changes

tilted by 23.5 degree to simulate topic of cancer
color code the checkbox
add distance = arc length drawing
add text as hints
add top Gnomon checkbox
add shadow checkbox

22June 2012
add angle visualization
add Google earth picture
add assessment for learning input fields with feedback for Radius of Earth and Circumference of Earth

25 june 2012
add // which is taken from http://www.shadedrelief.com/natural3/pages/textures.html // who release it to the public domain // Tom Patterson, www.shadedrelief.com. //resize to 2000x1000 by lookang using GIMP and quality reduce to 80% for 200+kb filesize to use materials that are in public domain
disabled the html as the pop box from NTNU seems to be too many, testing

credits to Todd Timberlake!
i didn't make this, just remixing to customize it to suit the Wikipedia entry http://en.wikipedia.org/wiki/Eratosthenes
[img]http://upload.wikimedia.org/wikipedia/commons/thumb/3/35/Eratosthenes.svg/250px-Eratosthenes.svg.png[/img]

Eratosthenes' measurement of the Earth's circumference
illustration showing a portion of the globe showing a part of the African continent. The sun is shown and arrows indicate rays of the sun hitting earth. Rays or arrows point to Alexandria (labeled "A") and Syrene (labeled "S"). Blue lines are drawn from A and S towards the equator. There is a line representing the Tropic of Cancer running to S. Two small curved arrows indicating a measurement are drawn from the Greek symbol phi. One ends midway between the blue lines from A and S and the other ends between the ray of light hitting A and an extension of the blue line passing through A into space.
Measurements taken at Alexandria (A) and Syene (S)

Eratosthenes calculated the circumference of the Earth without leaving Egypt. Eratosthenes knew that on the summer solstice at local noon in the Ancient Egyptian city of Swenet (known in Greek as Syene, and in the modern day as Aswan) on the Tropic of Cancer, the sun would appear at the zenith, directly overhead (he had been told that the shadow of someone looking down a deep well would block the reflection of the Sun at noon). He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the sun was 1/50th of a circle (712') south of the zenith on the solstice noon. Assuming that the Earth was spherical (360), and that Alexandria was due north of Syene, he concluded that the meridian arc distance from Alexandria to Syene must therefore be 1/50 = 712'/360, and was therefore 1/50 of the total circumference of the Earth. His knowledge of the size of Egypt after many generations of surveying trips for the Pharaonic bookkeepers gave a distance between the cities of 5000 stadia (about 500 geographical miles or 800 km). This distance was corroborated by inquiring about the time that it takes to travel from Syene to Alexandria by camel. He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. The exact size of the stadion he used is frequently argued. The common Attic stadion was about 185 m,[9] which would imply a circumference of 46,620 km, i.e. 16.3% too large. However, if we assume that Eratosthenes used the "Egyptian stadion"[10] of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 2%.[11]