2024-04-20

As Mars orbits the Sun, it completes one rotation every 24.6 hours, which is very similar to one day on Earth (23.9 hours). Martian days are called sols – short for "solar day." A year on Mars lasts 669.6 sols, which is the same as 687 Earth days.

- https://science.nasa.gov/mars/facts/

A year on Mars isn't 669 or 670 sols. It's 669.6! So how often does Mars have a leap year?

Each year is 0.6 sol longer than it should be. Or 0.4 less. I don't know. And I don't know why it "should" be an even number of sols either. But such is the cosmic law and the punishment for disobedience is leap years.

We have a choice here. Either we celebrate the new year at 670 sols and end up ahead, meaning we'd have to reduce the number of days some years. Or we celebrate at 669 sols and need to catch up some years.

I prefer the latter. That would mean we're 0.6 sol behind per year. Every two years we're 1.2 sols behind, and so on. This leaves us two choices:

- Wait until we're an even number of sols behind and then catch up. This would mean adding three sols to every fifth year.
- Add one sol every second year, which would catch us up a bit but still mean lagging behind 0.2 sol for every second year. That means adding a second extra sol every tenth year.

I owe a big thank you to brib for helping me out with the math. I managed to confuse both myself and others and brib set me straight!

I think I prefer the first option. Which one do you prefer?

-- CC0 Björn Wärmedal