How do I explain to my children why the Moon has phases? (Beginner)

I am a mother of three young children (12, 7 and 6) who have asked me the question, "Why does the moon have different faces (said the 6 yr old) phases (said the 12 year old). I can usually find an answer to appease their cornucopia of questions, however, this one stumped me. The question as I've deciphered it is why or how does the moon show on earth as a half-moon, 1/4 moon, or "smiley face" moon. And of course my oldest wants to know what covers it (if anything). I am not astronomically (?) inclined, so I don't even know if I have worded this question properly.

The question is worded just fine! I hope that I can explain the answer well enough. Nothing is covering the Moon, part of it is just not illuminated as it is facing away from the Sun.

There is a neat little practical demonstration that you could try at home if you wanted, and in any case it helps to explain what's going on if I use the analogy. The demonstration goes as follows: first find a ball and a flashlight (you need to be in a somewhat darkened room). One of you has to be the sun (the one with the flashlight) another is the moon (the one with the ball) and the third person is the Earth (any extra people can be the Earth too!). If the line between the sun and the moon is at a right angle to the line between the moon and the earth (ie. if the 3 of you stand at 3 corners of a square), half of the moon will be illuminated from the point of view of 'the Earth.' If 'the Moon' walks round 'the Earth' then the person who is the Earth will see more or less of the Moon illuminated. This is all that is needed to create the phases of the Moon. You'll notice that when the Moon is between the Sun and the Earth an eclipse occurs - for why we don't actually see an eclipse every new Moon see below.

Last Spring I tried a similar demonstration out with a 2nd grade class as part of the GSSOP at Cornell. It turns out 7 year olds take things very literally, and without a dark enough room or bright enough flashlight the demo doesn't work that well. They also had problems because we could see the pattern of the illumination on our "Moon" from the flashlight. So it's not a perfect demonstration, but it works OK.

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Website:  http://icg.port.ac.uk/~mastersk/

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