The closest planet to Earth? Probably not the one you’re thinking of

There has recently been some confusion over which of the planets of the Solar System is closest to Earth.

The order of the planets in the Solar System, moving outwards from the Sun, hasn’t changed.

Earth has Venus as the next planet inwards, towards the Sun, and Mars as the next planet going outwards from the Sun. Of those two, Venus comes closer to Earth than Mars at their respective closest approaches. So, it’s still correct to say that Venus comes closer to Earth than any other planet.

Confusion arises when we talk about the average distance between the planets.

Now, Mercury, being closer to the Sun than Venus, orbits the Sun more quickly than our ‘nearest’ neighbour. Furthermore, Mercury’s furthest distance from Earth (when it’s on the opposite side of the Sun) is much less than the furthest distance attained by Venus.

These facts mean that, if we average the distance between Earth and these two planets, Mercury is, on average, closer to Earth. It turns out that Venus is, on average, 1.14 astronomical units (AU) from Earth, but Mercury is, on average, only 1.04 AU from Earth. An AU is a unit of length equal to the average distance between Earth and the Sun.

The recent analysis showed that, for two bodies with roughly concentric and circular orbits on the same plane, the average distance between the two bodies decreases as the radius of the inner orbit decreases.

This appears to be counterintuitive. What it means is that not only is Mercury the closest planet, on average, to Earth, but it’s also the closest neighbour, on average, to each of the other seven planets in the Solar System!

But the fact that the planet Venus comes closest to Earth hasn’t changed.

Asked by: Conrad Owen, York

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