What shape is the moon in the sky?

Have you ever looked up at the night sky and seen a full moon? A full moon looks pretty much like a perfect circle:

So if a full moon looks like a circle, how would describe the shape of a crescent moon? Sure, it's a "crescent," but let's try to be more specific. Throughout each month, the shape of the moon changes, but the shape actually follows a specific mathematical pattern.

Here are sketches of three different "crescents," but only one of these is an accurate sketch of a crescent moon as it might appear in the sky. Which one of these do you think the moon could look like? Leave your vote in the comments below, and we'll reveal the correct answer in a later post!

Here's a little more background on the three sketches: each one starts with a circle, and then part of the circle is removed. In each sketch, the shape of the removed piece is another conic section. Conic sections are a group of mathematical functions including circlesellipsesparabolas, and hyperbolas.

In sketch A, an ellipse was removed, producing two crescents (as shown in the image below). Perhaps the moon could look like one of these crescents...

In sketch B, another circle of equal radius was removed (as shown below). Perhaps the moon looks like this crescent...

In sketch C, a parabola was drawn inside the circle, and everything to the right of the parabola was removed. Does the moon look like this crescent?

Again, vote below!


  1. Hi,
    I had a look to moon images on Google and I think it's B.

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