Height on the Unit Circle

The unit circle has radius 1 and its centre at the origin. The point at angle θ, measured anticlockwise from the positive x axis, has x = cos(theta) and y = sin(theta). So the sine of an angle is the height of its point above the x axis.

At 30° the point is half a unit up, so sin(30°) = 1/2. At 150° it is at the same height on the other side of the y axis, so sin(150°) = 1/2 as well. At 210° it is the same distance below the axis: sin(210°) = -1/2. Below the x axis a height is negative.

That is all a sine graph shows: the height of the point, angle by angle.