Shape of the EarthShape of the Earth

How a scientific proof works.How a scientific proof works.

What is Earth’s shape?What is Earth’s shape?

Is it flat like a coin, or spherical like a Is it flat like a coin, or spherical like a ball? Perhaps it’s like a tin can, flat ball? Perhaps it’s like a tin can, flat on the ends and round in the middle?on the ends and round in the middle?

Describe the evidence that best Describe the evidence that best supports you answer. supports you answer.

Spherical Earth?Spherical Earth?

Scientists claim that Earth is much Scientists claim that Earth is much like a slightly flattened ball - an like a slightly flattened ball - an oblate spheroid. oblate spheroid.

How do they know or do the merely How do they know or do the merely have faith in the claim? have faith in the claim?

What is the evidence that supports a What is the evidence that supports a scientific belief in a roughly spherical scientific belief in a roughly spherical Earth?Earth?

Ships appear to sink gradually Ships appear to sink gradually below horizon from bottom up.below horizon from bottom up.

Polaris, the Polaris, the North StarNorth Star is located practicallyis located practically

over the North Pole over the North Pole of of planet Earth.planet 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

Why do observations of Polaris Why do observations of Polaris help determine Earth’s shape?help determine Earth’s shape?

The North Star gradually appears The North Star gradually appears lower and lower in the sky as you lower and lower in the sky as you travel toward the equator. travel toward the equator.

When the North Star is visible at the When the North Star is visible at the horizon, you are located at the horizon, you are located at the equator.equator.

South of the equator the North Star is South of the equator the North Star is not visible at all.not visible at all.

These observations are possible only These observations are possible only for a roughly spherical Earth. for a roughly spherical Earth.

Lunar EclipsesLunar Eclipses Only a sphere can cast a shadow that Only a sphere can cast a shadow that

always appears round. During lunar always appears round. During lunar eclipses, Earth casts its shadow on eclipses, Earth casts its shadow on the moon and the shadow is always the moon and the shadow is always round.round.

Many other indications…Many other indications…

Circumnavigation of the globe.Circumnavigation of the globe. Existence of time zones.Existence of time zones. Earth-orbiting satellites.Earth-orbiting satellites. Photographs from space.Photographs from space. Eyewitness reports of Eyewitness reports of

astronauts.astronauts. Other objects - sun, moon, planets - Other objects - sun, moon, planets -

are roughly spherical.are roughly spherical.

Summary: How do we know Summary: How do we know the Earth to be spherical?the Earth to be spherical?

Ships appear to sink GRADUALLY below the Ships appear to sink GRADUALLY below the horizon as they travel away from observer horizon as they travel away from observer and over the horizon - in all directions.and over the horizon - in all directions.

Polaris (North Star) changes altitude (angle Polaris (North Star) changes altitude (angle measured in degrees above earth’s surface) measured in degrees above earth’s surface) with latitude – at all longitudes. with latitude – at all longitudes.

Lunar Eclipses - Earth’s shadow on the Lunar Eclipses - Earth’s shadow on the moon is always round.moon is always round.

Satellite imagery and much more.Satellite imagery and much more.

Is Earth perfectly spherical?Is Earth perfectly spherical? 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 equator due to its spinning.at the equator due to its spinning. Earth is very slightly out of round or Earth is very slightly out of round or

OBLATE. OBLATE. Saturn is an even Saturn is an even

better examplebetter example of being oblate.of being oblate.

What proof is there that the What proof is there that the Earth is slightly oblate?Earth is slightly oblate?

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

The pull of gravity is weaker The pull of gravity is weaker farther from the center of farther from the center of

EarthEarth

You weigh just a bit less at the equator than at the pole!

The The Oblate SphereOblate Sphere

The difference with a sphere is smallThe difference with a sphere is small We can not see it with the naked eyeWe can not see it with the naked eye Earth still appears like a sphereEarth still appears like a sphere

Why is Earth not perfectly round? Why is Earth not perfectly round?

Earth’s spinning (rotation) causes…Earth’s spinning (rotation) causes…– bulging at equator andbulging at equator and– flattening at the poles.flattening at the poles.

Be careful on multiple-choice tests! Be careful on multiple-choice tests!