Which is the closest planet to Earth?

What is the planet that is closest to earth most more than other planets, and what percentage of time is it closest?

This was was discussed on the BBC “more or less” stats podcast recently the answer is; Mercury at 45%.

This was surprising and counterintuitive to me until I thought about it; Mercury can’t get much further away from the Earth than the Sun is, while Mars can get much closer to Earth it can also be much further away. But still, it seemed a bit odd so I decided to calculate it myself based on planetary positions for the last 50 years.

It took about 15 minutes to write in R and my colleague Chris followed that up with a nice visualization of the results. The answer is, in fact, Mercury 45% of the time.

This was a classic bioinformatics / data science analysis undertaking; find a source of data, in this case the moonsun R package, figure out how to munge the data into the correct format then generate some summary statistics based on that.

update: and now a much more comprensive working through of the notion has been published in Physics Today