Proving the Earth's Shape: Satellite Imagery, Ships Sinking, Lunar Eclipses, and Polaris Altitude

1. Unit 2: Earth’s Size & Shape Aim: How can we prove the shape of the Earth?

2. What is the shape of the earth? • Nearly a perfect sphere • Oblate Spheroid – • bulges very slightly at the • equator and flattens very • slightly at the poles

3. The Oblate Sphere • The difference is small • We can not see it with the naked eye • It still appears like a sphere • Be careful on multiple choice ALWAYS pick the PERFECT circle √

4. How can we describe Earth’s surface? What causes oblateness? The Earth’s rotation • Mostly smooth • Even largest mountains are very small compared to Earth’s size.

5. How do we describe Earth? • MostlySMOOTH • NearlyperfectlyROUND. √ √ SOCCER BALL BASKETBALL MARBLE BILLIARD BALL

6. How do we PROVE the earth is round? 1. Satellite pictures are the best piece of evidence we have to prove Earths shape. https://www.youtube.com/watch?v=MtXxCNU34s8

7. 2. Ships appear to sink gradually below horizon https://www.youtube.com/watch?v=TVXsZJ39a6o

8. 3. Lunar Eclipse • Only a 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. https://www.youtube.com/watch?v=VW2xRR75lKE

9. LUNAR ECLIPSES

11. 4. Altitude of Polaris Polaris • This is the star that lies in space directly over the geographic North Pole of the earth. • If you stood at the North Pole, Polaris would be directly overhead.

12. Why do observations of Polaris help determine the Earth’s shape? • The North Star (Polaris) appears lower and lower in the sky as you travel toward the equator because of earth’s spherical shape. • At 90°N (at the North Pole) Polaris is directly overhead. • At 0° (at the equator) Polaris is just visible at the horizon.

13. Polaris Changes in Altitude as youchange latitude. 90º Polaris is a fixed point above the North Pole. ALTITUDE OF = POLARIS LATITUDE OF OBSERVER

14. Polaris is a Fixed Point above the North Pole • Because of this, in the Northern Hemisphere, the ALTITUDE (angle measured in degrees above the horizon) of Polaris tells an observer his LATITUDE position. • If an observer’s latitude changes in the Northern Hemisphere, the ALTITUDE of Polaris will EXACTLYMATCH the observer’s LATITUDE. ALTITUDE OF = POLARIS LATITUDE OF OBSERVER

17. 5. GRAVITY is stronger at the poles Uh Oh! I gained a few pounds! Yippee! My diet must be working!

18. Where on Earth would you weigh the LEAST and WHY? You would weigh the least at High Mountains at the equator because you are farther from the center of earth – gravity is less

19. Weight is a measure of gravitational force

20. We are further from the center of the Earth at the EQUATOR – gravity is less • This means we weigh LESS! • We are closer to the center of the Earth at the POLES • We weigh MORE!

21. What PROOF is there that we are slightly oblate? • GRAVITY MEASUREMENTS. Gravity is the force of attraction between any 2 objects. • Increase mass of objects = increase gravity • Decrease distances between two objects = increase gravity

22. What PROOF is there that we are slightly oblate? • 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. • Meaning, the gravitational force would be equal at the poles and equator. BUT IT IS NOT EQUAL!!

23. Summary: How did they prove the Earth to be round? • Satellite Imagery – absolute proof • Ships appear to sink GRADUALLY below the horizon as they travel away from observer • Polaris (North Star) changes altitude (angle measured in degrees above earth’s surface) directly with your latitude ALT OF POLARIS = LAT OF OBSERVER • Lunar Eclipses - Earth’s shadow on the moon • Gravity – stronger at poles, weaker at the equator