Re: Why isn't the cresent of the moon perpendicular to the sun?

This is almost certainly an effect of perspective, because the sun is so 
much further from the observer than the moon is. 

Look at the nearest corner of the room you are in. The corner line meets 
each of the two ceiling lines at right angles, but when you look at the 
corner you see angles between about 110 and 140 degrees, and the sum of the 
three angles is 360 degrees, since the three angles fill your entire visual 
circle.

In three dimensions, the line from the moon to the sun is certainly at an 
accurate right angle to the line joining the two points of the crescent 
moon. But in the visual field of an earthbound observer, the moon-sun line 
recedes from you, and is therefore oblique to the line joining the points 
of the crescent.

Your subjective adjustment of visual angles for perspective fails rather 
drastically for objects in the sky, because in the absence of other cues 
you judge the moon and the sun to be roughly the same distance away, and 
the line joining them to be much squarer to you than it really is.

So neither on a measured angle in the visual field, nor on a subjectively 
judged angle, will the crescent moon appear to be square to the sun.

There are two other effects, of relatively minor significance. One is 
atmospheric refraction -- this might produce distortion, especially when 
the moon is low in the sky, but only of the size of a degree or two at 
most. The other is a composite of lunar geography (selenography?) and 
viewing conditions that might make the actual position of the points very 
hard to judge accurately, especially for a thin crescent in bright light or 
hazy air.