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The Size of the Earth

The Size of the Earth. Sun’s Rays. Travel in straight lines So all light rays that hit Earth travels in parallel lines. Eratosthenes was a Greek astronomer and librarian in ancient times. Around 240 B.C. He made the first good measurement of the size of the earth . How did he do that?.

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The Size of the Earth

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  1. The Size of the Earth

  2. Sun’s Rays Travel in straight lines So all light rays that hit Earth travels in parallel lines

  3. Eratosthenes was a Greek astronomer and librarian in ancient times. Around 240 B.C. • He made the first good measurement of the size of the earth. • How did he do that?

  4. Watch it… • -video clip…

  5. Summer Solstice • The summer solstice occurs exactly when the Earth's semi-axis in a given hemisphere is most inclined towards the sun, at its maximum tilt of 23° 26'.

  6. Eratosthenes used the lengths of shadows to figure out how high in the sky the Sun was in a certain place on a certain day. • He knew of another place where there was no shadow at all on the same day. • That meant the Sun was straight overhead. He found out the distance between the two places, then used some geometry to figure out the rest. • Let's take a closer look!

  7. He got R = 6739 km Actual radius 6378 km http://www.windows2universe.org/citizen_science/myw/w2u_eratosthenes_calc_earth_size.html • Why do you think you would need to choose two cities that lie roughly on the same longitudinal line? Why couldn’t we use New York and Los Angeles to do a similar calculation?

  8. In your own words… Write a one paragraph biography explaining your understanding of who Eratosthenes was and his achievement in terms of finding the circumference of the earth.

  9. Proofs that the Earth is Round • Copy down any three: • Ships appearing to sink as they go over the horizon. • The Earth's shadow on the moon during an eclipse is always curved. This could only be possible if the Earth was a sphere. • Different time zones. • Different angles to Polaris as you travel N. or S. • Photos from space- the best proof.

  10. Using Some Geometry… • Use a protractor to measure angles • Use geometry to measure the height of various objects

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