What do you think of this fact?
Ask anyone which planet is closest to Earth and you’ll hear “Venus.” Fair enough: Venus can come within 0.28 astronomical units of us. One astronomical unit, or AU, is the distance from Earth to the Sun. But that’s only Venus at its closest. On average, the winner is Mercury.
Venus is often far away
The classic answer simply subtracts orbits: Venus circles the Sun at an average of 0.72 AU, Earth at 1 AU, so the gap is 0.28 AU. That only works when both planets are on the same side of the Sun. When Venus is on the far side, it can be up to 1.72 AU away, ZME Science explains.
Mercury, on the other hand, never strays far from the Sun. Its average orbital radius is just 0.39 AU. Over time, that adds up.
10,000 years in a computer
In 2019, Tom Stockman, Gabriel Monroe and Samuel Cordner laid this out in a commentary in Physics Today. They simulated all eight planets over 10,000 years and recorded their distances every 24 simulated hours. The result: on average, Venus is 1.14 AU from Earth, Mercury only 1.04 AU.
They also offered a simple shortcut, the “point-circle method.” Its results were less than 1 percent off from the simulation, while the traditional method was off by up to 300 percent. The rule behind it got a silly name: the “whirly-dirly corollary,” after an episode of “Rick and Morty,” ZME Science reports. It says the closer an inner orbit is to the Sun, the smaller its average distance to the outer orbit.
Bonus fact: The same logic makes Mercury the closest planet on average to every other planet in the solar system. According to ZME Science, Uranus is on average even closer to Mercury than to its neighbor Neptune.
So Earth’s closest neighbor is the one almost nobody guesses.