An introduction to Lunar Laser Ranging (L.L.R.)

(with the agreeable involvement of A. Handley)

The Moon, as always, fascinates people and influences many aspects of their daily life. Historically, its existence has helped scientists to acquire knowledge of our near universe.

Moon eclipses, allowed us to measure the earth's radius, and solar eclipses the discovery of the sun's corona. Up to the beginning of the sixties, the lunar theory, a major problem for astronomers, had reached a dead end and a breakthrough was needed. The space race provided the answer.Reaching for the moon opened the door for scientific and technical progress.

We developed the application of laser technology, using precision timing with the atomic clock and data processing, resulting in the Lunar Laser Ranging (L.L.R.) technique.

How do we use this technique to measure the earth-moon distance? The laser, a high energy, high density light beam, is aimed at targets on the moon's surface. These targets, reflectors, were put in place during the Apollo and Lunokhod missions. We measure the time taken by the light signal to travel to the moon and back. Of course these measurements need to be very precise (it only takes two and a half seconds for the light to cover the distance) so we use the atomic clock.

As the reflectors are relatively small and as a laser beam loses its intensity, only a minute part of the signal makes it back. However the information is sufficient for precise calculation of the earth and moon's movement: speed of rotation, axial variation and orbital deviation, of course taking into account the influence of other celestial bodies such as the sun.

One of the practical uses of this research is it's application to spacial and terrestrial navigation.

The next step could be to extend these studies to other parts of the solar system, for example, by putting reflectors on Mars.

Despite the increasing complexity of this relativity new technology, it appears that the limits of this experience are far from being reached. In 1985 to be within an accuracy of 18 cm was considered as a reasonable limit, today 5 mm is the going rate and we are talking about even less than one mm!

It is currently more accurate than measuring the time it takes Concorde to fly from Paris to New York within an accuracy of one tenth of a millionth of a second!

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