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Tuesday, 20 July, 1999, 16:24 GMT 17:24 UK
Apollo Moon experiment still working
The reflector is a mosaic of 100 glass half-cubes
The reflector is a mosaic of 100 glass half-cubes
By BBC News Online Science Editor Dr David Whitehouse

An experiment left on the lunar surface 30 years ago by the Apollo 11 Moonwalkers still continues to return valuable data about our satellite.

Pulses of laser light fired from the Earth are reflected by the lunar laser ranging reflector.

Three decades of scientific results
The idea was to determine the round-trip travel time of a laser pulse from the Earth to the Moon and back again, thereby calculating the distance between the two with incredible accuracy.

The data gathered has shown us that the Moon is receding from the Earth at about 3.8 centimetres (1.5 inches) every year. It has also measured minute changes in the shape of the Earth as landmasses gradually change after being compressed by the great weight of the glaciers in the last Ice Age.

Unlike the other scientific experiments left on the Moon, the reflector requires no power and is still functioning perfectly.

The reflector consists of a mosaic of 100 glass half cubes, all together about the size of an average television screen. They are called corner cubes and reflect a beam of light directly back towards its point of origin.

The laser beam ends up a kilometre-and-a-half wide on the Moon's surface
Three decades on, the McDonald Observatory Laser Ranging Station near Fort Davis in Texas, US, regularly sends a laser beam through an optical telescope to try to hit one of the reflectors.

They are far too small to be seen from Earth. Even when the beam is correctly aligned in the telescope, actually hitting a lunar reflector is quite challenging. At the Moon's surface the laser beam is a little over a kilometre-and-a-half wide. The reflected light is too weak to be seen with the human eye.

Thanks to the equipment, scientists know the average distance between the centres of the Earth and the Moon is 385,000 kilometres (239,000 miles).

This level of accuracy represents one of the most precise distance measurements ever made and is equivalent to determining the distance between London and Moscow to within quarter of a millimetre (one-hundredth of an inch).

Telescope picture courtesy McDonald Observatory

See also:

03 Jun 99 | Sci/Tech
Spaceprobe to smash into Moon
09 Jun 99 | Sci/Tech
Moon's tail spotted
10 Jul 99 | Sci/Tech
A silent death
21 Jul 99 | Sci/Tech
Frustration on Moonwalk anniversary
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