Proof of Earth's Shape and Size

What shape best describes What shape best describes earth? earth?

Describe the evidence that supports Describe the evidence that supports your answer.your answer.

Oceans are flatOceans are flat Airplanes all fly level on earthAirplanes all fly level on earth Ancient Egyptians: The sky was a tent Ancient Egyptians: The sky was a tent

canopy stretched between mountains canopy stretched between mountains at the four corners of the Earth. at the four corners of the Earth.

Inca: Called their land Tehuantinsuyu: Inca: Called their land Tehuantinsuyu: "The Four Corners of the Earth""The Four Corners of the Earth"

HOW DID THEY PROVE THAT HOW DID THEY PROVE THAT THE EARTH IS ROUND?THE EARTH IS ROUND?

Ships appear to sink gradually Ships appear to sink gradually below horizon below horizon

Polaris Polaris North StarNorth Star This is the star that This is the star that

lies in space lies in space practically over the practically over the geographic North geographic North Pole of the earth.Pole of the earth.

If you stood at the If you stood at the

North Pole, Polaris North Pole, Polaris would be almost would be almost directly overhead. directly overhead.

Polaris

Polaris – North StarPolaris – North Star If you can spot Polaris in the sky, you If you can spot Polaris in the sky, you

can always tell which way is north. can always tell which way is north. Because of this, Polaris was the most Because of this, Polaris was the most important star for navigating at sea. important star for navigating at sea.

Why do observations of Polaris Why do observations of Polaris help determine the Earth’s help determine the Earth’s

shape?????shape????? The North Star appears lower and The North Star appears lower and

lower in the sky as you travel toward lower in the sky as you travel toward the equator because of earth’s the equator because of earth’s spherical shape, where the North spherical shape, where the North Star is just visible at the horizon. The Star is just visible at the horizon. The latitude of the equator is 0°.latitude of the equator is 0°.

Polaris Changes in Altitude – Polaris Changes in Altitude – Polaris is a fixed point above Polaris is a fixed point above

the North Pole.the North Pole.

90º90º

Polaris – Fixed Point above the Polaris – Fixed Point above the North PoleNorth Pole

Because of this, in the Northern Hemisphere, Because of this, in the Northern Hemisphere, the altitude (angle measured in degrees the altitude (angle measured in degrees above the horizon) of Polaris tells observer above the horizon) of Polaris tells observer his latitude position.his latitude position.

If observer’s latitude changes in the Northern If observer’s latitude changes in the Northern Hemisphere, the altitude of Polaris will Hemisphere, the altitude of Polaris will exactly match observer’s latitude.exactly match observer’s latitude.

ALT OF POLARIS = LAT OF OBSERVERALT OF POLARIS = LAT OF OBSERVER

Locating Polaris – our latitude Locating Polaris – our latitude is 41°N , Polaris’ altitude will is 41°N , Polaris’ altitude will

be 41°be 41°

Lunar EclipseLunar Eclipse Only a sphere can cast a shadow that Only a sphere can cast a shadow that

appears round. During a lunar appears round. During a lunar eclipse, the earth casts its shadow on eclipse, the earth casts its shadow on the moon during the full moon the moon during the full moon phase.phase.

Summary: How did they Summary: How did they prove the Earth to be prove the Earth to be

round?round? Ships appear to sink GRADUALLY below the horizon as Ships appear to sink GRADUALLY below the horizon as they travel away from observerthey travel away from observer

Polaris (North Star) changes altitude (angle measured Polaris (North Star) changes altitude (angle measured in degrees above earth’s surface) directly with your in degrees above earth’s surface) directly with your latitude latitude

ALT OF POLARIS = LAT OF OBSERVERALT OF POLARIS = LAT OF OBSERVER Lunar Eclipses - Earth’s shadow on the moonLunar Eclipses - Earth’s shadow on the moon

Satellite Imagery – absolute proofSatellite Imagery – absolute proof

Is the Earth Perfectly Is the Earth Perfectly round?round?

The Earth is NOT a perfect sphereThe Earth is NOT a perfect sphere It is flattened at the poles and bulges It is flattened at the poles and bulges

at the equatorat the equator Earth is slightly out of round or Earth is slightly out of round or

OBLATE. OBLATE.

What proof is there that we are What proof is there that we are slightly oblate?slightly oblate?

Gravity measurements. Gravity is the force Gravity measurements. Gravity is the force of attraction between any 2 objects. of attraction between any 2 objects. Increase mass of objects = increase gravity Increase mass of objects = increase gravity Decrease distances = increase Decrease distances = increase gravity gravity

If Earth were a perfect sphere, it would be If Earth were a perfect sphere, it would be

expected to exert an equal force on objects expected to exert an equal force on objects at equal distances from the center of earth.at equal distances from the center of earth.

Weight!!! – measure of Weight!!! – measure of gravitational force gravitational force

We are further from the center of the We are further from the center of the Earth at the equator – gravity is lessEarth at the equator – gravity is less

This means we weigh LESS!!!This means we weigh LESS!!! We are closer at the poles…We are closer at the poles… We weigh MORE!!!!We weigh MORE!!!!

Center of Center of Earth= Earth= center of center of gravitygravity

•At the Poles we are closer to the At the Poles we are closer to the center = stronger gravity = more center = stronger gravity = more weightweight

The The Oblate SphereOblate Sphere The difference is smallThe difference is small We can not see it with the naked eyeWe can not see it with the naked eye It still appears like a sphereIt still appears like a sphere Be careful on multiple choiceBe careful on multiple choice

Why is the earth not perfectly Why is the earth not perfectly round? round?

Earth’s rotation causes…Earth’s rotation causes…

Bulging at equatorBulging at equatorFlattening at the poles Flattening at the poles

How big is Earth?How big is Earth? Having established the shape, what is the Having established the shape, what is the

size?size? Question: Question:

How do you measure something really big? How do you measure something really big?

Two centuries before the birth of Christ … Two centuries before the birth of Christ … (-200) Eratosthenes devised a way to (-200) Eratosthenes devised a way to

determine the size of earth. determine the size of earth. NOT CHRISTOPHER COLUMBUS!!!!!NOT CHRISTOPHER COLUMBUS!!!!!

EratosthenesEratosthenes Based his calculation on observations Based his calculation on observations

of the sun on June 21, the Summer of the sun on June 21, the Summer Solstice in Alexandria and Syene – 2 Solstice in Alexandria and Syene – 2 cities in Egypt. cities in Egypt.

On this date, the sun was straight On this date, the sun was straight overhead at noon in Syene, but not overhead at noon in Syene, but not quite overhead at Alexandriaquite overhead at Alexandria

Syene Syene AlexandriaAlexandria

Obelisk in Alexandria, EgyptObelisk in Alexandria, Egypt

The obelisk is calledThe obelisk is called “ “Cleopatra’s Needle”Cleopatra’s Needle” and it was moved toand it was moved to Central Park in 1881.Central Park in 1881.

ErathosthenesErathosthenes Made assumptions that…Made assumptions that… a. The earth is rounda. The earth is round b. The sun’s rays are parallelb. The sun’s rays are parallel

Using simple geometry he set up an Using simple geometry he set up an equation that determined the equation that determined the circumference of the earth.circumference of the earth.

Erathosthenes’ ResultErathosthenes’ Result 240,000 stadia (about 39,250 km) is 240,000 stadia (about 39,250 km) is

quite close to modern quite close to modern measurements.measurements.

Calculate the percent deviation Calculate the percent deviation (Equations are located on what page (Equations are located on what page of ESRT?) if the actual circumference of ESRT?) if the actual circumference is 40,070 kms.is 40,070 kms.

Eratosthenes' work was lost, except for a Eratosthenes' work was lost, except for a description of his method in an obscure source description of his method in an obscure source

140 AD: Ptolemy got an answer smaller by 28% 140 AD: Ptolemy got an answer smaller by 28% using a sound method with questionable data. using a sound method with questionable data.

Both results known by King Ferdinand’s Both results known by King Ferdinand’s experts. Columbus argued for the smaller value experts. Columbus argued for the smaller value for the distance from Spain to India but he was for the distance from Spain to India but he was turned down because they thought the turned down because they thought the distance too far.distance too far.

In the end Queen Isabella overruled the In the end Queen Isabella overruled the experts, and the rest is history.experts, and the rest is history.