the intersecting lines; Euclid adopted the third concept, although his definitions of right, acute, and obtuse angles are certainly quantitative.

Identifying angles

In mathematical expressions, it is common to use Greek letters to serve as variables standing for the size of some angle. Lower case Roman letters are also used, as are upper case Roman letters in the context of polygons. See the figures in this article for examples.
In geometric figures, angles may also be identified by the labels attached to the three points that define them. For example, the angle at vertex A enclosed by the rays AB and AC is denoted ∠BAC or. Sometimes, where there is no risk of confusion, the angle may be referred to simply by its vertex.
Potentially, an angle denoted, say, ∠BAC might refer to any of four angles: the clockwise angle from B to C, the anticlockwise angle from B to C, the clockwise angle from C to B, or the anticlockwise angle from C to B, where the direction in which the angle is measured determines its sign. However, in many geometrical situations it is obvious from context that the positive angle less than or equal to 180 degrees is meant, and no ambiguity arises. Otherwise, a convention may be adopted so that ∠BAC always refers to the anticlockwise angle from B to C, and ∠CAB to the anticlockwise angle from C to B.

Types of angles

Individual angles

There is some common terminology for angles, whose measure is always non-negative :
The names, intervals, and measured units are shown in a table below:

Equivalence angle pairs

When two straight lines intersect at a point, four angles are formed. Pairwise these angles are named according to their location relative to each other.
A transversal is a line that intersects a pair of lines and is associated with alternate interior angles, corresponding angles, interior angles, and exterior angles.

Combining angle pairs

There are three special angle pairs which involve the summation of angles:
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 or 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 :
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.

Angle addition postulate

The angle addition postulate states that if B is in the interior of angle AOC, then
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

Units used to represent angles are listed below in descending magnitude order. Of these units, the degree and the radian are by far the most commonly used. Angles expressed in radians are dimensionless for the purposes of dimensional analysis.
Most units of angular measurement are defined such that one turn is equal to n units, for some whole number n. The two exceptions are the radian and the diameter part.
;Turn : The turn, also cycle, full circle, revolution, and rotation, is complete circular movement or measure with circle or ellipse. A turn is abbreviated, cyc, rev, or rot depending on the application, but in the acronym rpm, just r is used. A turn of n units is obtained by setting in the formula above. The equivalence of 1 turn is 360°, 2 rad, 400 grad, and 4 right angles. The symbol can also be used as a mathematical constant to represent 2 radians. Used in this way allows for radians to be expressed as a fraction of a turn. For example, half a turn is.
;Quadrant : The quadrant is of a turn, i.e. a right angle. It is the unit used in Euclid's Elements. 1 quad. = 90° = rad = turn = 100 grad. In German the symbol ∟ has been used to denote a quadrant.
;Sextant : The sextant is of a turn. It was the unit used by the Babylonians, and is especially easy to construct with ruler and compasses. The degree, minute of arc and second of arc are sexagesimal subunits of the Babylonian unit. 1 Babylonian unit = 60° = /3 rad ≈ 1.047197551 rad.
;Radian : The radian is the angle subtended by an arc of a circle that has the same length as the circle's radius. The case of radian for the formula given earlier, a radian of n = 2 units is obtained by setting k = = 1. One turn is 2 radians, and one radian is degrees, or about 57.2958 degrees. The radian is abbreviated rad, though this symbol is often omitted in mathematical texts, where radians are assumed unless specified otherwise. When radians are used angles are considered as dimensionless. The radian is used in virtually all mathematical work beyond simple practical geometry, due, for example, to the pleasing and "natural" properties that the trigonometric functions display when their arguments are in radians. The radian is the unit of angular measurement in the SI system.
;Clock position : A clock position is the relative direction of an object described using the analogy of a 12-hour clock. One imagines a clock face lying either upright or flat in front of oneself, and identifies the twelve hour markings with the directions in which they point.
;Hour angle : The astronomical hour angle is of a turn. As this system is amenable to measuring objects that cycle once per day, the sexagesimal subunits are called minute of time and second of time. These are distinct from, and 15 times larger than, minutes and seconds of arc. 1 hour = 15° = rad = quad. = turn = grad.
; point or wind : The point, used in navigation, is of a turn. 1 point = of a right angle = 11.25° = 12.5 grad. Each point is subdivided in four quarter-points so that 1 turn equals 128 quarter-points.
;Hexacontade : The hexacontade is a unit of 6° that Eratosthenes used, so that a whole turn was divided into 60 units.
;Pechus : The pechus was a Babylonian unit equal to about 2° or °.
;Binary degree : The binary degree, also known as the binary radian, is of a turn. The binary degree is used in computing so that an angle can be efficiently represented in a single byte. Other measures of angle used in computing may be based on dividing one whole turn into 2n equal parts for other values of n.
;Degree : The degree, denoted by a small superscript circle, is 1/360 of a turn, so one turn is 360°. The case of degrees for the formula given earlier, a degree of n = 360° units is obtained by setting k =. One advantage of this old sexagesimal subunit is that many angles common in simple geometry are measured as a whole number of degrees. Fractions of a degree may be written in normal decimal notation, but the "minute" and "second" sexagesimal subunits of the "degree-minute-second" system are also in use, especially for geographical coordinates and in astronomy and ballistics.
;Diameter part : The diameter part is radian. One "diameter part" is approximately 0.95493°. There are about 376.991 diameter parts per turn.
;Grad : The grad, also called grade, gradian, or gon, is of a turn, so a right angle is 100 grads. It is a decimal subunit of the quadrant. A kilometre was historically defined as a centi-grad of arc along a great circle of the Earth, so the kilometer is the decimal analog to the sexagesimal nautical mile. The grad is used mostly in triangulation.
;Milliradian: The milliradian is defined as a thousandth of a radian, which means that a rotation of one turn consists of 2000π mil, and almost all scope sights for firearms are calibrated to this definition. In addition there are three other derived definitions used for artillery and navigation which are approximately equal to a milliradian. Under these three other definitions one turn makes up for exactly 6000, 6300 or 6400 mils, which equals spanning the range from 0.05625 to 0.06 degrees. In comparison, the true milliradian is approximately 0.05729578... degrees. One "NATO mil" is defined as of a circle. Just like with the true milliradian, each of the other definitions exploits the mil's handby property of subtensions, i.e. that the value of one milliradian approximately equals the angle subtended by a width of 1 meter as seen from 1 km away.
;Minute of arc : The minute of arc is of a degree = turn. It is denoted by a single prime. For example, 3° 30′ is equal to 3 × 60 + 30 = 210 minutes or 3 + = 3.5 degrees. A mixed format with decimal fractions is also sometimes used, e.g. 3° 5.72′ = 3 + degrees. A nautical mile was historically defined as a minute of arc along a great circle of the Earth.
;Second of arc : The second of arc is of a minute of arc and of a degree. It is denoted by a double prime. For example, 3° 7′ 30″ is equal to 3 + + degrees, or 3.125 degrees.
;Milliarcsecond : mas
;Microarcsecond : µas

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/or 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 or terminal side is defined by the measure from the initial side in radians, degrees, or turns. With positive angles representing rotations toward the positive y-axis and negative angles representing 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°. Although the final position is the same, a physical rotation of −45° is not 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 perpendicular to the plane in which the rays of the angle lie.
In navigation, bearings or azimuth are measured relative to north. By convention, viewed from above, bearing angles 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 angle of rotation.
The grade of a slope, or gradient is equal to the tangent of the angle, or sometimes the sine. A gradient is often expressed as a percentage. For very small values, the grade of a slope is approximately the measure of the angle in radians.
In rational geometry the spread between two lines is defined as the square of the sine of the angle between the lines. As 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 for 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.

Angles between curves

The angle between a line and a curve or between two intersecting curves is defined to be the angle between the tangents at the point of intersection. Various names have been given to particular cases:—amphicyrtic or cissoidal, biconvex; xystroidal or sistroidal, concavo-convex; amphicoelic or angulus lunularis, biconcave.

Bisecting and trisecting angles

The ancient Greek mathematicians knew how to bisect an angle using only a compass and straightedge, but could only trisect certain angles. In 1837 Pierre Wantzel showed that for most angles this construction cannot be performed.

Dot product and generalisations

In the Euclidean space, the angle θ between two Euclidean vectors u and v is related to their dot product and their lengths by the formula
This formula supplies an easy method to find the angle between two planes from their normal vectors and between skew lines from their vector equations.

Inner product

To define angles in an abstract real inner product space, we replace the Euclidean dot product by the inner product, i.e.
In a complex inner product space, the expression for the cosine above may give non-real values, so it is replaced with
or, more commonly, using the absolute value, with
The latter definition ignores the direction of the vectors and thus describes the angle between one-dimensional subspaces and spanned by the vectors and correspondingly.

Angles between subspaces

The definition of the angle between one-dimensional subspaces and given by
in a Hilbert space can be extended to subspaces of any finite dimensions. Given two subspaces, with, this leads to a definition of angles called canonical or principal angles between subspaces.

Angles in Riemannian geometry

In Riemannian geometry, the metric tensor is used to define the angle between two tangents. Where U and V are tangent vectors and gij are the components of the metric tensor G,

Hyperbolic angle

A hyperbolic angle is an argument of a hyperbolic function just as the circular angle is the argument of a circular function. The comparison can be visualized as the size of the openings of a hyperbolic sector and a circular sector since the areas of these sectors correspond to the angle magnitudes in each case. Unlike the circular angle, the hyperbolic angle is unbounded. When the circular and hyperbolic functions are viewed as infinite series in their angle argument, the circular ones are just alternating series forms of the hyperbolic functions. This weaving of the two types of angle and function was explained by Leonhard Euler in Introduction to the Analysis of the Infinite.

Angles in geography and astronomy

In geography, the location of any point on the Earth can be identified using a geographic coordinate system. This system specifies the latitude and longitude of any location in terms of angles subtended at the centre of the Earth, using the equator and the Greenwich meridian as references.
In astronomy, a given point on the celestial sphere can be identified using any of several astronomical coordinate systems, where the references vary according to the particular system. Astronomers measure the angular separation of two stars by imagining two lines through the centre of the Earth, each intersecting one of the stars. The angle between those lines can be measured, and is the angular separation between the two stars.
In both geography and astronomy, a sighting direction can be specified in terms of a vertical angle such as altitude /elevation with respect to the horizon as well as the azimuth with respect to north.
Astronomers also measure the apparent size of objects as an angular diameter. For example, the full moon has an angular diameter of approximately 0.5°, when viewed from Earth. One could say, "The Moon's diameter subtends an angle of half a degree." The small-angle formula can be used to convert such an angular measurement into a distance/size ratio.