[Andrej Kandl]

Andrej Kandl asked the Naked Scientists:

A week ago, I've noticed that if I draw a line perpendicular to a moon's shadow line (half-moon), this line won`t pass through the centre of the Sun. I assume this does not deny the laws of optics, but what is it then? Optical Illusion?

What do you think?

[Soul Surfer]

You have to allow for the effect of perspective.  when the moon is a thin crescent  the angle between the sun the earth and the moon is small and the bow of the crescent points reasonably well towards the sun.  when it is a half moon the angle between the sun the earth and the moon is a right angle and they are at very different positions in the sky.   If you left the earth and moved a long way away the half moon woould point towards the sun but you are just too close to see this and the moon points a bit away