• Following in the Footsteps of Eratosthenes: Measuring the Circumference of the Earth  http://www.mapmonde.org/eratos/ Very impressive. In several different languages no less.

Books
Here are some titles you will find in your favorite book store, library or on-line.

• Ferguson, Kitty. Measuring the Universe.
• Lasky, Kathryn. The Librarian who Measured the Earth. A must buy!
• Diggens, Julia. String, Straight Edge, and Shadow (p. 149-154)
• Sagan, Carl. Cosmos. The book and video from a PBS series of the same name.
• Coggins, Jack. By Star and Compass. The Story of Navigation (p. 64-69) Good explanation and diagrams.
• Moore, P. and Brinton, H. Exploring Maps. p. 13-15
• Gullberg, Jan. Mathematics: From the Birth of Numbers. p. 467  Did Eratosthenes really get the measurements right? According to Gullberg, historians have done some fudging to make Eratosthenes look good. Since his method was correct, why not give him the benefit of a doubt that he used the right length for his unit of measure - the stadion? Apparently there were at least seven different lengths for a stadion at that time.

• Adler, Irving. Shadows
• Gardner, Robert & Webster, David. Shadow Science (Doubleday, 1976). The authors take a fresh look and gives us clues about the world around us. Titles of chapters include: Telling Time with Shadows, Finding North with Shadows,  and Night is just a big Shadow. Contains a lot of puzzle pictures. Great book.
• Severn, Bill. You and Your Shadow. 

• Charischak, Ihor. In the Spirit of Eratosthenes: Measuring the Circumference of the Earth. Learning & Leading with Technology, V. 25, N. 6 (ISTE, March, 1998)
• Charischak, Ihor. Bringing the Vision of 21st Century Collaborative Projects Into Today's Mathematics Classrooms. Paper presented at the NCTM's Electronic Format's Group Conference, Reston, VA, June, 1998

• All about the Autumnal Equinox
• Four Seasons: The Movie Nice animation of the earth moving through the seasons as would be seen from the sun
• Noon Shadow Project - in Spanish
• Ken Cole's Noon Day Project - Excellent resources.
• NCTM's World's Largest Math Event (WLME 1) featured this measurement in one its activities
• The Analemma - time lapse photograph of the sun's position in the sky at the same time of day on each of 45 days throughout a year.

• Cary Rand used a gnomon on clear days over the course of a year to create this analemma.  Thanks to John Stetson for the photo!

• Activity: A Day in the Life of a shadow (from sunrise to sunset) - Predict where the shadows will be at different times in the day.
• A Day in the Life of a shadow - Shadows at different times of the day. Notice that the edges of the shadows all fall along a straight line. Do you think this would happen no matter what time of the year it is?
• Print Paper rulers for your measurements -  Hope Martin from Columbia, Missouri shared this one.
  

Measuring the Globe: An Historical Activity Eratosthenes' measurement of the earth, in a form easy to use by teachers. (You will need to register. Once in the Convergence site, search for Eratosthenes.)

• The Eratosthenes project - Sonoma State University (1999-2000)
• A similar project that is done by James D. Meinke, Assist. Professor of Educational Technology, Baldwin-Wallace College, Berea, Ohio.
• A Holiday Measurement of Earth's Circumference
• In honor of the centennial of Einstein's paper on special relativity and the 50th anniversary of his death,  several events are suggested. One of them is Measure the Earth with Shadows   http://www.physics2005.org/projects/eratosthenes/index.html

Participants Websites

• Noon Day Webquest, Gold Creek School, Australia (2002)
• Noon Day in Antilles High School, Fort Buchanan, Puerto Rico (2002-20004)
• Holy Trinity School in California. Participants in the Year of Physics celebration. 2005.
  

• http://www97.intel.com/education/odyssey/day_167/day_167.htm
  
  

Video

• Cosmos #1: The Shores of the Cosmic Ocean (1980) Carl Sagan Productions/PBS, 1/2" video Color 1 cass., 60 min. Explores the universe from clusters of galaxies to the Milky Way and earth. Discusses early scientific discoveries concerning measurement of the earth's circumference and its spherical nature. Journeys through time from the Big Bang to the present. Included in this video is a wonderful presentationn of Carl telling the Eratosthenes measurement story. See link below (with French subtitles.)
  
• http://video.google.com/googleplayer.swf?docId=-3973356878310464582&hl=en%22flashvars=%22

• What Causes Earth's Seasons? http://www.youtube.com/v/DuiQvPLWziQ

Aristotle believed the earth was round, but he low-balled how round it was. King Ptolemy of Egypt offered an estimate of 18,000 miles which Columbus used to his advantage in negotiating for his trip. (From a PBS interview with author of Don't Know Much About the Cosmos.)

Somewhere on or very near  the Tropic of Cancer or the Tropic of Capricorn. (Why?) Congratulations to the students of Mr. Langstaff at Hendrix Junior High in Chandler, AZ for being the first to come up with that.