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Wednesday, 16 January, 2002, 08:49 GMT
Measuring the Moon
The Moon (Associated Press)
Previous experiments show the Moon is slowly moving away
By BBC News Online's Helen Briggs

Imagine measuring the distance between the Moon and the Earth down to the last millimetre.

One US astronomer intends to spend the next five years doing just that.


It's like winning the lottery - very tall odds

Tom Murphy, University of Washington
We already know that the average distance between the centre of the Moon and the centre of the Earth is about 385,000 kilometres (239,000 miles). But Tom Murphy, a researcher at the University of Washington, wants to reduce the uncertainty as far as is technically possible.

His tape measure will be a giant telescope at Apache Point in New Mexico. Retroreflectors left on the surface of the Moon by various space missions, including the Apollo 11 lunar landing, will also come in handy.

Exact hit

The reflectors are made up of hundreds of special prisms that reflect light back to its source. Another tool is a powerful laser that will be used to create a bullet of light.

Moon, Nasa
This reflecting device was left on the Moon by the Apollo 11 team
Dr Murphy and colleagues will focus the beam at the surface of the Moon via the telescope.

By measuring the time it takes for the light to travel to the Moon and back, the scientists will be able to calculate its precise distance from the Earth.

But the task is not as simple as it sounds. The beam of light must hit the retroreflectors, each about the size of a suitcase, on the lunar surface.

This is made even trickier by the fact that the beam will be about 2 kilometres (1.2 miles) wide by the time it reaches the Moon.

Emulating Einstein

Dr Murphy said: "Only one in 30 million of the photons that you launch to the Moon will actually find the retroreflector. It's like winning the lottery - very tall odds."

The Moon
The only natural satellite of Earth
The second brightest object in the sky after the Sun
First visited by the Soviet spacecraft Luna 2 in 1959
The first manned landing was on July 20, 1969
Previous data show the Moon is receding from Earth at about 3.8 centimetres every year
By the time the light beam makes it back to Earth, it will have expanded to about 15 kilometres (9.3 miles) wide.

But the researchers hope to be able to detect individual photons of light with the technique, known as laser ranging.

Continuing improvements in lasers and electronics over the years have led to recent measurements of the Earth-Moon distance that are accurate to about two centimetres (less than one inch). Dr Murphy's group hope to do much better.

Fate of Universe

They will also perform some of the most sensitive ever gravity experiments.

"This is essentially measuring the weight of gravity, and this is the only type of project that can currently do that," said Dr Murphy.

The work, which is being funded by the US space agency Nasa, could shed light on whether the strength of gravity is changing with time, as some cosmologists predict.

Dr Robert Massey, an astronomer at the Royal Observatory, Greenwich, UK, said the Moon was moving away from the Earth by a few centimetres each year. He told BBC News Online: "The experiment will give much more accurate measurements of this changing distance."

He said the gravity research could ultimately help our understanding of the fate of the Universe.

"An understanding of gravity is fundamental to our understanding of the evolution of the whole Universe," he said.

See also:

20 Jul 99 | Sci/Tech
Apollo Moon experiment still working
23 Nov 01 | Asia-Pacific
China eyes the Moon
16 Oct 01 | Sci/Tech
Apollo samples reveal Moon's origin
15 Aug 01 | Sci/Tech
How the moon was made
10 Jan 01 | UK
Total eclipse of the Moon
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