The dwarf planet Eris is sometimes closer to Earth than Pluto is. That’s no secret. Wikipedia’s article on Eris even has a chart similar to the one below. But I feel like this fact has been strangely ignored in the whole is-Pluto-a-planet debate. Eris is a tiny bit smaller in diameter than Pluto, though it’s more massive. And it’s whiter, so it’s easier to see at a given distance. Though the odds were against it, there’s no reason Eris couldn’t have been discovered before Pluto.
I wanted to know when Eris was last closer, and the next time it will be closer. But, apparently, I don’t know the right place to look to get such information. Even knowing the magic word “ephemeris” wasn’t enough. I’m sure the data is out there, but it’s probably written in a language that only professional astronomers understand.
Also, I’m not sure how well we even know the orbital parameters of Eris. It might be asking too much to get an accuracy better than about 1 Earth year per Eris orbit.
NASA-JPL hosts a calculator called HORIZONS that looks like just the thing. But it only supports Pluto from the years 1900 to 2100, and Eris from 1600 to 2500. I’m baffled by this limitation. I assume NASA knows where Pluto will be 100 years from now. I wonder why they don’t want us to know.
Anyway, I took what data they had, and added just enough data points from another source to make this crude chart:
Note that this is based on distances to the Sun, even though what I really care about is distances to the Earth. But at this resolution and data quality, it would be misleading to use distances to the Earth. Just be aware that, on a yearly basis, the Earth distances wobble up and down by up to 1 AU from the Sun distances.
I conclude that Eris was last closer than Pluto from roughly 1662 to 1692. Unfortunately, the next time Eris gets close to the Sun, Pluto will be even closer. The minimum distance difference will be about 4 AU, around the year 2284. We have to wait another 558-year orbit for Eris to be closer, which it will be from about 2796 to 2865.
These windows of time are probably not entirely continuous. Near the edges of them, the Earth’s orbit may take it closer to Eris, then closer to Pluto, back and forth for a few years.