Brian Dredge, of Palmerston North, asks :-

Has the Earth's gravity changed over time?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

There is no evidence that Earth's gravity has altered appreciably since the earth condensed.

The gravity pull of any planet is proportional to the planet's mass. Gravity weakens with distance. So the gravity pull at the surface of a planet depends on how big the planet is; how far the surface is from the centre. Jupiter is 320 times earth's mass but the gravity at the top of its clouds -- its "surface" -- is only 2.6 times the gravity at earth's surface. That is because the cloud tops are 70,000 km from Jupiter's centre. We are 6000 km from Earth's centre.

Most planets spin. That makes a centrifugal (outward) force that slightly lessens the gravity pull near the equator. Jupiter spins once in 10 hours, so its centrifugal force is quite strong. It reduces Jupiter's gravity pull by 9 per cent at the equator compared to the gravity pull at the poles. The effect is much smaller on Earth: the change is about 0.5 per cent.

The Earth's spin is slowing down gradually due to the drag of tides on the globe. The spin energy (angular momentum) lost by Earth goes into the moon, making its orbit bigger. Asteroid collisions like the one that killed the dinosaurs are much too tiny to have any appreciable affect on Earth's spin.

Tiny variations in gravity over Earth's surface ("gravity anomalies") are caused by rocks of different densities under the surface. These variations can be used to aid searches for oil and minerals.

There have in the past been theories that gravity is changing slowly as the universe expands. There is no evidence for gravity changing.