I usually bristle when someone says “They didn’t need an expensive study to figure this out! It’s common sense!” Because very often studies overturn common sense. Very often, “common sense” is based on very superficial, simple observations, while in-depth scientific studies can look at more complex machinations — not to mention increasing the general knowledge base of humanity. But in the case of a study published recently, I will join the chorus of “We didn’t need a study for this.”

In brief, the study supposedly “overturns” the popular notion that Venus is the closest planet to Earth — spawning dozens of science-y news stories like this one, all of which wrote of the study’s results as if it really discovered something amazing. C’mon, people. This isn’t difficult, and certainly shouldn’t require mechanical engineers from the Army and NASA (with help from Ph.D. student in Alabama).

Four Inner Planets

I made this simple illustration to help frame the problem. Here, the orbits of the four inner planets are roughly to scale, as is the Sun in the center. The planets are NOT to scale, because if they were you couldn’t see them. It shows how people typically visualize the distances between these planets and the Sun — as if the planets are always lined up like this. In the VERY rare instances that the planets all line up in a row, then yes, Venus is the closest planet to Earth — about 36 million miles away. Mars, then, would be the second-closest, at 49 million miles away, and Mercury is third-closest at 68 million miles. But the planets are almost never lined up in a row.

Note that none of the planetary orbits are perfectly circular, as I’ve depicted them in my illustration above. All the planets get a little closer to the Sun at times and all of them then get a little further away again. I’m using the average distances here.

The problem is that most people think of the planets as if they exist in space the way they’re drawn in children’s books — all lined up in rows. As illustrated above, this means that Venus, yes, is the closest to Earth, and Mars is second. But as I said in the caption above, the planets are rarely lined up in a row. Mercury goes around the Sun once every 88 days; it laps us a few times every year. Venus takes 225 days to get around the Sun, so it too is passing us every year — sometimes once and sometimes twice. But it rarely catches up to us at the same time Mercury does. (And Mars requires nearly two of our years to complete a full orbit.)

All of that means that about half the time each planet is on the other half of its orbit, heading around the other side of the Sun from us. Now look at the illustration below.

More Realistically

In this second illustration, Earth is in the same position it was above. But I’ve moved Mercury and Venus into two other possible positions, and Mars into a more realistic spot. Guess which planet is closest? That’s right. Mercury.

On average, Mercury is the closest planet to Earth. If you chose twenty days at random from the next calendar year, statistically most of those days will see Mercury closer to Earth than Venus is. And Mars will almost always be farther than both of them.

In the same way, these scientists figured (and anyone else could have done simply by drawing a few sketches on a legal pad) that the same is true for any other planet. No matter which of the planets you stand on, Mercury is going to be the closest one to you — on average. If you get much further out that these four rocky inner planets and stand on, say, Neptune, then yes Uranus will be the closest planet to you when it swings by, but it almost never swings by. Neither does Saturn or Jupiter. Those planets take many years to orbit the Sun and are very often on the opposite side from each other. But Mercury is always there, right next to the center, whizzing around its tiny orbit every 88 days.

Conclusion

There really isn’t much more to say about this. These scientists didn’t discover anything, prove anything, disprove anything, nor add to the meaningful dialog about space in any way. All that was necessary was a simple correction of language. Instead of people saying “Venus is the closest planet to Earth”, they should remember to say that Venus’s orbit is closest to Earth’s orbit. The planets themselves are usually not as close to each other as Earth and Mercury are.