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updated: 2/17/2014 1:50 PM

Earth's size means you can't see its surface curving

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  • Students learn about the earth with a 20-foot-tall balloon. At this size, it's easy to see the spherical shape of the planet, but the full-size planet can look flat when you're standing on its surface.

      Students learn about the earth with a 20-foot-tall balloon. At this size, it's easy to see the spherical shape of the planet, but the full-size planet can look flat when you're standing on its surface.
    Daily Herald file, 2002

 

Why is the Earth's surface flat and not a sphere shape?" asked a young writer at the Vernon Area Library District's "Write Away!" program.

Around the world, from the beginning of time, people have tried to imagine what the Earth looked like.

Without being able to see the entire thing, it was almost impossible to imagine what type of object everyone was perched on.

Could it be a flat disc floating in water? Or an oval encased in a dome? And what about the sun -- why does it always rise in the east and set in the west? Why do the constellations in the night sky shift predictably as the seasons change?

We now know the Earth actually is a sphere, a round globe floating in space. So why doesn't it look like that to us? Why does the ground looking out as far as the horizon look flat?

"The explanation is that the Earth is very large, and so the amount of curving is very small over short distances," said Geza Gyuk, Adler Planetarium director of astronomy. "Add in the fact the Earth is so bumpy and you'll never notice the curving."

Have you ever watched something travel off into the distance, like a car or a ship? When it starts off, it seems very large, and as it ventures farther away it becomes much smaller. Instead of just getting smaller, objects also eventually disappear under the horizon.

If the Earth really were flat, that car or ship would appear as an ever smaller spot off in the distant vista, but it would never disappear entirely.

Observing that the sun and the moon are round, about 2,500 years ago Greek philosopher Anaximander figured the Earth must also be round.

Gyuk described why our eyes can't tell us the Earth actually curves.

"Let's imagine for a moment that the Earth were completely smooth. If you then stood in the middle of a large field, say a mile in radius, then the amount the edge of the field would curve down compared to a flat Earth would only be about eight inches. That would be very difficult to notice given that the Earth is actually quite bumpy.

"And how frequently does one have a smooth, uninterrupted mile to look over without a building or tree or something blocking one's sight? Over a half mile distance the curving is only two inches."

Other philosophers, mathematicians and astronomers from various cultures thought about this same question over time.

The Earth is not yet old enough to have a completely smooth surface since shifting of the Earth's crust and volcanic forces are constantly crafting new hills and mountains. Venus has a smoother surface with some volcanoes and chasms.

To learn more about our universe, visit the Adler Planetarium in Chicago. Find out more at www.adlerplanetarium.org.

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