1. Astronomy and the Greeks

2. Eratosthenes was a Greek scholar who worked in Alexandria. While working in the Library in Alexandria, he noted that, at noon during the Summer solstice, the Sun was directly overhead (or at its zenith) in a city called Syene in southern Egypt. He also knew that, at the exact same time in Alexandria, the Sun was about 7 0 south of its zenith. He also knew that the distance between Alexandria and Syene was about 800 km

4. Using this information, how do you think Eratosthenes was able to calculate the circumference of the Earth? 7 degrees = 800 km 1 degree = 800/7 = 114 km 360 degrees = 360 x 114 = 41,100 km - very close to the right answer !

5. Measuring the circumference of the Earth 1.Open up Google Earth and find the location of school on the Earth. 2.Position a marker on school and then find another location on the same line of longitude outside of Ireland. 3.Calculate the angular separation between their school and the second location they have chosen by subtracting their latitudes. 4.Using Google Earth it is also possible to find the distance between these two locations by placing a second marker. 5.Now, knowing the angular separation and overland distance it is possible to find a value for the circumference of the Earth.

6. Google Earth Google Earth at http://earth.google.com/download-earth.html