The BBC radio4 programme More or Less (broadcast weekly on Fridays; repeated on a Sunday) deals with claims involving numbers or statistics. Unusually, the episode broadcast on 11 January 2019 contained an astronomical question (‘Which is our nearest planet?). That arose from a remark by (Prof) Chris Lintott in a Sky at night TV programme, where he claimed that Mars is our nearest neighbour. That was odd since surely everyone knows that the orbit of Venus is nearer to us that the orbit of Mars.

Consequently the programme, presented by Tim Harford, went to work on the answer. Since the inner planets constantly change in their relationship to one another the answer is not obvious. The question had to be adjusted to ‘Which planet spends more time as our nearest neighbour for more time than any other planet?’

This task was given to a statistical expert (‘Oliver’) who wrote a computer program to calculate which planet on average is nearest to Earth than any other. This was then commented on by David Rothery, Professor of Planetary Geoscience at the Open University. He was as surprised as anyone that the answer turned out to be Mercury! The calculation showed that the nearest planet is Mars 18 per cent of the time, Venus 36 per cent of the time, but Mercury 46 per cent of the time. This may be because Mercury’s short orbital period means that it is more frequently on our side of the sun than the two other candidates, which often spend long periods on the far side.

Actually, at the time Chris Lintott made his claim, Mars was the nearest planet.

A podcast of the radio programme is available from the More of Less website, where the discussion starts 22 minutes in.

Steuart Campbell

This is a slightly expanded version of a presentation Steuart Campbell gave to the ASE on 1 February 2019. Steuart is a member of ASE and a science writer.