Today the distances to Venus and to the Sun are known with great accuracy. The breakthrough came in 1961. For the first time scientists were able to measure the distance in a direct way using radar signals emitted from the ground.
Large antennae in the Goldstone facility. In 1961 two 26 meter
antennae in this facility were used to catch radar echos from Venus. By measuring the time the signal used on its way from the Earth to Venus and back, the distance could be determined very accurately.
Already in 1958 a group of researchers claimed that they had been able to measure the distance in this way. But they finally had to realize that the results for some unknown reason were wrong. They had not observed reflections from the surface of Venus.
On March 10, 1961 the gigantic, dish formed radio antennae in the Goldstone facility turned towards Venus. The equipment was mainly built for communication with spaceships and had the best sensitivity of any receivers on Earth.
The experiment lastet for 2 months and 238 hours of radar data were collected. And we know that they really managed to receive radar echos from Venus. By measuring the time the signals spent travelling between the Earth and Venus and back with the speed of light, a news and very accurate value of the Astronomical Unit (AU) be determined: 149 599 000 kilometers.
The following decades new measurements from the ground and observations performed with space probes improved the value further. The currently accepted value is 1 AU = 149 597 870 kilometers.
Link to the Goldstone facility
Major celestial events in Norway 2010-2015