Our Moon has the so-called locked (or synchronous) rotation, i.e. it rotates once per orbiting the Earth. Consequently, in theory, it shows always the same "face" to us and we can never observe its half, which is "behind". However, due to many physical reasons (e.g., the elliptical orbit, an un-even mass distribution, etc), the Moon also oscillates like a pendulum around an axis, which constantly changes its position. This particular motion is called the LIBRATION. Due to this property we can "peek around the edges" and observe around 59 per cent of the Moon's surface. The effect of libration is observable also by naked eye. Take a good atlas of the Moon, find Mare Crisium located on the Moon's north-eastern part (remember, the Moon's East is to its RIGHT on the Northern hemisphere!), and observe it during the month. You will notice that its form changes. Sometimes it is perfectly circular, sometimes it is more elliptical and closer to the Moon's north-eastern edge. This is the consequence of the Moon's oscillation, when Mare Crisium moves either towards the middle of the Moon or in the opposite direction.