What shape best describes earth?
n Describe the evidence that supports your
n Oceans are flat
n Airplanes all fly level on earth
n Ancient Egyptians: The sky was a tent
canopy stretched between mountains at
the four corners of the Earth.
n Inca: Called their land Tehuantinsuyu:
"The Four Corners of the Earth"
HOW DID THEY PROVE THAT
THE EARTH IS ROUND?
Ships appear to sink gradually
n This is the star that
lies in space
practically over the
geographic North Pole
of the earth.
n If you stood at the
North Pole, Polaris
would be almost
Polaris – North Star
n Ifyou can spot Polaris in the sky, you can
always tell which way is north. Because of
this, Polaris was the most important star
for navigating at sea.
Why do observations of Polaris help
determine the Earth’s shape?????
n The North Star appears lower and lower in
the sky as you travel toward the equator
because of earth’s spherical shape, where
the North Star is just visible at the
horizon. The latitude of the equator is 0°.
Polaris Changes in Altitude – Polaris
is a fixed point above the North
Polaris – Fixed Point above the
n Because of this, in the Northern Hemisphere, the
altitude (angle measured in degrees above the
horizon) of Polaris tells observer his latitude
n If observer’s latitude changes in the Northern
Hemisphere, the altitude of Polaris will exactly
match observer’s latitude.
ALT OF POLARIS = LAT OF OBSERVER
Locating Polaris – our latitude is
41°N , Polaris’ altitude will be 41°
n Onlya sphere can cast a shadow that
appears round. During a lunar eclipse, the
earth casts its shadow on the moon during
the full moon phase.
Summary: How did they prove
the Earth to be round?
n Ships appear to sink GRADUALLY below the horizon as
they travel away from observer
n Polaris (North Star) changes altitude (angle measured in
degrees above earth’s surface) directly with your latitude
ALT OF POLARIS = LAT OF OBSERVER
n Lunar Eclipses - Earth’s shadow on the moon
n Satellite Imagery – absolute proof
Is the Earth Perfectly round?
n The Earth is NOT a perfect sphere
n It is flattened at the poles and bulges at
n Earth is slightly out of round or OBLATE.
What proof is there that we are
n Gravity measurements. Gravity is the force of
attraction between any 2 objects.
Increase mass of objects = increase gravity
Decrease distances = increase gravity
n 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.
Weight!!! – measure of
n We are further from the center of the
Earth at the equator – gravity is less
n This means we weigh LESS!!!
n We are closer at the poles…
n We weigh MORE!!!!
The least amount of gravity is
farther from the center of Earth
High Mountains farther from the center of earth – gravity is
The Oblate Sphere
n The difference is small
n We can not see it with the naked eye
n It still appears like a sphere
n Be careful on multiple choice
Why is the earth not perfectly
n Earth’s rotation causes…
Bulging at equator
Flattening at the poles
How big is Earth?
n Having established the shape, what is the size?
n How do you measure something really big?
n Two centuries before the birth of Christ …
(-200) Eratosthenes devised a way to determine
the size of earth.
NOT CHRISTOPHER COLUMBUS!!!!!
n Based his calculation on observations of
the sun on June 21, the Summer Solstice
in Alexandria and Syene – 2 cities in
n Onthis date, the sun was straight
overhead at noon in Syene, but not quite
overhead at Alexandria
Obelisk in Alexandria, Egypt
n Theobelisk is called
and it was moved to
Central Park in 1881.
n Made assumptions that…
a. The earth is round
b. The sun’s rays are parallel
Using simple geometry he set up an
equation that determined the
circumference of the earth.
n 240,000 stadia (about 39,250 km) is quite
close to modern measurements.
n Calculate the percent deviation (Equations
are located on what page of ESRT?) if the
actual circumference is 40,070 kms.
n Eratosthenes' work was lost, except for a
description of his method in an obscure source
n 140 AD: Ptolemy got an answer smaller by 28%
using a sound method with questionable data.
n Both results known by King Ferdinand’s experts.
Columbus argued for the smaller value for the
distance from Spain to India but he was turned
down because they thought the distance too far.
n In the end Queen Isabella overruled the experts,
and the rest is history.