W Turner of South Dunedin asks :-

Is the Moon drawing closer to the Earth?

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

No, the moon is moving away from the earth. The movement is very slow, just 38 mm per year or 38 km per million years. The moon's average distance from Earth is 380,000 km. So in a million years the moon's distance increases by 1 part in 10,000.

The moon is receding from us because it is slowing down the earth's spin. The moon raises tides on the oceans. Mostly the tides have little effect on the earth. However, where the ocean passes through narrow channels and over shallow sea beds the water drags on the earth. This drag slows the earth's rotation. The slowing rate is very small. The day is lengthened by around 2 milliseconds (0.002 second) per 100 years. So a day is 20 seconds longer now than it was a million years ago.

The spin energy or angular momentum lost by the earth goes into making the moon's orbit bigger. That makes the lunar month longer. This effect was predicted in 1897 by the mathematician George Darwin, a son of Charles Darwin.

Early confirmation of Darwin's prediction came from coral fossils. Some corals living in waters with strong tides show daily growth bands. Fossils from the Devonian era, 370 million years ago, show that a day was about 22 hours long. Also the time from new moon to new moon was about 36 hours less than it is today.

These days it is possible to measure both effects directly. Reflectors put on the moon by the Apollo programme and by Soviet unmanned spacecraft allow laser ranging from earth. By timing how long a laser pulse takes to return from the moon scientists can measure the moon's distance to a few millimetres. Other technologies allow the earth's rotation speed to be measured with great precision.