SCIENCE Transits | The AU | The AU using transit method | The AU using radar

Measurement of the distance of Venus by radar

By the late 1950's there were measurements of the Astronomical Unit derived from a number of differing methods. The problem was that each had relatively small errors, but that the differences between them were far larger than the errors assigned to them. This cannot be the case in reality and it indicated that there were systematic errors in all the measurements that had not been properly accounted for.

With the construction of large radio telescopes, such as the Mk1 telescope completed at Jodrell Bank in 1957, an entirely new way of measuring the astronomical unit became possible - that of making a direct measurement by radar of the distance of Venus at its closest approach of about 41 million km. Kepler's third law allows a very precise map to be made of the solar system, but it has no scale. Once one reasonably large distance within the solar system has been measured accurately, the scale is known so all the distances can be calculated include that of the Astronomical Unit (AU). (It is often quoted as being the mean distance of the Earth from the Sun, but that is not quite right - the AU is slightly less than the mean distance, a result of its elliptical orbit.)

In time for the closest approach of Venus in 1961, suitably powerful radars were installed on the Mk 1 telescope and also at the 70m Goldstone dish in New Mexico. These were b pulsed radars in which, in principle, a pulse of radio energy was transmitted towards Venus reaching there about 2 minutes later. The reflected pulse would then travel back towards the Earth to be detected after a further 2 minutes. If the travel time of the pulse can be measured accurately then the distance is immediately found given the known value of the velocity of light (and radio waves).

In fact it would not be possible to detect a single pulse so a train of pulses is transmitted for a period of just less than the return trip time (about 4 minutes). This is rather like juggling with many balls in the air at once rather than just one. First the transmitter is switched to the feed on the radio telescope and a sequence of pulses transmitted. The transmitter is then turned off and the receiver switched to the feed for just over 4 minutes and the returned pulses detected and averaged up so that the combined signal can exceed the noise level. At Jodrell Bank the whole sequence was then repeated over and over again for 5 minute periods. After averaging the results from a number of 5 minute periods the returned echo could be seen and hence the distance calculated. It was about 41.4 million km. When Venus is closest to us we knew from the "map" of the solar system that the Sun would be around 3.6 times further away and hence (using the precise values) a value of the astronomical unit was calculated. The Jodrell Bank value was 149,600,000 km.

At the next closest approach of Venus we used a different approach in which the relative motion of Venus and the Earth was measured over a 6 week period about the time of closest approach. This used a "carrier wave" or CW radar similar to that used by police speed cameras to measure the speed of cars passing them. The result of this experiment was 149,596,600 +/- 900 km.

Over the years observations by powerful radars have the refined the value so that now it is given as 149,597,870 km. The precision is now essentially limited by our knowledge of the velocity of light!

Internet Links
The Lovell Telescope (Jodrell Bank)
Jet Propulsion Laboratory (NASA)
How Far to the Sun? (Ohio State University)