ASTRONOMICAL UNIT

(AU) length of the semi-major axis of the Earth to the Sun, the value of which is 149,597,870 km (92,955,808 miles). It also is often defined simply as the average distance from the Earth to the Sun. The most obvious method for obtaining the value of the astronomical unit would seem to be to measure simultaneously from two places on the Earth the position of the Sun against the background of the distant stars. The two locations would have to be at opposite ends of the Earth to achieve the greatest parallax displacement. The angle subtended by the Earth's radius as viewed from the Sun is called the solar parallax. Its value is about 8.794 seconds of arc. Because the radius of the Earth is known, the distance to the Sun can thus be obtained, in principle. This method, however, does not work, because the glare of the Sun's radiation drowns out the light from the stars against which the Sun would have to be projected. Astronomers have found a way out of the dilemma. On the basis of the geometry of the motions and positions of the planets and of the Sun under Newton's inverse square law of universal gravity, a model for planetary positions and motions can be devised without the use of a scale. If at any time the distance from the Earth to a particular planet (or asteroid) can be determined, then the scale for the whole system is fixed and so is the astronomical unit. The planet Mars was used initially, but it proved to be too large and mountainous for precise measurements. The asteroid Eros, which at closest approach comes to within one-seventh of an astronomical unit to the Earth, was found to be ideal for the purpose. The close approach of Eros in 1932 yielded what seemed at the time a very precise value for the unit in kilometres. Since 1958 even more precise values for the length of the astronomical unit have been obtained by timing radar reflections from Venus and by laser ranging of the Moon. The latter technique involves bouncing laser signals off a mirror placed on the lunar surface by American astronauts. The timing of the return signal is so accurate that the distance between the observatory that transmits the signal and the reflecting surface on the Moon can be determined to within 2.5 cm (1 inch).