Deduction here is that the lower Sun is the shadow is longer, disappearing when Sun is on the horizon. So when angle is 0, sin(0) = 0 and we have "assumed" result that the length of the shadow is 0, because there is no shadow.
Note that the measure of void ( \phai) is zero, as an elementary mathematics on measure.
It seems that you do not have the important concept of zero!!
For this concept, India is very great!! Meanwhile, Greek is very poor.