Sciency Words: Synodic Month

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Science fiction uses scientific language as a form of artistic expression.  With that in mind, today’s post is part of a series here on Planet Pailly called Sciency Words.  Every Friday, I’ll bring you a new and interesting scientific word to help us all expand our scientific vocabularies.  Today’s word is:


You’re probably aware of the fact that a month is approximately the amount of time it takes for the Moon to complete one full orbit around the Earth.  But if we’re talking about science, “approximately” doesn’t cut it.  That’s why we have the synodic month.

Image courtesy of
Image courtesy of

A synodic month is exactly the amount of time it takes the Moon to complete one full orbit around the Earth (keeping in mind that the Earth is also orbiting the Sun).  It’s a bit easier to remember than the regular month.  There’s nothing about February being shorter than June or June being shorter than July.  Unfortunately, twelve synodic months don’t add up to a solar year, so we still have to keep using the calendars we already have.

The length of a synodic month is a little more than twenty-nine and a half days, give or take a fraction of a second.  Over time, however, the synodic month will grow longer and longer.  This is because the Moon is slowly moving away from the Earth.  One day, the gravitational pull between the Earth and the Moon might balance out, and the synodic month will stabilize at roughly 47 days.  Or perhaps, billions of years from now, the Moon might escape Earth’s gravity all together and fly off into space by itself.

So the next time you see the Moon glowing pale white in the sky, be sure to take a moment and appreciate it because it might not be there forever.

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Did you already know this word?  If so, please share another sciency word in the comments below.  That way, we can all keep expanding our sciency vocabularies together!

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