The sine is the opposite side / hypotenuse and the hypotenuse is the longest side. So the ratio must be less than 1, and as the lengths are positive the sine must be positive.
The sine ratio is the length of the side opposite a given acute angle divided by the length of the hypotenuse. Because the hypotenuse is the side opposite the largest angle, the 90° angle, it has to be the longest side. Thus, the ratio will have a denominator that is larger than the numerator, and the ratio will be less than 1.
