Round Earth Day
— February 22nd —

Although most elementary school children know that the earth is spherical, some do not. Round Earth Day is a good day to make sure that all kids are up to speed. And for those smart-aleck students who roll their eyes, Round Earth Day is a good time to remind everyone that once upon a time most if not all cultures on the planet believed (rather strongly) that the Earth was flat. Adults believed it! And it is a good day to remind them that even today, people in some areas on Earth that have been relatively unreached by modern technology and knowledge still retain a belief in a flat earth.

How to celebrate:

1. Send your students, teacher, friends, or family a cheery, "Have a great Round Earth Day" e-mail greeting.

2. Buy a bunch of spheres and have students decorate them with continents and oceans. As they are finishing up,

b. Ask them how they know the Earth is round. If parents told them, did they believe their parents? Would you believe everything your parents told you? What if your parents had told you that the Earth is shaped like a enormous pretzle? A pancake? If you didn't trust them, how might you figure out the true shape of the Earth? [Answers: go into space and take a look, wait for a lunar eclipse, check out an astronomy book at the library, among other answers.]

3. Conduct a "shape of the moon" activity. Once you think about the shape of our moon, you could ask students to imagine how people on the moon would view the Earth. [This activity determines the date for Round Earth Day each year -- the date in Februrary that has a good "waning crescent" moon is chosen.]

b. Go outside and find the moon (if it's not too cloudy, naturally).

c. If your students (and you can't find it), pull out your pre-made cheat-sheet for the position of the moon which you have cleverly printed out the day before using,

http://aa.usno.navy.mil/data/docs/AltAz.html
(For example, in Swarthmore, Pennsylvania (US) at 10:00 AM on February 22 is: altitude = 31.1 degrees, azimuth (E of N) =140.4 degrees. Each minute thereafter the moon will rise in the sky as it progresses on a clockwise path.)

http://www.jgiesen.de/SME/
(Use this site to generate a graphic that shows the position of the sun and moon at different times of the day.)

c. Ask your students why the moon is crescent-shaped. Some students might answer, "Because the Earth's shadow makes it that way," which is the answer that many adults make. The correct answer, of course, is that the Sun is only illuminating part of the moon. For a review on why there is a crescent phase, see:

http://www.noao.edu/education/phases/phases_demo.html

d. Ask them what this observation says about the shape of the moon. Answer: the fact that a crescent is formed is good evidence that the moon is spherical. Indeed, this observation and inference was probably one of the early reasons why people abandoned the idea that the Earth is flat.

4. For advanced classes (3rd or 4th grade), have a guided inquiry about what the shape of the Earth really is, and why. [The Earth is oblate spheroid, and this deviation from a true sphere is due to the planet's spin, which minimizes the effects of gravity at the equator.] Once your students have determined (and you have confirmed for them) that the Earth is not a sphere, ask them if the "spherical Earth theory" (first proposed by the Greeks) is wrong. If it is false, then is it therefore true that the Earth is, in fact, flat? Of course not -- students should appreciate that the "spherical Earth theory" was pretty much correct, and only when scientists could make more accurate measurements did the theory need some minor refinement.

Resources:

How We Learned the Earth is Round (by Patricia Lauber, illustrated by Megan Lloyd)

Flat Earth? Round Earth? (by Theresa Martin)

Goodnight Moon (by Margaret Wise Brown)

Activity: "Proof that the Earth is spherical"
http://www.uncfsu.edu/msec/nova/timmod1k.htm

Children's views of earth: http://www.psych.uiuc.edu/~wbrewer/earth.html

The Round Earth and Christopher Columbus
http://www.phy6.org/stargaze/Scolumb.htm

Phases of the moon
http://www.geocities.com/CapeCanaveral/6389/Da_Moon.htm

Excellent summary
http://en.wikipedia.org/wiki/Flat_earth