Take a look at the animation below. Do you see the side labeled sine?

In this magic picture the circle has a radius of 1. It is really convenient in trigonometry to use a circle with radius 1. You can see why if you take a look at the sine. The sine is defined as the side opposite a given angle divided by the hypotenuse. Here the division by the hypotenuse can be left out, since the hypotenuse has a length of 1 and any number divided by 1 results in itself. The same is true,of course, for the cosine.