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Someone (I’m sorry I can’t remember who) tweeted out a link to this great video.

This is but one of many great videos from the the vlog Veritasium which is filled with many beautiful and simple explanations of complex scientific ideas. This is definitely a blog to add to your RSS reader, and I think someone is working on writing a post how it is a wonderful example what anti-pseudteaching looks like.

When I saw this video, I instantly got an idea for a fun little lesson in scale. I grabbed a tennis ball, basketball, and a lego person, and brought them to my class and basically waked through the same questions as the video: “Let this tennis ball be the moon, and the basketball be the earth.” If this is the case, then how far away is the moon?

Students were puzzled, but offered guesses mostly the same as the video. I wanted to push them further, to see that they can begin to develop a sense of scale for themselves, and so I gave them the lego person, and asked them if the lego man could be to the same scale as the basketball and the earth. Instantly, everyone saw this couldn’t be the case, since the lego man was about 1/2 the diameter of the moon.

I then asked them how we knew the the basketball and the tennis ball were in the proper scale? Students said we could just check this, and thanks to the wonders of Wolfram Alpha, we were able to find that the diameters of the moon and earth are in almost the same ratio as the diameters of the tennis ball and basketball. Cool. The guy who did these videos really knows his stuff.

So how far away is the moon? I asked the students to imagine themselves on the earth, and think about the fraction of the night sky that is obscured by the moon. It it’s only an arms length away, then a very sizable fraction of the sky would be obscured by the moon. And from there, we were able to see that the moon must be very far away. It would have been great to actually set up a small web camera and display its feed live as the tennis ball is walked away from the basketball, and stop when it looks approximately moon sized.

All this gets me thinking that I need to do more to get my students thinking about ideas of scale—it would be wonderful for students to begin to get a sense of scale through intuitive little questions like this. How often in chemistry class do you get students build a model atom by imagining the nucleus to be a tennis ball. Where would the electrons be? Could students understand that it would have to be 200 km away? (Note: there’s a great activity in Physics for the 21st century that does just this with google maps). As much as I love Powers of Ten, I think there’s a call here for something more interactive and substantial, but not requiring them to memorize all the various metric prefix (does anyone care if you know which is smaller, a femtosecod? or an attosecond?)

If you have ideas for how to teach a sense of scale to students, and in particular how to pull it through the year, rather than just a unit in the beginning, I’d love to hear them.