Definition:Angle

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Definition

Given two intersecting lines or line segments, the amount of rotation about the intersection required to bring one into correspondence with the other is called the angle between them.


In the words of Euclid:

A plane angle is the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line.

(The Elements: Book $\text{I}$: Definition $8$)


Rectilineal

In the words of Euclid:

And when the lines containing the angle are straight, the angle is called rectilineal.

(The Elements: Book $\text{I}$: Definition $9$)


Thus the distinction is made between straight-line angles and curved-line angles.

Most of the time the fact that angles are rectilineal is taken for granted.


Subtend

Let $AB$ be a line segment and $C$ be a point:

Subtend.png

The line segment $AB$ is said to subtend the angle $\angle ACB$.


Adjacent

Two angles are adjacent if they have an intersecting line in common:

AdjacentAngles.png


Containment

The two lines whose intersection forms an angle are said to contain that angle.


Vertex

The point at which the lines containing an angle meet is known as the vertex of that angle.


Measurement

The usual units of measurement for angle are as follows:


Degree

The degree (of arc) is a measurement of plane angles, symbolized by $^\circ$.

\(\displaystyle \) \(\) \(\displaystyle 1\) $\quad$ degree $\quad$
\(\displaystyle \) \(=\) \(\displaystyle 60\) $\quad$ minutes $\quad$
\(\displaystyle \) \(=\) \(\displaystyle 60 \times 60 = 3600\) $\quad$ seconds $\quad$
\(\displaystyle \) \(=\) \(\displaystyle \dfrac 1 {360}\) $\quad$ full angle (by definition) $\quad$


Minute

The minute (of arc) is a measurement of plane angles, symbolized by $'$.

$1$ minute $= 60$ seconds (of arc) $= \dfrac 1 {60}$ of a degree of arc.


Second

The second (of arc) is a measurement of plane angles, symbolized by $''$.

$1''$ is defined as $\dfrac 1 {60}$ of a minute of arc, or $\dfrac 1 {3600}$ of a degree of arc.


Radian

The radian is a measure of plane angles symbolized either by the word $\mathrm{rad}$ or without any unit.

Radians are pure numbers, as they are ratios of lengths. The addition of $\mathrm{rad}$ is merely for clarification.


$1 \ \mathrm{rad}$ is the angle subtended at the center of a circle by an arc whose length is equal to the radius:

Radian.png


Types of Angle

Angles can be divided into categories:

Zero Angle

The zero angle is an angle the measure of which is $0$ regardless of the unit of measurement.


Acute Angle

An acute angle is an angle which has a measure between that of a right angle and that of a zero angle.


Right Angle

A right angle is an angle that is equal to half of a straight angle.


Obtuse Angle

An obtuse angle is an angle which has a measurement between those of a right angle and a straight angle.


Straight Angle

A straight angle is defined to be the angle formed by the two parts of a straight line from a point on that line.


Reflex Angle

A reflex angle is an angle which has a measure between that of a straight angle and that of a full angle.


Full Angle

A full angle is an angle equivalent to one full rotation.


It is possible to consider angles outside the $\left[{0^\circ \,.\,.\, 360^\circ}\right]$ or $\left[{0 \,.\,.\, 2 \pi}\right]$ range.

However, in geometric contexts it is usually preferable to convert these to angles inside this range by adding or subtracting multiples of a full angle.


Directed versus Undirected Angles

The most basic definition of angle is an undirected angle on the interval $\left[{0^\circ \,.\,.\, 180^\circ}\right]$ or $\left[{0 \,.\,.\, \pi}\right]$.

This definition is often insufficient, in cases such as the external angles of a polygon.

Therefore, angles are most commonly defined in one of two ways:

$(1): \quad$ Undirected angles on the interval $\left[{0^\circ \,.\,.\, 360^\circ}\right]$ or $\left[{0 \,.\,.\, 2 \pi}\right]$.
$(2): \quad$ Directed angles, with the positive direction being counterclockwise from a given line (or, if no line is specified, from the $x$-axis).
This definition is more commonly found in applied mathematics, such as in surveying, navigation, or, more colloquially, in a $720^\circ$ degree spin in skateboarding, skiing, etc.


Also see

  • Results about angles can be found here.