Sciency Words: Sol

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


If you’ve read or seen The Martian or pay any close attention to NASA’s ongoing Mars missions, you probably already know what this word means. A sol is a Martian day. The word itself is a shortening of the term solar day, the period of time it takes for a planetary body to rotate once in relation to the Sun.

Technically we could apply this term to any planetary body that experiences regular day/night cycles. A solar day on Earth is 24 hours, as you probably already knew. A solar day on Jupiter is about 10 hours long, and a solar day on Venus is roughly 2,802 hours (or 116.75 Earth days).

But for whatever reason, the shortened form “sol” seems to be used only in reference to Mars, where it equals 24 hours, 39 minutes, and 35 seconds. It’s weird how close a Martian sol is to an Earth day, isn’t it?

Starting with the Spirit and Opportunity missions, NASA has actually made some of its scientists and engineers work on Mars time rather than Earth time. That extra 39 minutes and 35 seconds obviously messes with people’s sleep cycles, eating habits, and social calendars. But it’s important for the crew in mission control to be in sync with the robotics activities taking place on Mars.

Since humans have a natural tendency to be playful with language, a few clever new words have emerged as a result:

  • Yestersol: the sol prior to the current sol, formed by analogy with yesterday.
  • Tosol: the current sol, formed by analogy with today.
  • Solmorrow: the sol after the current sol, formed by analogy with tomorrow.
  • Nextersol: an alternative to solmorrow, presumably formed by analogy with yestersol. Personally I like the sound of solmorrow better.

What really pleases me about these terms is that we haven’t even landed the first humans on Mars yet, and we’re already coming up with vernacular lingo for the Red Planet. Could this be a preview of how Mars colonists might actually speak one day… I mean, one sol? And how else might we adapt human culture to the new environment? I for one am looking forward to Mars’s version of Shakespeare.

Next time on Sciency Words: it’s one thing to know that a Martian day is called a sol, but how the heck are you supposed to find tosol’s date on a calendar?

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