: I just learned that mercury is in fact the closest planet to the earth. What is this madness and since when?
In: 3690
So you are probably picturing the planets lined up in order of their distance from the sun – lined up from left to right?
Yeah don’t do that. Picture them orbiting the sun all at different speeds.
What’s the closest planet to Mercury, on average? Venus, right?
Mercury is the closest planet, on average, to *all* the other planets, because, half the time, any given planet is on the other side of the sun from you, and the one with the smallest orbit will suffer (in terms of being close to any given planet) least from that. And the distance-to-the-planet-when-it’s-on-the-other-side-of-the-sun always outweighs a brief, temporary, distance-to-the-planet-when-it’s-passing-nearby.
It’s closest to Earth on average over time. Mercury is the closest to the Sun, so it is never much farther away from Earth than the Sun is. Venus, on the other hand, is much further from the sun, so when it’s on the opposite side of the Sun from us, it’s much further away than Mercury is.
This is true for any planet in any solar system. The first planet is, on average, the closest to any other given planet, so it’s a bad way to measure closeness.
In terms of space travel, Venus is the easiest to get to because of how close its orbit is and its large size compared to Mars, but Mars’ orbit is just about as close to us as Venus’.
[Relevant CGP Grey](https://youtu.be/SumDHcnCRuU)
Since the planets all go around the sun at different speeds, we have to consider all the locations of their orbit.
But to keep things simple, just consider 4 locations.
Earth is at 6 o’clock and the Sun is in the middle. When Venus and Mercury are also at 6 o’clock, Venus is of course closer to Earth.
But what happens when Venus and Mercury are on the far side of the sun at 12 o’clock? Mercury is closer to Earth and by the same amount!
Now let’s look at 3 o’clock and 9 o’clock. This is a little harder to imagine, but if you draw it you can see that Mercury is just a little bit closer to Earth than Venus is.
Average the distances for the four locations together and you find that on average Mercury is a bit closer to Earth than Venus is.
CGP Grey has a wonderful video about this which explains it very nicely
So uh… not gonna lie, I clicked on this to read it because I misread the title. My brain really expected “Mercury…is the closest planet to the sun” and I was all “I have got to know what they’ve been thinking was going on before learning this”. Instead, TIL something. 😂
Hi there. I published the result you are talking about in Physics Today a few years ago. Here is a video I put together about it with diagrams that might help. Happy to answer questions. [https://youtu.be/GDgbVIqGADQ?si=OuK31xfE-HIPnUBE](https://youtu.be/GDgbVIqGADQ?si=OuK31xfE-HIPnUBE)
The ELI5 I think is this. Imagine 2 points in Mercury’s orbit relative to Earths: when it is near and when it is far. The average distance from Earth to those two points is the distance to the Sun. Imagine the same 2 points for Venus. Again, the average distance is the Sun. So if we just consider the nearest and farthest positions, it looks like Venus and Mercury have the same average distance to Earth.
Now consider two other points 90 degrees from the original two so that lines drawn between the point in Mercury’s orbit, Earth, and the Sun would form a right triangle. The distance to those points is shorter for Mercury (with its smaller orbit) than for Venus with (its larger orbit). So if we consider the average distance from Earth to all 4 points in each planet’s orbit, Mercury is closer.
As we include more and more points, the average distance becomes more and more clear – Mercury is closer on average. It has nothing to do with speed or even orbital mechanics. It’s just geometry.
You probably think Venus or Mars are closer.
Well yes, their *orbits* are closer to the Earths orbit than Mercury’s but the planets themselves travel quite slowly round those orbits and spend a lot of their time on the other side of the Sun to the Earth. So, during this time they really are quite a long way away.
Mercury, meanwhile travels quickly round the Sun and so is on the same side as us a lot of the time. Even when it is on the opposite side of the Sun, it will still be positioned nearer to us than either of the other planets when they are on the other side
If you ask “What planet is closest to Earth?” that question may actually be several different question depending on how it is understood.
##What planet is closest to Earth *on average*?
**Mercury**
##What planet is closest to Earth *right now*?
**Mercury**
(Mars and Venus can both also be the answer here at any given point in time)
##What planet is closest to Earth *most of the time*?
**Mercury**
##What planet *gets* closest to Earth *ever*?
**Venus**
##What planet’s *maximal* distance to Earth *is the lowest*?
**Mercury**
The thing is that the orbit of Venus is closer to the orbit of Earth than the orbit of any other planet, but Venus and Earth end up on opposite sides of the sun half the time.
Mercury and Earth also end up on opposite sides of the sun half the time but mercury is much closer to the sun so adding Mercury’s distance to the sun to earth’s distance from the sun doesn’t add up to as much possible total distance.
So on average Mercury is closer to Earth than any other planet, is closer to us for the most time and moves away from us the least distance when it is farthest away..
Surprisingly this means is also true for all the other Planets. Mercury is the closest planet for Mars, Jupiter, Saturn and Neptun too.
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