What shape best describes earth? n Describe the evidence that supports your answer. n Oceans are flat n Airplanes all fly level on earth n Ancient Egyptians: The sky was a tent canopy stretched between mountains at the four corners of the Earth. n Inca: Called their land Tehuantinsuyu: "The Four Corners of the Earth" HOW DID THEY PROVE THAT THE EARTH IS ROUND? Ships appear to sink gradually below horizon Polaris Polaris North Star n This is the star that lies in space practically over the geographic North Pole of the earth. n If you stood at the North Pole, Polaris would be almost directly overhead. Polaris – North Star n Ifyou can spot Polaris in the sky, you can always tell which way is north. Because of this, Polaris was the most important star for navigating at sea. Why do observations of Polaris help determine the Earth’s shape????? n The North Star appears lower and lower in the sky as you travel toward the equator because of earth’s spherical shape, where the North Star is just visible at the horizon. The latitude of the equator is 0°. Polaris Changes in Altitude – Polaris is a fixed point above the North Pole. 90º Polaris – Fixed Point above the North Pole n Because of this, in the Northern Hemisphere, the altitude (angle measured in degrees above the horizon) of Polaris tells observer his latitude position. n If observer’s latitude changes in the Northern Hemisphere, the altitude of Polaris will exactly match observer’s latitude. ALT OF POLARIS = LAT OF OBSERVER Locating Polaris – our latitude is 41°N , Polaris’ altitude will be 41° Lunar Eclipse n Onlya sphere can cast a shadow that appears round. During a lunar eclipse, the earth casts its shadow on the moon during the full moon phase. Summary: How did they prove the Earth to be round? n Ships appear to sink GRADUALLY below the horizon as they travel away from observer n Polaris (North Star) changes altitude (angle measured in degrees above earth’s surface) directly with your latitude ALT OF POLARIS = LAT OF OBSERVER n Lunar Eclipses - Earth’s shadow on the moon n Satellite Imagery – absolute proof Is the Earth Perfectly round? n The Earth is NOT a perfect sphere n It is flattened at the poles and bulges at the equator n Earth is slightly out of round or OBLATE. What proof is there that we are slightly oblate? n Gravity measurements. Gravity is the force of attraction between any 2 objects. Increase mass of objects = increase gravity Decrease distances = increase gravity n If Earth were a perfect sphere, it would be expected to exert an equal force on objects at equal distances from the center of earth. Weight!!! – measure of gravitational force n We are further from the center of the Earth at the equator – gravity is less n This means we weigh LESS!!! n We are closer at the poles… n We weigh MORE!!!! The least amount of gravity is farther from the center of Earth High Mountains farther from the center of earth – gravity is less The Oblate Sphere n The difference is small n We can not see it with the naked eye n It still appears like a sphere n Be careful on multiple choice Why is the earth not perfectly round? n Earth’s rotation causes… Bulging at equator Flattening at the poles How big is Earth? n Having established the shape, what is the size? Question: n How do you measure something really big? n Two centuries before the birth of Christ … (-200) Eratosthenes devised a way to determine the size of earth. NOT CHRISTOPHER COLUMBUS!!!!! Eratosthenes n Based his calculation on observations of the sun on June 21, the Summer Solstice in Alexandria and Syene – 2 cities in Egypt. n Onthis date, the sun was straight overhead at noon in Syene, but not quite overhead at Alexandria Syene Alexandria Obelisk in Alexandria, Egypt n Theobelisk is called “Cleopatra’s Needle” and it was moved to Central Park in 1881. Erathosthenes n Made assumptions that… a. The earth is round b. The sun’s rays are parallel Using simple geometry he set up an equation that determined the circumference of the earth. Erathosthenes’ Result n 240,000 stadia (about 39,250 km) is quite close to modern measurements. n Calculate the percent deviation (Equations are located on what page of ESRT?) if the actual circumference is 40,070 kms. n Eratosthenes' work was lost, except for a description of his method in an obscure source n 140 AD: Ptolemy got an answer smaller by 28% using a sound method with questionable data. n Both results known by King Ferdinand’s experts. Columbus argued for the smaller value for the distance from Spain to India but he was turned down because they thought the distance too far. n In the end Queen Isabella overruled the experts, and the rest is history.
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