We study it in school, we read it in astronomy manuals and we answer almost automatically when someone asks us: among all the planets of the Solar System, Venus is the closest to the Earth.

And yet, it is not true. In reality, the planet closest to us is not the Dawn Star, but Mercury. And although it is true that Venus is the world that comes closest to the Earth at specific times of its orbit, it is also true that Mercury is the one that, on average, spends the most time being the nearest world.

In a commentary published last week in Physics Today, in fact, Tom Stockman, Gabriel Monroe and Samuel Cordner state that “due to some kind of carelessness, ambiguity or group thinking, those who have to divulge Science have disseminated information based on an erroneous assumption about what is the average distance between planets”.

Several educational websites,” the authors write, “such as The Planets and Space Dictionary, publish the distances between each pair of planets, and all show that Venus is the closest to the Earth as an average. But they’re all wrong. Even NASA literature tells us that Venus is ‘our nearest neighbor planetrio’, which is true only if we’re talking about which planet has the closest approximation to Earth, but not if we want to know which planet, on average, is closest to us.

Normally, when calculating the distance between two planets, the average distances of those worlds from the Sun are usually subtracted. But that is precisely where the error is, since that calculation only serves to know at what distance two planets are when they are at the closest possible point from each other. And it turns out that sometimes Venus is just on the opposite side of the Sun, because the two planets (Venus and the Earth) move at different speeds.

Point-circle method
In their commentary, the researchers explain how they devised a new mathematical technique, called the “point-circle method,” to more accurately measure how far two planets actually are from each other. The method averages the distance at which the planet is at a large number of points along its orbit, and also takes into account how much time each planet spends at a particular distance from the others.

Calculating in this way, it turns out that it was Mercury that was closest to the Earth for most of the time. And not only that, but Mercury also turned out to be the planet closest to Saturn, Neptune, Mars and all the other planets in the Solar System. To arrive at this surprising conclusion, the researchers verified their findings by plotting the concrete positions of all the planets in the Solar System in their orbits every 24 hours for a period of 10,000 years.

By using a more precise method to estimate the average distance between two bodies in orbit,” say the researchers in their article, “we find that this distance is proportional to the relative radius of their internal orbits. In other words, Mercury is closer to Earth, on average, than Venus, because it orbits closer to the Sun. In addition, Mercury is also the closest neighbor, on average, to each of the other seven planets of the Solar System.

To finish understanding, imagine that your next-door neighbor spends eight months a year abroad. If asked who your nearest neighbor is, what would you say? When you are at home, the nearest neighbor would undoubtedly be the neighbor to the next door, but each time you leave on a trip your nearest neighbor would become the neighbor to the next door. And since your immediate neighbor spends eight months away from home each year, it turns out that it would be the next door neighbor who, for the longest time, is actually your nearest neighbor.