Because of its large mass, the Earth exerts a considerable force of of gravitational attraction on objects close to it.

As yet, there is still no fully satisfactory theory to explain why gravity acts. However, even in such a complex task as the navigation of a space probe, the laws of gravitational attraction formulated by Isaac Newton more than three centuries ago are still relevant.

Since the force with which the Earth attracts an object depends on the mass of the object (and the square of its distance from the Earth), it is more useful to think in terms of the object's acceleration due to gravity. Near the Earth's surface, a falling object increases its speed by about 9.8m/sec in every second of its fall, if air resistance is ignored.

The gravity field of the Earth in space approximates fairly well to that of a uniform non-rotating sphere. It is, however, complicated by a variety of factors, for instance the overall shape of the Earth and the presence of irregular features on its surface.

On the surface itself, gravity is also affected by the daily rotation of our planet. These factors are only significant in precise measurements, however, and if the Earth were to cease rotating, people would not experience any change in gravitational force.

Gravity and the Earth's rotation

The rotation of the Earth affects gravity in two quite distinct ways. Firstly, the spin produces a centrifugal acceleration which tends to oppose gravity. This effect is strongest at the equator and diminishes to zero at the poles where there is no movement of rotation. From this cause alone, the value of g (acceleration due to gravity) at the equator is about 0.35% less than at the poles.

Secondly, the rotation of the Earth distorts its spherical shape, giving it a slightly flattened appearance. As a result, the polar diameter is about 43km less than the equatorial diameter (The equator itself corresponds very closely to a circle). This flattened shape has the effect of still further reducing g at the equator while the value at the poles is slightly increased. Ignoring local effects, the value of g varies from about 9.780m/sec2 at the equator to about 9.832m/sec2 at the poles, giving a maximum variance of a little more than 0.5 per cent.

Gravity above and below the Earth's Surface

The Earth's gravitational field can be detected far out into space, and it is the gravitational attraction between the Earth and the Moon that keeps the latter in its elliptical orbit and always presenting the same side toward Earth. Studies of lunar motion first led Newton to formulate his inverse square law for gravitational attraction. But the lunar orbit is also affected by the tremendous gravitational pull of the Sun. Even at the distance of the nearby planets of Venus and Mars, however, the Earth produces measurable irregularities, or perturbations, in these planets' orbital motions.

The flattening of the Earth also distors its gravitational field and this has a noticeable effect on the orbital motion of the Moon. Closer to Earth, the orbits of artificial satellites are influenced by irregularities in the Earth's gravitational field, with the result that the whole orbit spins or precesses in space. Because of this phenomenon, the precise flattening of the Earth is best calculated from studies of the motion of artificial satellites. Such studies also enable anomalies over the Earth's surface to be mapped in detail from the minor perturbations caused in a satellite's orbit by local variations in the gravity field.

If the Earth were of uniform density throughout, gravity would gradually decrease below ground level towards the centre of the Earth. However, studies of the passage of earthquake waves through the Earth's interior show that the density of the rocks increases rapidly with depth. This is partly because of the pressure of the material above, but is also due to the presence of heavy minerals such as compounds of iron. The value of g reaches a maximum of about 10.5m/sec2 at the boundary of the liquid core - some 2,900km down. At the Earth's center, g is zero, because the attractive force is equal in all directions.