The sine function returns the sine of an angle provided in radians. In geometric terms, the function returns the vertical component of the point formed by the angle on the unit circle.
sin(θ)
Name  Description 

θ (theta)  The radian angle 
Returns the sine of an angle provided in radians.
The sine function returns the sine of a number provided in radians. In geometric terms, the function returns the vertical component of the point formed by the corresponding angle (theta) on the unit circle. For example, given the angle of radians (one third of a turn) the function returns the value .
Visually the return value is equal to the ycomponent of the point on the unit circle corresponding to the angle of radians. This is shown in the figure below which illustrates the unit circle in the XY
coordinate plane. Given the angle which represents a third of a rotation around the circle, the sine function returns the ycomponent of the resulting position on the unit circle.
These examples show the return value of sine for some common angles formed from dividing the unit circle into equal parts. Note, the circle constant (tau) represents a full rotation in radians.
These examples show the return value of sine for some common angles formed from dividing the unit circle into equal parts.
sin(0*TAU/8) = 0
sin(1*TAU/8) = 0.7071... // sqrt(2)/2
sin(2*TAU/8) = 1
sin(3*TAU/8) = 0.7071... // sqrt(2)/2
The sine function is part of a group of trigonometric functions that return ratios related to the geometry of the circle. The sine function is one of the three main functions: sine, cosine and tangent which take in an angle as input and return a ratio as output. The graph of the sine function is shown below which demonstrates the possible output of the sine function for the domain of to , where . This plot demonstrates the output of one full rotation around a circle.
The sine function is periodic, meaning that the output of the function repeats indefinitely. Geometrically, the function’s output can be visualized as the vertical component of a point on the unit circle corresponding to the input angle. This can be seen from the definition of the function shown below in terms of the circle.
Conceptually, the sine of an angle is defined by the ratio of a corresponding point’s coordinate over the radius (distance from the origin). Geometrically, the function returns the coordinate of the point formed by the angle on the unit circle. This can be seen by substituting into the definition above.
The sine function can be defined using calculus by a Taylor Series approximation^{[1]}.

Derive Sine Function (Taylor Series) Example