As the Moon moves on from this position towards the New Moon position, even less of the illuminated hemisphere will be visible from the Earth and it is said to wane (waning crescent).
The part of the Moon that we can see is the fraction of the side that is lit up, which is visible from Earth. Half of the Moon is always lit up, but how much of this we see depends on where the Moon is in its orbit. As the Moon travels around the Earth, the fraction first grows larger, until the Moon is full (directly opposite the Sun). The fraction then grows smaller again, until the Moon is on the same side of the Earth as the Sun.
In the course of one night, the 24 hour spinning motion of the Earth takes us past one of these distinctive phases of the Moon, and as the month progresses (and the Moon moves farther around its orbit of the Earth) the observed phase gradually changes.
These simple diagrams raise a question. Why is the full Moon not in the shadow of the Earth? In other words, when the Moon travels around to the opposite side of the Earth to the Sun, why does the Earth not block the light from the Sun?
The answer to this is that the Moon does not orbit the Earth in the same plane as the Earth orbits the Sun (the plane of the ecliptic). The consequence is that the Earth rarely stops light from reaching the Moon. When it does a lunar eclipse results.
The slightly odd thing about the Moon is that it rotates once on its axis in exactly the same time (27.3 days) that it takes to orbit the Earth. The result is that it always keeps the same face towards us so that we never see the other side of the Moon. The effect is not so strange as it may seem. It is caused by the interaction between the Earth, the Moon and the tides. The Moon did not always spin at this rate.
dark side of the Moon is not totally dark as it is illuminated by reflected light. It has been observed by satellites and space probes and by astronauts who have orbited the Moon.