What’s the nearest planet to Earth? On average, it’s Mercury. What’s the nearest planet to Venus, Mars, Jupiter, Saturn, Uranus, and Neptune? Also Mercury.
We all know the order of major planets in terms of proximity to the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and then Neptune. Intuition tells us, therefore, that our closest neighbours are Venus and Mars. Intuition, alas, is wrong.
Oh sure, if you trace the line of our orbit the two nearest orbits are indeed Venus and Mars. And when we are at our closest to each other, Venus gets closer to Earth than any other planet. But when Venus is on one side of the sun and Earth is on the other we are actually relatively far apart. And if we average out distance between us and another planet over time, accounting for both those near passes and the big gaps, Venus is not the closest. Instead, it’s Mercury.
This makes sense to me. Mercury orbits very close to the sun, so wherever the Earth is on its own orbital path it cannot be too far away from Mercury. Here’s the funny thing: this is true not just for Earth, but for every other planet in the solar system – for the same reason. Sure, sometimes Saturn and Jupiter have a little orbital flyby and are close neighbours, but on average over the course of time both of them are closer to Mercury than to each other.
[Thanks to Quite Interesting for drawing my attention to this fact.]