Stephanie White, of Kakanui School, asks :-

How do scientists measure the distances between planets?

Alan Gilmore, an astronomer at the Mt John University Observatory, responded.

These days they use radar. The time taken for a radar signal to travel to a planet and back tells us how far away the planet is. Of course the radar signal is being reflected off a large part of the planet so one measurement isn't very precise. It's better to have a spacecraft on the planet, or orbiting around it, to give a sharp radar echo.

In the 1970s the Viking Spacecraft on Mars echoed radar signals. They made it possible to measure the distance from Earth to Mars to better than one metre!

Before radar was invented astronomers had to use methods based on trigonometry (measuring angles) to find the distances to planets. This was first done in 1604 by the mathematician Johannes Kepler. He was able to find the distances of planets in terms of earth's distance. Kepler found that Venus was 0.72 earth's distance from the sun. Mars, Jupiter and Saturn were 1.4, 5.0 and 9.0 times earth's distance respectively.

But Kepler couldn't give the planets' distances in everyday units, like miles or kilometres. The only way to do this was by looking at a planet from different places on earth and seeing how different was its position in the sky. The difference is a very tiny angle, hard to measure accurately. This problem occupied astronomers for a long time.

Captain Cook went to Tahiti in 1769 to measure the time taken for Venus to cross (transit) the sun. Timings of the transit from different places on the earth would tell the distance to Venus. The method worked, but not very well. Early this century a similar method was used to measure the distance of a nearby asteroid in the night sky. There was still some uncertainty in the distances to planets right up till the 1960s when radar was first used.