Blinded by Science blog #1: Why isn’t the moon always a circle?

Well friends, and hopefully random internet person, here is the first entry in my Blinded by Science blog (name shamelessly based on the song “She Blinded Me with Science” by Thomas Dolby which I am actually listening to while I write this).  Welcome!

Now, before I get to the main part of the entry, I thought I would begin with a quick explanation of the purpose of this blog.  I am writing this to improve my ability to communicate science and hopefully to answer science-related questions while I am at it.  That’s where you, the reader, come in.  If you have a science question, send it my way and if I like the question, I will try to answer it as best as I can.  Keep in mind that my wheelhouse is biology, and while I would eventually like to either have guest bloggers or interview people to answer those questions that I am not able to, for now I may just skip the question entirely.  This way, if/when I get questions like this: “What is the Schwarzchild radius, and why does it depend on the spin and charge of a black hole? – Jeff M”, I won’t have to just link the Wikipedia article.

Though if anyone is curious (as I was) and too lazy to actually click on that link (it’s the Internet, so I wouldn’t be surprised), here is a quick explanation of the Schwarzchild radius.  If the mass of an object were to be condensed into a sphere so dense that the gravity at the surface would prevent light from escaping, the Scharwarzchild radius would be the radius of that sphere.  As for the part about spin and charge, I’m just going to have to shrug my shoulders and move on while hopefully distracting you with something shiny.

In this case the shiny is a cool picture of the Eagle Nebula taken by the Hubble Telescope.

Actually, in a bit of a roundabout way, that brings me to the question for today’s entry, submitted by my three year old godson G.  G asks, “Why isn’t the moon always a circle?”  And to add just a little “Dawww” to the blog, when asked what he thought was the reason, G said, “Because it’s in the clouds.”  Now allow me a slight moment of godfatherly pride because his answer is actually pretty plausible, all things considered.

Well G, to answer your question, the moon doesn’t actually change shape; as the moon circles the Earth, the part of the moon that is lit by the sun and visible from the Earth changes.  That is why it looks like the moon is changing shape.

That explanation might be a little confusing, so let’s ask your mom to help us with a little demonstration.  All you need is a flashlight and a ball (or something ball-shaped).  We will pretend the ball is the moon and the flashlight is the sun.  Place the ball in the center of the room, lie down on the floor facing the ball, then have your mom turn off the lights in the room and put the flashlight on the side of the ball and turn it on like in the picture at position 1. 

You will see that only half of the ball is lit up while the other half is dark (1).  When the moon looks like this, it is called a first quarter moon.  Next, have your mom move the flashlight* so it is between you and the ball.  You will notice that the entire part of the ball that you see is lit up (2) and when the moon looks like this, we call it a full moon. 

Now have your mom move the flashlight to the left side of the ball (3).  When the moon looks like this, we call it a third quarter moon.  Finally, have your mom move the flashlight so it is behind the ball.  The part of the ball that is facing you is now all dark (4).  When the moon looks like this, we call it a new moon.

This is why the moon looks like it is changing shapes.

*This was merely for the ease of the demonstration.  For all intents and purposes, the sun stays in a fixed position and it is the movement of the moon that causes the phases to occur.