The closest planet to the Earth - really?

Well, who knew? When asked which planet is the nearest to the Earth, as contestants on an episode of British quiz show QI were recently, I would probably have gone for Venus. Others may have guessed Mars (and did). One contestant guessed the Moon, which is a real non-starter. But only one contestant guessed correctly: Mercury!

The problem is the way the planets are normally shown in models of the Solar System, which have Mercury closest to the Sun, followed by Venus, then Earth and then Mars. But planetary orbits are not as well behaved as that model suggests.

The minimum distance from Earth to Venus is 24 million miles, to Mars 34 million miles, and to Mercury 48 million miles. But planetary orbits, particularly Mercury's, are more or less eccentric, and Venus, for example, spends much if its time on the opposite side of the Sun to the Earth. Anyway, computer modelling reveals that Mercury is closest to the Earth 46% of the time, Venus is closest 36% of the time, and Mars just 18% of the time, making Mercury the closest on average.

Of course, the answer to the question depends on your definition of "closest" - I would still argue that Venus is closest, because it comes closest! - but it's interesting how our preconceptions can be upset.