Why is only one side of the Moon visible from Earth?
Why do we only ever see the same side of the moon?
If this is to do with gravity are there any variables which mean we might one day see more than we have before?
The reason for this is what we call tidal locking:
Tidal locking (or captured rotation) occurs when the gravitational
gradient makes one side of an astronomical body always face another,
an effect known as synchronous rotation. For example, the same side of
the Earth's Moon always faces the Earth. A tidally locked body takes
just as long to rotate around its own axis as it does to revolve
around its partner. This causes one hemisphere constantly to face the
partner body. Usually, at any given time only the satellite is tidally
locked around the larger body, but if the difference in mass between
the two bodies and their physical separation is small, each may be
tidally locked to the other, as is the case between Pluto and Charon.
This effect is employed to stabilize some artificial satellites.
Fig. 1: Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. (Source: Wikipedia)
Fig. 1, cont.: Except for libration effects, this results in the Moon keeping the same face turned towards the Earth, as seen in the figure
on the left. (The Moon is shown in polar view, and is not drawn to
scale.) If the Moon were not spinning at all, it would alternately
show its near and far sides to the Earth while moving around our
planet in orbit, as shown in the figure on the right.
Fig. 2: Lunar librations in latitude and longitude over a period of one month (Source: Wikipedia)
Libration is manifested as a slow rocking back and forth of the Moon
as viewed from Earth, permitting an observer to see slightly different
halves of the surface at different times.
There are three types of lunar libration:
Libration in longitude results from the eccentricity
of the Moon's orbit around Earth; the Moon's rotation sometimes leads
and sometimes lags its orbital position.
Libration in latitude results
from a slight inclination between the Moon's axis of rotation and the
normal to the plane of its orbit around Earth. Its origin is analogous
to how the seasons arise from Earth's revolution about the Sun.
Diurnal libration is a small daily oscillation due to the Earth's
rotation, which carries an observer first to one side and then to the
other side of the straight line joining Earth's and the Moon's
centers, allowing the observer to look first around one side of the
Moon and then around the other—because the observer is on the surface
of the Earth, not at its center.
I might add that the tidal effects of the moon are slowing down the Earth's rotation as well, leading to "leap seconds" and the like. The energy of the lost Earth's angular momentum causes the moons distance to recede. In a few billion years the Earth will also be tidally locked with the much more distant Moon.