3.3 Earth's Shape

Before spacecraft, how did people know that Earth is spherical?

The ancient Greeks knew that Earth was round by observing the arc shape of the shadow on the Moon during a lunar eclipse. Was there other evidence of Earth's roundness available to people before spacecraft gave us a bird's eye view?

Earth’s Shape

Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the Equator. To be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the Equator). Half of the sphere is a hemisphere. North of the Equator is the northern hemisphere and south of the Equator is the southern hemisphere. Eastern and western hemispheres are also designated.

What evidence is there that Earth is spherical? What evidence was there before spaceships and satellites?

Try to design an experiment involving a ship and the ocean to show Earth is round. If you are standing on the shore and a ship is going out to sea, the ship gets smaller as it moves further away from you. The ship’s bottom also starts to disappear as the vessel goes around the arc of the planet (Figure below). There are many other ways that early scientists and mariners knew that Earth was not flat. 

The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are spherical as well.

Earth's actual shape is not spherical but an oblate spheroid. The planet bulges around the equator due to mass collecting in the middle due to rotational momentum.

Summary

• Ancient Greeks knew that Earth was round because of the shadow the planet cast on the Moon during a lunar eclipse.
• A boat does not get smaller with distance but sinks below the horizon - more evidence for Earth's roundness.
• Earth is divided into hemispheres: northern, southern, eastern, and western.

Review

1. Describe where you live, in terms of hemispheres.
2. If you met up with someone who claimed that Earth is flat, what evidence would you present to them that their assertion is not true?
3. What evidence do you have that our planet is flat? Which of these ideas do you believe and why?

Explore More

Use this resource to answer the questions that follow. 

1. What is the #10 reason that we know that Earth is spherical?
2. What is the #9 reason that we know that Earth is spherical?
3. What is the #8 reason that we know that Earth is spherical?
4. What is the #7 reason that we know that Earth is spherical?
5. What is the #6 reason that we know that Earth is spherical?
6. What is the #5 reason that we know that Earth is spherical?
7. What is the #4 reason that we know that Earth is spherical?
8. What is the #3 reason that we know that Earth is spherical?
9. What is the #2 reason that we know that Earth is spherical?
10. What is the #1 reason that we know that Earth is spherical?