2. Planetary Science Round Earth / Flat Earth

3. Planetary Science Vocabulary horizon - where sky and earth appear to meet line of sight - the straight unimpeded path taken by light from an object to an eye terrestrial - pertaining to the earth extraterrestrial - outside the earth

4. Historical Observations of a Round Earth Ships apparently rising up out of the sea or sinking into it Lunar eclipses - the leading and trailing edge of the earth’s shadow were curved causing us to assume the object casting the shadow was round As an observer increased altitude he could see farther over the earth’s curvature

5. Eratosthenes - Greek mathematician, 3rd century BC Read of the city of Syene on the Nile River, 1 day each year on summer solstice the sun shone directly down an abandoned well illuminating the bottom He assumed that the sun was very far away and therefore all of its light struck the earth in parallel beams He reasoned that if the earth were flat, vertical poles on the earth’s surface would cast no shadow at noon on that day But he found that poles outside the city did cast shadows, and the farther north they were placed, the longer the shadows

6. Page 4 Cont. He reasoned therefore that the earth must be round He measured that the sun shone at an angle of 7.2º in the city of Alexandria 7.2º is about 1/50 of a 360º circle therefore the circumference of the earth = 50 times the distance between Alexandria and Syene Our best guess is that he was within 6000 km of todays measured circumference

7. Latitude In the northern hemisphere, the latitude of a place is equal to the altitude of the north star (Polaris) Christopher Columbus followed the 25th parallel to try to find a shortcut to Persia but ran into the Carribean Islands He had no idea how far he was around the planet

8. Longitude There’s no “east star” You need an accurate clock that would run for weeks at a time While at sea you observe the exact time the sun reaches its zenith ( highest point = local noon) Compare this to the exact time at home 360º / 24 hrs = 15º / hour 4 minutes = 1º Chronometers guided sailors for nearly 200 years Today sailors and pilots use GPS