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Speaker 1: Welcome to brain Stuff from how stuff works dot com

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Speaker 1: where smart Happens. Hi. I'm Marshall Brain with today's question.

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Speaker 1: Is there any way that people could know that the

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Speaker 1: world is a sphere while standing on the Earth? In

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Speaker 1: other words, is the only way to know that the

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Speaker 1: world is a sphere to be up in space looking

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Speaker 1: down on that sphere? Or could you somehow figure this

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Speaker 1: out while standing on a planet that appears at least

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Speaker 1: superficiently to be flat. If you didn't know that the

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Speaker 1: Earth is a sphere, there are two common observations you

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Speaker 1: could use to convince yourself that it is. The first

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Speaker 1: common observation is the shape of the moon. You see

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Speaker 1: the moon up there every night, and the face of

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Speaker 1: the full moon is circular. That might lead you to

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Speaker 1: believe that it's a sphere rather than a big, round,

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Speaker 1: flat disk. By extrapolation, you could assume that the Earth

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Speaker 1: is a sphere also. But there's a second way that's

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Speaker 1: more convincing. You would look at how objects in the

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Speaker 1: distance disappear as they get further away. For example, if

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Speaker 1: you had a hundred foot tall sailing ship that's fifteen

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Speaker 1: miles away, it's not visible even if you try to

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Speaker 1: see it with the telescope. That's because it's blocked by

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Speaker 1: the curvature of the sphere. As the ship approaches it rises.

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Speaker 1: First the tip of the mast is visible, then more

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Speaker 1: and more of the ship comes into view as the

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Speaker 1: ship gets closer. Anyone looking at ships at sea would

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Speaker 1: notice this and they would be able to tell that

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Speaker 1: the Earth is a sphere. Do you have any ideas

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Speaker 1: or suggestions for this podcast? If so, please send me

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Speaker 1: an email at podcast at how stuff works dot com.

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Speaker 1: For more on this and thousands of other topics, go

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Speaker 1: to how stuff works dot com.