Moonlock kingdom

Spooky winter full moon. Photo by Johannes Plenio on Unsplash.

Here’s a crazy thing: the earth’s gravity pulls so strongly on the moon that it has almost stopped it from spinning. Now settled into “tidal lock,” the moon only spins once in the same amount of time it takes to orbit our planet: 27 days.

The whole spin-equals-orbit thing means we only ever see one side of the moon. The one with the face, or the bunny, or the toad, depending on where you hail from.

So you can see that the idea of a dark side of the moon is actually a myth. (Sorry Pink Floyd. We still love that spacey song.) Of course there is a dark side, but it keeps changing, just like night keeps rolling around our planet. Let’s call the side we never see the “far side” (thanks Gary Larson). Sometimes that far side is dark, sometimes light.

So a 27-day orbit and only one spin. By comparison, our planet is a spinning top, rotating around once every 24 hours.

Try to picture this. As the moon slowly makes its month-long circular journey eastward around the Earth, we (wherever we are on the planet) just keep spinning past it like a revolving viewing gallery. It’s our spinning that makes the moon seem to rise and set every night or day. All earthlings (if they’re looking up) see the same moon shape on the same night. It’s just tilted differently in different places. Check it out: Google “equator smile moon.”

Over the month, as you watch the moon drift like a big round opal across a diamond-studded velvety expanse (to get poetic), you might also notice that every night it’s sitting a little bit more to the east against its starry backdrop, and that every moonrise is a little later.

Now back to the tidal lock: the earth has successfully stopped the moon. Yay Earth. But guess what? The moon is also slowing us down.

Eventually, it’ll slow us down so much that both space balls will be tidally locked with each other, the same two faces gazing lovingly at each other forever. Luckily, that will only happen about 50 billion years from now, and only if both of us make it through the sun’s fiery red-giant phase.

As we wait for that, why don’t we get out there and soak up all this spinning?