11.2 How the Earth Was Shown to be Round

Before learning this lesson you may want to take the background lessons.  They are

1) Ancient Greece

2) Angles and Parallel Lines

People used to believe that the earth was flat and the sun moved around it.  Some people used to think the earth was square with 4 corners. Some thought that the oceans poured off the ends of the earth. The belief that the earth was flat was a reasonable belief because when we look in the distance the earth looks flat.  The belief that the sun moves around the earth was also reasonable because we don't feel like the earth is rotating and we see the sun rise in the east and set in the west.   The earth is spinning very fast but we don't feel it because the earth is so big that it takes 24 hours for us to make a full circle.  You can understand that if you magine standing on a spinning floor that takes 24 hours to make a full circle.  It changes direction so slowly that you wouldn't feel anything. 

Here is a video showing what people thought about the earth and the sky in ancient times. 


 As long ago as 300 BC the ancient Greeks had determined that lunar eclipses were caused by the earth blocking the sun from shining on the moon.  Figure 1 is a diagram showing the earth blocking sunlight from getting to the moon.  Notice how in the diagram the moon isn't black.  That's because some of the sunlight is being scattered by the earths atmosphere toward the moon.  Sunlight is composed of many colors.  The reason the moon is red when the earth is blocking the sun is that red light scatters more than blue light.  The red light of the sun is scattered onto the moon during a lunar eclipes but the blue light isn't.


Figure 1

 Here is a picture taken at different times of a lunar eclipse.

Figure 2

Back in 300 BC a brilliant man named Aristotle who lived in Greece.  Aristotle wrote about many subjects including physics, biology, zoology, metaphysics, logic, ethics, aesthetics, poetry, theatre, music, rhetoric, psychology, linguistics, economics, politics, meteorology, geology, and government.  One of the things Aristotle wrote about was the shape of the earth.  Aristotle noticed that the edge of the earth's shadow on the moon was curved.  He also noticed that if he traveled that some new stars appeared on one side of the sky and some stars disappeared on the other.  That would happen if the world was round.  Both these two observations led him to conclude that the earth was round.

Forty six years after Aristotle died another brilliant man, Eratosthones was born in the Greek city of Cyrene.  He too wrote about many subjects.  Despite the fact that the Greeks of the time did not have modern technology he managed to calculate how big the earth is.  He measured the shadow cast by a stick a known distance away from a place where no shadow was cast (Syene).  He knew that the reason the shadow was cast in one place and not in the other was because in one place the stick was not facing the sun directly because of the curvature of the earth.   You can see this in the diagram below.

The size of the shadow depended on how fast the earth was curving and how far he was away from the city of Syene.  From the length of the shadow Erasthonese knew he was 7.2 degrees away from Syene.  There are 360 degrees in a circle.  If we know the distance of 7.2 degrees on the earths surface and we know how many times 7.2 degrees goes into 360 degrees we can calculate the circumference of the earth's surface.  The number of times 7.2 degrees goes into 360 degrees is 360/7.2 = 50.  If we knew the distance on the earth of 7.2 degrees we can get the entire circumference of the earth by multiplying that distance by 50.  Erastosthones  had a walker determine the distance between those places and it was 497 miles though in those days they measured in stadia instead of miles.  The size of the earth can then be calculated by multiplying 50 * 497 miles.  The answer is 24,850 miles.


One way to prove the earth is round is to go around it.  If you keep going in the same direction and get back to where you were you've proved that the world is round.

Magellan's crew made it all the way around the earth in the 1500s.  The story is told in the video below.


Nowadays we have photos of the earth from space that show the earth is round as shown in the video below.



Amazingly there are still people who believe the earth is flat. 



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Every day the sun seems to go around the earth yet we're told that the earth is going around the sun.  Who came up with the idea that the earth is going around the sun and why?

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