PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY
Earth-Moon distance measurement. The dome of the laser astrometer at the Observatoire du Cote d'Azur. The green colour of the interior of the telescope is due to the laser beam it projects toward the Moon. Pulses of light, 0.4 nanoseconds long, are aimed at reflectors on the Moon, left behind by US Apollo and Russian 'Lunakhod' missions. Exact timing of the return trip yields a distance measurement accurate to 3 centimetres. Laser astrometry gives very accurate orbital data for the Earth and Moon which have implications in many Earth and Space sciences. The Observatory is near Grasse, southern France.
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