I live at roughly $(52.4^\circ,-2.1^\circ)$. On sunny evenings I've often looked at the Moon and the Sun and noticed that the light part of the Moon does not appear to line up with the Sun. For example, at about 17:00 GMT on 13 Mar 2011, I noticed the half Moon was facing toward a point roughly $10^\circ-20^\circ$ above where the Sun appeared to be. Why?
Astronomy – Why the Light Side of the Moon Appears Misaligned with the Evening Sun
astronomygeometrymoonsun
Related Solutions
This is a gravitational phenomenon known as tidal lock. It is closely related to the phenomenon of tides on Earth, hence the name.
Tidal locking is an effect caused by the gravitational gradient from the near side to the far side of the moon. (That is, the continuous variation of the gravitational field strength across the Moon.) The end result is that the Moon rotates around its own axis with the same period as which it rotates around the Earth, causing the face of one hemisphere always to point towards the Earth.
The problem with most of the earth-moon pictures is that they show the Earth and moon very close together - which suggests that the moon is in the earth's shadow for almost half of the time.
So in the picture linked to above - it looks like a full moon should be dark.
The real picture is more like this
Best Answer
This is what you expect, in terms of the moon pointing towards the sun:
That is, the line across the moon appears perpendicular to the line towards the sun.
Now the above is a flat drawing. The sky appears curved (i.e. the dome of the starry sky). So that curve may introduce some apparent distortion.
To make a drawing that avoids the curvature issue, consider a drawing that only includes the sun, the moon, and a small amount of sky around the line connecting the two. (By the way, even for the curved bowl of the sky, the shortest line connecting the two is well defined except in the case of a new or full moon.) That drawing is approximately flat and will show the above relation.