MODERN MEASUREMENTS OF THE DISTANCE TO VENUS

by Knut Jørgen Røed Ødegaard

Today we know the distances to Venus and the Sun with great precision. The breakthrough came in 1961. For the first time scientists succeeded in measuring the distance directly by sending radar signals from ground.

Huge antennae in the Goldstone system. In 1961 two antennae with diameter 26 meters used to register radar echoes from Venus. By timing the signals this gave for the first time the distance with very great precision. Foto: JPL

Already in 1958 another group of scientists claimed to have measured the distance in the same way, but they finally had to realize that the results for some strange reason were incorrect - no reflections from the surface of Venus had been observed.

On March 10, 1961 the gigantic dish shaped antennae in Goldstone in the Mojave desert in California, USA were turned towards Venus. The equipment was mainly constructed for communication with spaceships and was the most sensitive receivers in the world.

The experiment lasted for 2 months and 238 hours of radar echoes were collected. And they did indeed observe radar echoes from Venus. By measuring the time the signals spent (with the speed of light) on the round trip, a new and very accurate value for the Astronomical Unit (AU - the distance from the Earth to the Sun) could be determined: 149 599 000 kilometers.

New measurements with radar from ground and observations with spaceships and other instruments have improved the value further. Today the accepted value is 1 AU = 149 597 870 kilometers.

Link to Goldstone plant

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