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Today’s Big Objective • Describe 3 ancient pieces of evidence of a round earth. Shape of Earth? Old adage • The Earth was Flat • Not true, the intellectual, the observant minds of thousands of years ago, knew that the Earth was not flat. • They based this on simple observations. • Those observations lead them to know that the earth was round. • But which round? Are these items round? Ancient Evidence #1 Position (altitude) of North Star Changes with change in latitude That change occurs because the earth is NOT Flat. The North Star gets higher (bigger angular altitude) as an observer moves north. It gets lower in the sky as an observer moves south. So, how do you find the North Star? Polaris • The axis of Earth’s Rotation extends into outer space. • Earth’s North Pole extends into the north celestial Pole, and that is where the Polaris is! • Therefore all stars seem to move around Polaris during a given night. • Long exposure photography reveals the following… Ancient Evidence #2, Sailing Ship As a ship moves away from shore the hull always disappears before the mast, and when an arriving ship appears, the mast is always visible before the hull This is because of the curvature of the Earth. The Earth is blocking part of the sight line of part of the ship because it is below the horizon. Ancient Evidence #3, Lunar Eclipse A lunar eclipse occurs when the full moon passes behind the earth, relative to the sun. The moon enters the shadow. So Earth’s shadow falls onto the surface of the moon. Therefore you can see the silhouette (shape) of the Earth as its outline falls on the moon. When the Earth enters the shadow, you see one side and when it leaves, you see the other side. The shape is always circular. Shape of Earth Summary Video http://www.youtube.com/watch?v=YNVZNFUx atU&safe=active Assignment Read P 552 about the first successful attempt to establish the size of the earth. Study the diagram on P 551 1. Who is credited with the first successful measurement of the earth’s circumference? 2. When did he do this? 3. What two variables did he need to measure in order to calculate the circumference of the earth? 4. Why do you think he chose the noonday sun? 5. What two cities did he use? Why did he use these two cities?