One degree (shown in red) and
ninety degrees (shown in blue)

A degree (in full, a degree of arc, arc degree, or arcdegree), usually denoted by ° (the degree symbol), is a measurement of plane angle, representing ¹⁄₃₆₀ of a full rotation; one degree is equivalent to π/180 radians. It is not an SI unit, as the SI unit for angles is radian, but it is mentioned in the SI brochure as an accepted unit.^[1]

History[link]

A circle with an equilateral chord (red). One sixtieth of this arc is a degree. Six such chords complete the circle.

The original motivation for choosing the degree as a unit of rotations and angles is unknown. One theory states that it is related to the fact that 360 is approximately the number of days in a year.^[2] Ancient astronomers noticed that the sun, that follows through the ecliptic path over the course of the year, seems to advance in that path by approximately one degree, each day. Some ancient calendars, such as the Persian calendar, used 360 days for a year. The Mayans used 20 cycles of 18 plus 5 unlucky days in one of their Maya calendars. The use of a calendar with 360 days may be related to the use of sexagesimal numbers.

Another theory is that the Babylonians subdivided the circle using the angle of an equilateral triangle as the basic unit and further subdivided the latter into 60 parts following their sexagesimal numeric system.^[3] The earliest trigonometry, used by the Babylonian astronomers and their Greek successors, was based on chords of a circle. A chord of length equal to the radius made a natural base quantity. One sixtieth of this, using their standard sexagesimal divisions, was a degree.

Aristarchus of Samos and Hipparchos seem to have been among the first Greek scientists to exploit Babylonian astronomical knowledge and techniques systematically.^[4] Timocharis, Aristarchus, Aristillus, Archimedes, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minutes (DIO 14 ‡2 p.19 n.24). Eratosthenes used a simpler sexagesimal system dividing a circle into 60 parts.

The division of the circle into 360 parts also occurred in ancient India, as evidenced in the Rigveda:^[5]

Twelve spokes, one wheel, navels three.
Who can comprehend this?
On it are placed together
three hundred and sixty like pegs.
They shake not in the least.

— Dirghatamas , Rigveda 1.164.48

Another motivation for choosing the number 360 may have been that it is readily divisible: 360 has 24 divisors, making it one of only 7 numbers that have more divisors than any number twice itself (sequence A072938 in OEIS)^[6][7]. Furthermore, it is divisable by every number from 1 to 10 except 7.^[8] This property has many useful applications, such as dividing the world into 24 time zones, each of which is nominally 15° of longitude, to correlate with the established 24-hour day convention.

Finally, it may be the case that more than one of these factors has come into play. According to that theory, the number is approximately 365 because of the apparent movement of the sun against the celestial sphere and that it was rounded to 360 for some of the mathematical reasons cited above.

Subdivisions[link]

For many practical purposes, a degree is a small enough angle that whole degrees provide sufficient precision. When this is not the case, as in astronomy or for latitudes and longitudes on the Earth, degree measurements may be written with decimal places like 40.1875° with the degree symbol behind the decimals.

Alternatively, the traditional sexagesimal unit subdivision can be used. One degree is divided into 60 minutes (of arc), and one minute into 60 seconds (of arc). These units, also called the arcminute and arcsecond, are respectively represented as a single and double prime: for example, 40.1875° = 40° 11′ 15″ . Sometimes single and double quotation marks are used instead: 40° 11' 15" .

If still more accuracy is required, current practice is to use decimal divisions of the second like 40° 11′ 15.4″ . The older system of thirds, fourths, etc., which continues the sexagesimal unit subdivision, was used by al-Kashi and other ancient astronomers, but is rarely used today. These subdivisions were denoted^{[citation needed]} by writing the Roman numeral for the number of sixtieths in superscript: 1^I for a "prime" (minute of arc), 1^II for a second, 1^III for a third, 1^IV for a fourth, etc. Hence the modern symbols for the minute and second of arc, and the word "second" also refer to this system.^{[citation needed]}

Alternative units[link]

A chart to convert between degrees and radians

In most mathematical work beyond practical geometry, angles are typically measured in radians rather than degrees. This is for a variety of reasons; for example, the trigonometric functions have simpler and more "natural" properties when their arguments are expressed in radians. These considerations outweigh the convenient divisibility of the number 360. One complete turn (360°) is equal to 2π radians, so 180° is equal to π radians, or equivalently, the degree is a mathematical constant: 1° = ^π⁄₁₈₀.

The turn (or revolution, full circle, full rotation, cycle) is used in technology and science. 1 turn = 360°.

With the invention of the metric system, based on powers of ten, there was an attempt to define a "decimal degree" (grad or gon), so that the number of decimal degrees in a right angle would be 100 gon, and there would be 400 gon in a circle. Although this idea was abandoned already by Napoleon, some groups have continued to use it and many scientific calculators still support it.

An angular mil, which is most used in military applications, has at least three specific variants, ranging from ¹⁄₆₄₀₀ to ¹⁄₆₀₀₀, each approximately equal to one milliradian. However, ¹⁄₆₀₀₀ used by the Russian Army originated in Imperial Russia, where an equilateral chord was divided into tenths to give a circle of 600 units (this may be seen on a protractor, circa 1900, in the St Petersberg Museum of Artillery).

Conversion of some common angles[link]

See also[link]

• Gradian
• Radian
• Turn
• Square degree
• Steradian
• Compass
• Geographic coordinate system
• Meridian arc

Notes[link]

External links[link]

Wikimedia Commons has media related to: Degree (angle)

• Degrees as an angle measure, with interactive animation
• Degree at MathWorld

v t e SI units Base units Ampere · Candela · Kelvin · Kilogram · Metre · Mole · Second Derived units Becquerel · Coulomb · Degree Celsius · Farad · Gray · Henry · Hertz · Joule · Katal · Lumen · Lux · Newton · Ohm · Pascal · Radian · Siemens · Sievert · Steradian · Tesla · Volt · Watt · Weber Accepted for use with SI Dalton (Atomic mass unit) · Astronomical unit · Day · Decibel · Degree Celsius · Degree of arc · Electronvolt · Hectare · Hour · Litre · Minute · Minute of arc · Neper · Second of arc · Tonne Atomic units · Natural units See also SI prefixes · Systems of measurement · Conversion of units · New SI definitions · History of the metric system Book:International System of Units  · Category:SI base units

Look up degree in Wiktionary, the free dictionary.

Degree may refer to:

As a unit of measurement[link]

• Degree symbol, (°), a notation used in science, engineering and mathematics
• Degree (angle), a unit of angle measurement
• Degree in geographic coordinate system
• Degree (temperature), a unit of temperature measurement
• Degree API, a measure of density in the petroleum industry
• Degree Baumé, a pair of density scales
• Degree Brix, a measure of sugar concentration
• Degree Gay-Lussac, a measure of the alcohol content of a liquid by volume, ranging from 0° to 100°
• Degree proof, or simply proof, the alcohol content of a liquid, ranging from 0° to 175° in the UK, and from 0° to 200° in the U.S.
• Degree of curvature, a unit of curvature measurement, used in civil engineering
• Degrees of freedom (mechanics), the number of displacements and/or rotations needed to define the position and orientation of a body
• Degrees of freedom (physics and chemistry), a concept describing dependence on a countable set of parameters
• Degree of frost, a unit of temperature measurement
• Degree of unsaturation, in organic chemistry, also known as the index of hydrogen deficiency or rings plus double bonds
• dGH, degrees of general hardness of water
• Degree of carbonate hardness of water (degree KH)

In mathematics[link]

• Degree (mathematics), with several meanings
• Degree of a polynomial, the exponent of the term with the highest exponent
• Degree of a field extension
• The degree of an algebraic number field, its degree as a field extension of the rational numbers
• Degree (graph theory), or valency, the number of edges incident to a vertex of a graph
• Degree of a continuous mapping
• Degrees of freedom (statistics), the number of values in the final calculation of a statistic that are free to vary

In education[link]

• Academic degree, an academic rank, title or award, including:
  • Foundation degree
  • Associate's degree
  • Bachelor's degree
  • Master's degree
  • Doctorate
  • Engineer's degree
  • Specialist degree
  • Ad eundem degree
  • Honorary degree
  • Lambeth degree
  • External degree
• Vocational degree, an award in vocational education

Other measures[link]

• Degree (music), identification of a note in a scale by its relation to the tonic
• Degree of inventiveness in inventions and patents
• Degree of separation in connectivity between groups (first degree is closest)
• Degree of relationship, in kinship between individuals (first degree is closest)
  • Consanguinity, or level of kinship
• The positive, comparative, and superlative degrees, in linguistics (e.g. "good", "better", and "best", respectively)
• The severity of a crime, e.g., first degree murder (first degree is worst)
• The intensity of a burn (the higher the worse)
• A level of initiation, often used in fraternal organizations
• A ranking of black belt, in certain martial arts

Other uses[link]

• Degree (deodorant), a brand of antiperspirant
• degree (freemasonry)
• degree of Knights Templar (Freemasonry), a special case of a degree in freemasonry

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

∠, the angle symbol

In 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.^[1] Angles are usually presumed to be in a Euclidean plane, but are also defined in non-Euclidean geometry.

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 by its radius. In the case of an angle (figure), 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 point and its image by the rotation.

The word angle comes from the Latin word angulus, meaning "a corner". The word angulus is a diminutive, of which the primitive form, angus, does not occur in Latin. Cognate words are the Greek ἀγκύλος (ankylοs), meaning "crooked, curved," and the English word "ankle". Both are connected with the Proto-Indo-European root *ank-, meaning "to bend" or "bow".^[2]

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.^[3]

Measuring angles[link]

The size of an angle is characterized by the magnitude of the smallest rotation that maps one of the rays into the other. Angles that have the same size are called congruent angles.

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.

The measure of angle θ is the quotient of s and r.

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 length of the arc s is then divided by the radius of the arc r, and possibly multiplied by a scaling constant k (which depends on the units of measurement that are chosen):

Failed to parse (Missing texvc executable; please see math/README to configure.): \theta = k \frac{s}{r}.

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.

Units[link]

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 (i.e. one full circle) is equal to n units, for some whole number n. The two exceptions are the radian and the diameter part. For example, in the case of degrees, n = 360. A turn of n units is obtained by setting k = n/(2π) in the formula above. (Proof. The formula above can be rewritten as k = θr/s. One turn, for which θ = n units, corresponds to an arc equal in length to the circle's circumference, which is 2πr, so s = 2πr. Substituting n for θ and 2πr for s in the formula, results in k = nr/(2πr) = n/(2π).)

• The turn (or full circle, revolution, rotation, or cycle) is one full circle. A turn can be subdivided in centiturns and milliturns. A turn is abbreviated Failed to parse (Missing texvc executable; please see math/README to configure.): \tau

or rev or rot depending on the application, but just r in rpm (revolutions per minute). 1 turn = 360° = 2π rad  = 400 grad = 4 right angles.

• The quadrant is 1/4 of a turn, i.e. a right angle. It is the unit used in Euclid's Elements. 1 quad. = 90° = π/2 rad = 1/4 turn = 100 grad. In German the symbol ^∟ has been used to denote a quadrant.
• The angle of the equilateral triangle is 1/6 of a turn. It was the unit used by the Babylonians^{[citation needed]}, 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° = π/3 rad ≈ 1.047197551 rad.

θ = s/r rad = 1 rad.

• The radian is the angle subtended by an arc of a circle that has the same length as the circle's radius (k = 1 in the formula given earlier). One turn is 2π radians, and one radian is 180/π 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 (derived) unit of angular measurement in the SI system.
• The diameter part (occasionally used in Islamic mathematics) is 1/60 radian. One "diameter part" is approximately 0.95493°.
• The astronomical hour angle is 1/24 of a turn. Since this system is amenable to measuring objects that cycle once per day (such as the relative position of stars), the sexagesimal subunits are called minute of time and second of time. Note that these are distinct from, and 15 times larger than, minutes and seconds of arc. 1 hour = 15° = π/12 rad = 1/6 quad. = 1/24 turn ≈ 16.667 grad.
• The point, used in navigation, is 1/32 of a turn. 1 point = 1/8 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.
• Eratosthenes used a unit of 6° so that a whole turn was divided in 60 units.
• The Babylonians sometimes used the unit pechus of about 2° or 2½°.
• The binary degree, also known as the binary radian (or brad), is 1/256 of a turn.^[4] The binary degree is used in computing so that an angle can be efficiently represented in a single byte (albeit to limited precision). Other measures of angle used in computing may be based on dividing one whole turn into 2ⁿ equal parts for other values of n.^[5]
• The degree, denoted by a small superscript circle (°), is 1/360 of a turn, so one turn is 360°. 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 (e.g. 3.5° for three and a half degrees), 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:
• The grad, also called grade, gradian, or gon, is 1/400 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.
• The mil is approximately equal to a milliradian. There are several definitions ranging from 0.05625 to 0.06 degrees (3.375 to 3.6 minutes), with the milliradian being approximately 0.05729578 degrees (3.43775 minutes). In NATO countries, it is defined as 1/6400th of a circle. Its value is approximately equal to the angle subtended by a width of 1 metre as seen from 1 km away (2π / 6400 = 0.0009817… ≒ 1/1000).
• The minute of arc (or MOA, arcminute, or just minute) is 1/60 of a degree = 1/21600 turn. It is denoted by a single prime ( ′ ). For example, 3° 30′ is equal to 3 + 30/60 degrees, or 3.5 degrees. A mixed format with decimal fractions is also sometimes used, e.g. 3° 5.72′ = 3 + 5.72/60 degrees. A nautical mile was historically defined as a minute of arc along a great circle of the Earth.
• The second of arc (or arcsecond, or just second) is 1/60 of a minute of arc and 1/3600 of a degree. It is denoted by a double prime ( ″ ). For example, 3° 7′ 30″ is equal to 3 + 7/60 + 30/3600 degrees, or 3.125 degrees.

Positive and negative angles[link]

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, angles are typically defined relative to the positive x-axis 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 as they commonly are, with the x-axis rightward and the y-axis upward, positive rotations are counterclockwise 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 "counterclockwise" 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 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[link]

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 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[link]

Astronomers measure angular separation of objects in degrees from their point of observation.

• 1° is approximately the width of a little finger at arm's length.
• 10° is approximately the width of a closed fist at arm's length.
• 20° is approximately the width of a handspan at arm's length.

These measurements clearly depend on the individual subject, and the above should be treated as rough approximations only.

Identifying angles[link]

In mathematical expressions, it is common to use Greek letters (α, β, γ, θ, φ, ...) to serve as variables standing for the size of some angle. (To avoid confusion with its other meaning, the symbol π is typically not used for this purpose.) Lower case roman letters (a, b, c, ...) are also used. 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 (i.e. the lines from point A to point B and point A to point C) is denoted ∠BAC or BÂC. Sometimes, where there is no risk of confusion, the angle may be referred to simply by its vertex ("angle A").

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 (see Positive and negative angles). 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 (positive) angle from B to C, and ∠CAB to the anticlockwise (positive) angle from C to B.

Types of angles[link]

Right angle.

The complementary angles a and b (b is the complement of a, and a is the complement of b).

Acute (a), obtuse (b), and straight (c) angles. Here, a and b are supplementary angles.

Reflex angle.

• An angle equal to 1/4 turn (90° or π/2 radians) is called a right angle.

  Two lines that form a right angle are said to be perpendicular or orthogonal.

• Angles equal to 1/2 turn (180° or two right angles) are called straight angles.
• Angles equal to 1 turn (360° or four right angles) are called full angles.
• Angles that are not right angles or a multiple of a right angle are called oblique angles.
• Angles smaller than a right angle (less than 90°) are called acute angles ("acute" meaning "sharp").
• Angles larger than a right angle and smaller than a straight angle (between 90° and 180°) are called obtuse angles ("obtuse" meaning "blunt").
• Angles larger than a straight angle but less than 1 turn (between 180° and 360°) are called reflex angles.
• Angles that have the same measure (i.e. the same magnitude) are said to be equal (UK) or congruent (USA). An angle is defined by its measure and is not dependent upon the lengths of the sides of the angle (e.g. all right angles are congruent).
• Two angles opposite each other, formed by two intersecting straight lines that form an "X"-like shape, are called vertical angles or opposite angles or vertically opposite angles. These angles are equal in measure.
• Angles that share a common vertex and edge but do not share any interior points are called adjacent angles.
• Two angles that sum to one right angle (90°) are called complementary angles.

  The difference between an angle and a right angle is termed the complement of the angle.

• Two angles that sum to a straight angle (180°) are called supplementary angles.

  The difference between an angle and a straight angle (180°) is termed the supplement of the angle.

• Two angles that sum to one turn (360°) are called explementary angles or conjugate angles.
• An angle that is part of a simple polygon is called an interior angle if it lies on the inside of that simple polygon. A concave simple polygon has at least one interior angle that exceeds 180°.

  In Euclidean geometry, the measures of the interior angles of a triangle add up to π radians, or 180°, or 1/2 turn; the measures of the interior angles of a simple quadrilateral add up to 2π radians, or 360°, or 1 turn. In general, the measures of the interior angles of a simple polygon with n sides add up to [(n − 2) × π] radians, or [(n − 2) × 180]°, or (2n − 4) right angles, or (n/2 − 1) turn.

• The angle supplementary to the interior angle is called the exterior angle. It measures the amount of rotation one has to make at this vertex to trace out the polygon. If the corresponding interior angle is a reflex angle, the exterior angle should be considered negative. Even in a non-simple polygon it may be possible to define the exterior angle, but one will have to pick an orientation of the plane (or surface) to decide the sign of the exterior angle measure.

  In Euclidean geometry, the sum of the exterior angles of a simple polygon will be one full turn (360°).

• Some authors use the name exterior angle of a simple polygon to simply mean the explementary (not supplementary!) of the interior angle.^[6] This conflicts with the above usage.
• The angle between two planes (such as two adjacent faces of a polyhedron) is called a dihedral angle. It may be defined as the acute angle between two lines normal to the planes.
• The angle between a plane and an intersecting straight line is equal to ninety degrees minus the angle between the intersecting line and the line that goes through the point of intersection and is normal to the plane.
• Alternate angles, corresponding angle, interior angles and exterior angles are associated with a transversal of a pair of lines by a third.
• A reference angle is the acute version of any angle determined by repeatedly subtracting or adding 180 degrees, and subtracting the result from 180 degrees if necessary, until a value between 0 degrees and 90 degrees is obtained. For example, an angle of 30 degrees has a reference angle of 30 degrees, and an angle of 150 degrees also has a reference angle of 30 degrees (180-150). An angle of 750 degrees has a reference angle of 30 degrees (750-720).^[7]

Angles between curves[link]

The angle between the two curves at P is defined as the angle between the tangents A and B at P

The angle between a line and a curve (mixed angle) or between two intersecting curves (curvilinear angle) is defined to be the angle between the tangents at the point of intersection. Various names (now rarely, if ever, used) have been given to particular cases:—amphicyrtic (Gr. ἀμφί, on both sides, κυρτός, convex) or cissoidal (Gr. κισσός, ivy), biconvex; xystroidal or sistroidal (Gr. ξυστρίς, a tool for scraping), concavo-convex; amphicoelic (Gr. κοίλη, a hollow) or angulus lunularis, biconcave.^[8]

Dot product and generalisation[link]

In the Euclidean plane, the angle θ between two vectors u and v is related to their dot product and their lengths by the formula

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{u} \cdot \mathbf{v} = \cos(\theta)\ \|\mathbf{u}\|\ \|\mathbf{v}\|.

This formula supplies an easy method to find the angle between two planes (or curved surfaces) from their normal vectors and between skew lines from their vector equations.

Inner product[link]

To define angles in an abstract real inner product space, we replace the Euclidean dot product ( · ) by the inner product Failed to parse (Missing texvc executable; please see math/README to configure.): \langle\cdot,\cdot\rangle , i.e.

Failed to parse (Missing texvc executable; please see math/README to configure.): \langle\mathbf{u},\mathbf{v}\rangle = \cos(\theta)\ \|\mathbf{u}\|\ \|\mathbf{v}\|.

In a complex inner product space, the expression for the cosine above may give non-real values, so it is replaced with

Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{Re}(\langle\mathbf{u},\mathbf{v}\rangle) = \cos(\theta)\ \|\mathbf{u}\|\ \|\mathbf{v}\|.

or, more commonly, using the absolute value, with

Failed to parse (Missing texvc executable; please see math/README to configure.): |\langle\mathbf{u},\mathbf{v}\rangle| = \cos(\theta)\ \|\mathbf{u}\|\ \|\mathbf{v}\|.

The latter definition ignores the direction of the vectors and thus describes the angle between one-dimensional subspaces Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{span}(\mathbf{u})

and  Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{span}(\mathbf{v})
spanned by the vectors Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{u}
and  Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{v}
correspondingly.

Angles between subspaces[link]

The definition of the angle between one-dimensional subspaces Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{span}(\mathbf{u})

and  Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{span}(\mathbf{v})
given by

Failed to parse (Missing texvc executable; please see math/README to configure.): |\langle\mathbf{u},\mathbf{v}\rangle| = \cos(\theta)\ \|\mathbf{u}\|\ \|\mathbf{v}\|

in a Hilbert space can be extended to subspaces of any finite dimensions. Given two subspaces Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{U},\mathcal{W}

with Failed to parse (Missing texvc executable; please see math/README to configure.): \operatorname{dim}(\mathcal{U}):=k\leq \operatorname{dim}(\mathcal{W}):=l

, this leads to a definition of Failed to parse (Missing texvc executable; please see math/README to configure.): k

angles called canonical or principal angles between subspaces.

Angles in Riemannian geometry[link]

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

Failed to parse (Missing texvc executable; please see math/README to configure.): \cos \theta = \frac{g_{ij}U^iV^j} {\sqrt{ \left| g_{ij}U^iU^j \right| \left| g_{ij}V^iV^j \right|}}.

Angles in geography and astronomy[link]

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 (usually) the Greenwich meridian as references.

In astronomy, a given point on the celestial sphere (that is, the apparent position of an astronomical object) 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.

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.

See also[link]

• Angle bisector
• Argument (complex analysis)
• Astrological aspect
• Central angle
• Clock angle problem
• Complementary angles
• Great circle distance
• Inscribed angle
• Protractor
• Solid angle for a concept of angle in three dimensions.
• Supplementary angles
• Irrational angle
• Angular velocity

Notes[link]

References[link]

• Heiberg, Johan Ludvig (1908). Heath, T. L.. ed. Euclid. The thirteen books of Euclid's Elements. 1. http://books.google.com/books?id=UhgPAAAAIAAJ. 

Attribution

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Angle". Encyclopædia Britannica (11th ed.). Cambridge University Press. 

External links[link]

Wikimedia Commons has media related to: Angles

• Angle Bisectors in a Quadrilateral at cut-the-knot
• Constructing a triangle from its angle bisectors at cut-the-knot
• Angle Estimation – for basic astronomy.
• Angle definition pages with interactive applets.
• Various angle constructions with compass and straightedge
• GonioLab DD – Convert between DecDeg and DegMinSec and vice-versa (requires Java Web Start)