Earth Earth ScienceScience

Aim: What is the size and Aim: What is the size and shape of Earth?shape of Earth?

Objectives: Objectives:

Describe the size and shape of EarthDescribe the size and shape of Earth Explain the evidence that determines Explain the evidence that determines

the shape of Earth.the shape of Earth. Understand Polaris’ apparent change Understand Polaris’ apparent change

in altitude at different latitudesin altitude at different latitudes

Vocabulary:Vocabulary:

Oblate Sphere or SpheroidOblate Sphere or Spheroid AxisAxis GravityGravity HorizonHorizon PolarisPolaris AltitudeAltitude Circumpolar StarsCircumpolar Stars Star TrailsStar Trails

1. How do we know the 1. How do we know the Earth is round?Earth is round?

There is some evidence…There is some evidence…direct (some we can see) direct (some we can see) and indirect (some we and indirect (some we infer)infer)

a. Ships appear to sink a. Ships appear to sink gradually below gradually below horizonhorizon

HORIZON

Ships don’t fall off the “edge”

Who was the first to Who was the first to circumnavigate the globe?circumnavigate the globe?

Ferdinand Magellan….and he did not Ferdinand Magellan….and he did not FALL OFF the edge.FALL OFF the edge.

Christopher Columbus simply tried to Christopher Columbus simply tried to find a shorter route to India by sailing find a shorter route to India by sailing west….when he ran into the West west….when he ran into the West Indian Islands.Indian Islands.

b. Lunar Eclipseb. Lunar Eclipse

During a lunar eclipse, the earth During a lunar eclipse, the earth casts a rounded shadow on the casts a rounded shadow on the moon during the full moon phase. moon during the full moon phase. Only a sphere can cast a round Only a sphere can cast a round shadow.shadow.

Earth’s shadow on the Moon during a lunar eclipse

c. Satellite photos from c. Satellite photos from spacespace

Clearly… its round!

d. Polaris d. Polaris a.k.a North a.k.a North StarStar This is the star that This is the star that

lies lies in space in space practically 90 practically 90oo overoverthe geographic the geographic North Pole .North Pole .

If you stood at the If you stood at the

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

Polaris

Polaris – North Star is a fixed point Polaris – North Star is a fixed point in the night sky above the North in the night sky above the North

Pole.Pole. If you can spot Polaris in the sky, you can always If you can spot Polaris in the sky, you can always

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

90º90º

2. Why do observations of 2. Why do observations of Polaris help determine the Polaris help determine the

Earth’s shape?Earth’s shape? If you travel from the If you travel from the

equator to the North equator to the North Pole, Polaris appears Pole, Polaris appears higher and higher in higher and higher in the sky because of the sky because of Earth’s spherical shapeEarth’s spherical shape

Altitude of Polaris at:Altitude of Polaris at:– Equator (0Equator (0oolatitude) = 0latitude) = 0oo

above the horizonabove the horizon– North Pole (90North Pole (90ooN latitude) N latitude)

= 90= 90oo above the horizon above the horizon

Polaris – Altitude above horizonPolaris – Altitude above horizondetermines latitudedetermines latitude

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

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

ALTITUDE OF POLARIS = LATITUDE OF OBSERVERALTITUDE OF POLARIS = LATITUDE OF OBSERVER

5. Is the Earth perfectly 5. Is the Earth perfectly round?round?

a. The Earth is NOT a a. The Earth is NOT a perfect sphere.perfect sphere.

b. It is flattened at b. It is flattened at the poles and the poles and bulges at the bulges at the equatorequator

c. Earth is anc. Earth is an OBLATE OBLATE SPHEROIDSPHEROID

6. Why is the Earth not 6. Why is the Earth not perfectly round? perfectly round?

a.a. The Earth rotates The Earth rotates

about its about its axisaxis at 1000 at 1000 mph mph at the equator.at the equator.

b.b. The force of the rapid The force of the rapid spinning causes…spinning causes…

**Bulging at equator**Bulging at equator

**Flattening at the poles**Flattening at the poles

c. The diameter and c. The diameter and circumference from pole circumference from pole to pole is LESS than to pole is LESS than around the equator.around the equator.

Polar Polar diameter=12,715 kmdiameter=12,715 km

Equatorial Equatorial Diameter=12,756kmDiameter=12,756km

7. What proof is there that 7. What proof is there that EarthEarth

is slightly oblate? is slightly oblate?a.a. Gravity measurements. Gravity measurements. Gravity is the force of Gravity is the force of

attraction between any 2 objects. attraction between any 2 objects.

b.b. Two factors affect the gravitational attraction between Two factors affect the gravitational attraction between objectsobjects

- Mass :- Mass :Increase mass of objects (heavier) = increase Increase mass of objects (heavier) = increase

gravitygravity

- Distance:- Distance:Decrease distances (closer) = increase gravity Decrease distances (closer) = increase gravity

c.c. If Earth were a perfect sphere, it would be expected to If Earth were a perfect sphere, it would be expected to exert an equal force on objects at any point on Earth …. exert an equal force on objects at any point on Earth …. BUTBUT