Shape of the Earth
How a scientific proof works.
What is Earth’s shape?
Is it flat like a coin, or spherical like a
ball? Perhaps it’s like a tin can, flat on
the ends and round in the middle?
 Describe the evidence that best
supports you answer.

Spherical Earth?
Scientists claim that Earth is much like
a slightly flattened ball - an oblate
spheroid.
 How do they know or do the merely have
faith in the claim?
 What is the evidence that supports a
scientific belief in a roughly spherical
Earth?

Ships appear to sink gradually
below horizon from bottom up.
Polaris, the
North Star

is located practically
over the North Pole
of planet Earth.

If you stood at the
North Pole, Polaris
would be almost
directly overhead.
Polaris
Why do observations of Polaris
help determine Earth’s shape?
The North Star gradually appears lower
and lower in the sky as you travel
toward the equator.
 When the North Star is visible at the
horizon, you are located at the equator.
 South of the equator the North Star is
not visible at all.
 These observations are possible only for
a roughly spherical Earth.

Lunar Eclipses

Only a sphere can cast a shadow that
always appears round. During lunar
eclipses, Earth casts its shadow on the
moon and the shadow is always round.
Many other indications…
Circumnavigation of the globe.
 Existence of time zones.
 Earth-orbiting satellites.
 Photographs from space.
 Eyewitness reports of
astronauts.
 Other objects - sun, moon, planets - are
roughly spherical.

Summary: How do we know
the Earth to be spherical?
Ships appear to sink GRADUALLY below the
horizon as they travel away from observer
and over the horizon - in all directions.
 Polaris (North Star) changes altitude (angle
measured in degrees above earth’s surface)
with latitude – at all longitudes.
 Lunar Eclipses - Earth’s shadow on the moon
is always round.
 Satellite imagery and much more.

Is Earth perfectly spherical?
The Earth is NOT a perfect sphere
 It is flattened at the poles and bulges
at the equator due to its spinning.
 Earth is very slightly out of round or
OBLATE.
 Saturn is an even
better example
of being oblate.

What proof is there that the
Earth is slightly oblate?

Gravity measurements. Gravity is the force
of attraction between any two objects.
Increase mass of objects = increase gravity
Decrease distances = increase gravity

If Earth were a perfect sphere, it would be
expected to exert an equal force on objects
at equal distances from the center of earth.
The pull of gravity is weaker
farther from the center of Earth
You weigh just a bit less at the equator than at the pole!
The Oblate Sphere
The difference with a sphere is small
 We can not see it with the naked eye
 Earth still appears like a sphere

Why is Earth not perfectly round?

Earth’s spinning (rotation) causes…
– bulging at equator and
– flattening at the poles.

Be careful on multiple-choice tests!