By Jeff Ray

Friday night was a full moon, the second in August. About every 18 months you get two full moons inside the same calendar month. That’s called a “Blue Moon” even though the moon isn’t actually blue.

Your kids might ask you about that when they see it tonight. If they are young enough you might get some of the standard questions as well. Those would probably be:

How big is the moon?” or, “How far away is the moon?”

If you have a couple of minutes the easiest thing to do is watch the video on this page on how to answer these two questions. You might be able to pull off that great “my dad/mom is so smart” moment.

The moon is one-quarter of the diameter of the earth but those relationships are difficult for the young.

Show them the top of your coffee cup and tell them that is the diameter of the earth (that that ball would fit [barely] inside of the mug. Tell them that is what is called the diameter of a circle. Then take out a penny and hold it next to the mug. Tell them the circle on the top of the mug is the earth and the penny is the size of the moon.

They might not get that. If not, get a piece of paper and trace the coffee cup (if it’s a standard size cup you have drawn about a 3” diameter circle). Now just place the penny inside the circle, since both are flat they might have a better chance of getting your point.

How far away is the moon? Take a basketball and a tennis ball and a tape measure (one that goes at least 25ft). That size relationship is about right between the two, if the earth was the size of a basketball the moon would be the size of a tennis ball.

The two objects on this scale would be 23 feet and four inches apart. Have them be the moon and grab the end of the tape and start walking away from you. Tell them to stop at that distance.

In the video on this page I talk about a common misconception in people’s minds about this distance. Most hold the basketball and moon apart at arm’s length. This is because the textbooks often show a diagram of the moon phases drawing the moon and earth right next to each other. Plus the moon does look rather close when you look at when it’s full.

Like I said, watch the video on this page if you have a moment. If the question comes up tonight and you are able to have a teaching moment on it you might just get a chance to seem like “smart Dad/Mom” . At least until you tell them it’s bed time.

Jeff Ray is the Science/Environmental Reporter and weekend Meteorologist for CBS11. He holds a master’s degree in Science Education as well as a B.S. in Broadcast Meteorology. You can email him at