In right triangle trigonometry, the sine of an angle $A$ is defined as the ratio of two lengths, the opposite leg $a$ and the hypotenuse $c$, that's to say, $\sin A= \frac{a}{c}$?
My question is: supposing that the hypotenuse is not equal to $1$, how to see this ratio geometrically? I'm looking for a geometric interpretation of the ratio of two lengths, and how this construction can be done, if these lengths are the sides of a right triangle.