Only the Shadow Knows


Using shadows to measure the world

I have wanted to use social media software for a lesson in my physics and astronomy class for some time now.  One natural idea I had for an online connection was a modern-day re-enactment of Eratosthenes experiment for measuring the size of the Earth.  This experiment is a natural for the physics/earth science/astronomy/math/science/inquiry classroom because it addresses so many of the issues addressed in these units of study.

For example this experiment attacks such  misconceptions as:

  • The idea that the world is round is a new idea created around the time of  Columbus
  • Simple tools cannot be used to figure out complex ideas
  • Measurements must be made using only tools that are “official” such as a ruler or meter stick
  • Only complex math concepts can be used to solve seemingly complex experiments

But any science experiment can address science issues.  Where does an experiment address other issues?  By using tools like Skype, Google Earth(or Maps) and Google Docs a student starts to see their place in the world. They can see that what is happening in their community is not the same as elsewhere.  They may even see that their view of science and math is universal(or not…) Also this allows students to do the one thing they love to do – talk to other kids.  More is accomplished in 15 minutes of students actively talking with each other than in a 40 minutes classroom where they are being preached to.  When students are allowed to see the connections they can accomplish an amazing amount of learning.

So now that I have an idea, I sent out questions to my Twitter PLN and within minutes, found out a few other teachers had tried this experiment.  Teachers John Burk and Frank Noschese did it back in September 2010. It seemed to work for them with few hiccups,  so I decided to go a step further.  I want to include as many teachers and classrooms as possible.  So here is my starting idea.  I have created a basic lab using a structure that my students are familiar with.  Before you read it, I want to warn you that this is as close to cookbook as my labs get.  I usually let the students come up with their own procedure. In this case I walk them through the procedure of gathering data but not how or what they must do with it. For the sake of labeling, let’s call it guided inquiry (it’s not, but that’s okay for now.) Eratosthenes Shadow Lab. You may want to do things in a slightly different way, but I address that later in this blog.  For participating in the lab, I have created a Google form Eratosthenes Shadow Participation Form.  My basic form for entering your data, this is the form I have created – Eratosthenes Data Form.  I really want this to be a collaborative effort so please either comment on this post below or feel free to add to the Eratosthenes Colab Document.

I really would like to see the idea of this lab grow to include classrooms of students of all grade levels, covering a whole range of subjects.  Why not use science as the excuse to have students do a geography lesson with students at a different latitude through Skype or Google Chat?  Why not have language classes try communicating with students of the language they are learning where both are trying to do the same basic science lab?   How about students learning Physics of light talking to students learning Earth Science?

Can you see the connections which could be made?

I would like to have this lab happen all over the world at local solar noon on the day of the Spring Equinox which is March 20th 2011.  In order for that to happen, we need to start talking.

What do you think?

Can we make this happen?

Only the Shadow knows…

Update March 16th, 2011.

I have updated and shared the project with anyone who has signed up. Ideally everyone will perform this experiment on the same day at local noon.  Due to local weather conditons and school schedules, this may not be possible. But if everyone performs the experiment within a few days of each other, you can use the differences to have conversations about the error involved and how much change would occur during this time.  I think you may be surprised at how little change there will be.

I have provided(below)  my guided inquiry lab sheet that my students will use for your perusal and use.

If you do not sign up ahead of time, you may still participate by doing the experiment with your class and submitted the info on the data entry form and choosing “Other” Please make sure you provide your location in the provided textbox at the end of the form.

And Lastly. Feel free to view and use the information with your class, once gathered, even if you did not perform the experiment!


To join this lab project:   Eratosthenes Shadow Participation Form

My lab write-up: Eratosthenes Shadow Lab

The  data entry form: Eratosthenes Data Form

If you have an Eratosthenes lab document that you would like to share please link it to this collection: Shared Eratosthenes Documents

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10 thoughts on “Only the Shadow Knows

  1. John makes a good point. I would suspect data uncertainty would increase as you approach the equator, because the shadow would be getting shorter…but you might be able to counter that with lots of data points.

    People at different latitudes could also do it, as long as they all agree to go out at the same local time…I assume local noon would be easiest b/c that’s when the shadow would be shortest and it woubl be easy to look up on the internet.

    If you could look up local noon, why not just look up the radius of the earth? So I guess each location could plot the length of the shadow at, say, half-hour or 1 hour intervals during the day, make a graph, and use the graph to estimate the shortest length of the shadow.

    OR, you could set up a camera in front of the shadow, take a photo say one a minute or once every 5 minutes for the day and use the picture with the shortest shadow.

    I hope this all makes sense. Good luck!

  2. This lab could be expanded to last throughout a whole semester so students can see the change in the Sun’s altitude (elevation) as the year progresses. As mentioned by Frank, it could also be expanded to track a shadow throughout the day. Or if you do the same clock time throughout the year you could recreate the analemma.

  3. Hi, Gene.

    I love this idea, but I have a couple of questions.

    Since March 20 is a Sunday, would this be something students would do on their own, or would you gather together on a non-school day? I’m trying to figure out how I could participate with my students.

    Thanks! Janelle

  4. Thanks Janelle, Thank you for bringing this up. Since the Spring Equinox occurs on March 20 @ 7:21 P.M. EDT, I think that doing the experiment on Monday, March 21st would be okay. Actually as long as everyone does the experiment on the same day, the results will be fine. I just like the idea of doing the experiment on the Equinox for a couple of reasons. 1 – The sun is directly overhead at the Equator, meaning no shadow there – making math easier. And 2 – It is another cool idea to bring up with kids of all ages (including those my age)

    But again, thank you for double checking! That is why I wanted to make this a team project. Many eyes and brains thinking about it!

  5. Have some questions about the diffraction that occurs at the end of (and the sides of the shadow). The end of the shadow has a darker and then a lighter (fuzzier) and more diffuse end. There should be some discussion of the errors this may generate. My shadow angle gave a latitude of about 1.4 degrees MORE than it should have been.

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