In mathematics, a unit circle is a circle of unit radius—that is, a radius of 1. Frequently, especially in trigonometry, the unit circle is the circle of radius 1 centered at the origin in the Cartesian coordinate system in the Euclidean plane. In topology, it is often denoted as because it is a one-dimensional unit -sphere. If is a point on the unit circle's circumference, then and are the lengths of the legs of a right triangle whose hypotenuse has length 1. Thus, by the Pythagorean theorem, and satisfy the equation Since for all, and since the reflection of any point on the unit circle about the - or -axis is also on the unit circle, the above equation holds for all points on the unit circle, not only those in the first quadrant. The interior of the unit circle is called the open unit disk, while the interior of the unit circle combined with the unit circle itself is called the closed unit disk. One may also use other notions of "distance" to define other "unit circles", such as the Riemannian circle; see the article on mathematical norms for additional examples.
The trigonometric functionscosine and sine of angle may be defined on the unit circle as follows: If is a point on the unit circle, and if the ray from the origin to makes an angle from the positive -axis,, then The equation gives the relation The unit circle also demonstrates that sine and cosine are periodic functions, with the identities for any integer. Triangles constructed on the unit circle can also be used to illustrate the periodicity of the trigonometric functions. First, construct a radius OA from the origin to a point on the unit circle such that an angle with is formed with the positive arm of the -axis. Now consider a point and line segments. The result is a right triangle with. Because has length, length, and length 1, and. Having established these equivalences, take another radius OR from the origin to a point on the circle such that the same angle is formed with the negative arm of the -axis. Now consider a point and line segments. The result is a right triangle with. It can hence be seen that, because, is at in the same way that P is at. The conclusion is that, since is the same as and is the same as, it is true that and. It may be inferred in a similar manner that, since and. A simple demonstration of the above can be seen in the equality. When working with right triangles, sine, cosine, and other trigonometric functions only make sense for angle measures more than zero and less than. However, when defined with the unit circle, these functions produce meaningful values for any real-valued angle measure – even those greater than 2. In fact, all six standard trigonometric functions – sine, cosine, tangent, cotangent, secant, and cosecant, as well as archaic functions like versine and exsecant – can be defined geometrically in terms of a unit circle, as shown at right. Using the unit circle, the values of any trigonometric function for many angles other than those labeled can be calculated without the use of a calculator by using the angle sum and difference formulas.
s can be identified with points in the Euclidean plane, namely the number is identified with the point. Under this identification, the unit circle is a group under multiplication, called the circle group; it is usually denoted. On the plane, multiplication by gives a counterclockwise rotation by. This group has important applications in mathematics and science.
