Moon rising over the earth as viewed from space.
We only see one side of the moon. Credit: NASA

The other day I was thinking about the moon (because I love thinking about space) and did the most sensible thing which is to download this app called My Moon Phase. After looking at it for a minute I was curious about what exactly the age of the moon meant. I then became totally fascinated by NASA’s guide on the moon phases and their incredible visualizations. I also learned a few things I feel like I should have known but didn’t really grasp. Here they are.

  1. The moon orbits the earth (sure, I knew that). But I didn’t realize that it takes the moon ~28 days to orbit the earth and it also rotates at EXACTLY the same rate as well, so we only ever see one side of the moon. It’s kind of a strange phenomenon and is referred to as tidal locking.
  2. The moon is always illuminated by the sun (sure, I knew that too). But I didn’t realize that the moon’s age refers to the number of days since the moon was between the earth and the sun (referred to as the new moon). When a moon is new, we here on earth only see a side of the moon that is getting no sunlight (and thus, it is not visible). As the moon ages, it moves away from the sun and we start to see the light reflecting off the edge more and more until the earth is between the moon and sun and the moon is full.
  3. Every two weeks, we have to look in the opposite direction to see the moon (because it has rotated to the other side of the sky as it orbits the earth), and because of the tilt of the earth on its axis, when the moon is on one side we’re seeing it from a slightly different angle than from the other.
  4. Everyone any where in the world will see the same moon phase (i.e. a full moon is a full moon everywhere), the difference is that the northern and southern hemispheres will see the phases opposite to each other (i.e. the mirror image).

Let this serve as a little reminder that what you are worrying about right now is kind of insignificant in relation to the sun and moon and, you know, the motion of planetary bodies. As Seinfeld said “People always say [the scale of the universe] makes them feel insignificant, but I don’t find being insignificant depressing. I find it uplifting.”

I loved learning about this for no other reason than it fills me with wonder.

You’re welcome.

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