Nothing’s more naturally predictable than the progression of a day, right? The Sun comes up in the East, rises through the sky, then sets in the West. That’s part of the fascination of places where it isn’t quite so.


However, one of the places with the craziest days is rather unaccessible: Mercury.

It’s the closest planet to the Sun, so it feels the star’s gravitational pull the most. This formidable force put Mercury in what’s called a tidal lock. Think of the Moon: it turns around itself as fast as it revolves around the Earth, showing us always the same face.

Mercury’s situation is similar, but more interesting: the ratio between orbit time and rotation, instead of 1:1 (as it is for the Moon), is 3:2. In other words, the planet turns one and a half times per (local) year, and the Sun takes two (local) years to do one full round through the sky.

To complicate matters, Mercury speeds up and down a lot orbiting the Sun—because its trajectory isn’t very round—but rotates on its axis always at the same time. As a consequence, sometimes the rotation drives the Sun’s apparent motion (as it is on Earth), sometimes it’s orbital motion. Which, needless to say, is weird.

This makes for very peculiar (and rather long) days. If we stood on Mercury, we’d see the Sun rise through the sky as usual. But after a while, once the orbital speed gets high enough, we’d see the Sun stop, and even go back for a while! Then, as rotation goes back in the driver’s seat, the Sun would resume its previous route and set normally.

There’s a cool animation of the day on Mercury in this episodes of Crash Course Astronomy.

Everywhere in the universe there is a kind of magic.


Cover photo: Wind in the Desert With Sun Flare, CC-BY-NC-ND Bill Gracey, via Flickr. Some rights reserved

Questions? Comments? Let's talk about that!

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s