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Today’s Big Objective
• Describe 3 ancient pieces of evidence of a
round earth.
Shape of Earth?
Old adage
• The Earth was Flat
• Not true, the intellectual, the observant minds
of thousands of years ago, knew that the Earth
was not flat.
• They based this on simple observations.
• Those observations lead them to know that
the earth was round.
• But which round?
Are these items round?
Ancient Evidence #1
Position (altitude) of North Star Changes with
change in latitude
That change occurs because the earth is NOT Flat.
The North Star gets higher (bigger angular altitude)
as an observer moves north.
It gets lower in the sky as an observer moves south.
So, how do you find the North Star?
• The axis of Earth’s Rotation extends into outer
• Earth’s North Pole extends into the north
celestial Pole, and that is where the Polaris is!
• Therefore all stars seem to move around
Polaris during a given night.
• Long exposure photography reveals the
Ancient Evidence #2, Sailing Ship
As a ship moves away from shore the hull always
disappears before the mast, and when an
arriving ship appears, the mast is always
visible before the hull
This is because of the curvature of the Earth.
The Earth is blocking part of the sight line of
part of the ship because it is below the
Ancient Evidence #3, Lunar Eclipse
A lunar eclipse occurs when the full moon
passes behind the earth, relative to the sun.
The moon enters the shadow. So Earth’s
shadow falls onto the surface of the moon.
Therefore you can see the silhouette (shape) of
the Earth as its outline falls on the moon.
When the Earth enters the shadow, you see one
side and when it leaves, you see the other
side. The shape is always circular.
Shape of Earth Summary Video
Read P 552 about the first successful attempt to
establish the size of the earth.
Study the diagram on P 551
1. Who is credited with the first successful
measurement of the earth’s circumference?
2. When did he do this?
3. What two variables did he need to measure in
order to calculate the circumference of the earth?
4. Why do you think he chose the noonday sun?
5. What two cities did he use? Why did he use these
two cities?