The Motions of the Earth

Evidence of Movement and the

Consequences

How do we know that the Earth is Round?

• First, the sky is curved. This is a reflection of the Earth’s curvature.

• We can see the Earth’s shadow during a lunar eclipse.

• Different constellations are visible in Southern Hemisphere than here.

• Sun’s height changes with latitude.

Lunar Eclipse: Earth’s shadow on the Full Moon

We see different constellations than South Africa

Sun Height March 21

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Ht. of Sun (°)

Rochester Key West Fairbanks Kampala Buenos Aires

The closer to the equator you are, the higher the sun is in the sky. The Sun’s rays are most direct on the equator, and most slanted in polar regions.

Rays on Sun on Earth

Paths of Sun Across the Sky

east south west

0 °lat.

43 °N lat

80 °N lat

What proof is there that the Earth rotates in 24 hours?

• The Sun, the Moon, the Planets, and the “fixed” stars all move from east to west.

• The stars leave trails across the sky that go around the celestial poles.

• Physical evidence comes from the Foucault pendulum.

• When it’s noon here, it’s midnight in Japan.

Star Trails With Polaris Near the Center

33°

20°

Similar Pattern in the

Southern Hemisphere

South Celestial Pole

…except in the opposite direction to the North!

Foucault Foucault PendulumPendulum

All dominoes fall down in a 24 hour period.

Different Time Zones: one of the results of Earth’s rotation

North Pole

6 AM

6 PM

12 Mid

12 noon

Portsmouth Rochester

MuskegonMuskegon Madison

43rd

Parallel

Sun Height 43 °N

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Ht. of Sun (°)

Portsmouth, NH Rochester, NY Muskegon, MI Madison, WI

The farther west you go, the later the sun rises and sets. Solar noon is later too.

Sundial - Tracking the Sundial - Tracking the SunSun

What is wrong What is wrong with this with this picture?picture?

(It’s not the (It’s not the watchwatch!)!)

Time Zone MapsTime Zone Maps

The Earth’s Revolution!The Earth’s Revolution!

How do we know that the Earth revolves around the

Sun in 365 days?• The Sun completes one trip “through”

the zodiac in one year.

• The sky looks the same on Sept. 23, 2010 as it did on Sept. 23, 2009.

• The sun returns to the same place at the beginnings of each season, which can easily be marked.

The Sun appears to move through the 12 signs of the Zodiac in one year cycle.

Animation

Orion dominates our southern sky every February.

For centuries people have marked sun rise positions for midsummer and midwinter.

The Sun Dagger

• In Chaco Canyon, New Mexico, there is a rock painting aligned with the equinoxes and solstices.

• This gave the Anasazi people a way to check their calendar and make corrections as needed.

• Other cultures had similar monuments to help them keep track with the yearly cycle of the sun.

The Effects of the Earth’s Tilt

• The Earth tilts 23.5° but it always points toward the North Star (Polaris).

• Because the two poles tilt in the opposite direction, seasons are reversed in Northern and Southern Hemispheres.

• The height of the Sun changes with the seasons.• Length of daylight is long in summer and short

in winter.

Seasons Animation

The poles always point in the same direction, which means the Earth either leans toward the sun or away from it.

Seasons are reversed in the two hemispheres

summer winter

summerwinter

The higher the sun’s path, the higher the intensity…

SUMMERWINTER

Ht. of Sun - Rochester, NY

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Time of Day

Altitude of Sun (°)

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Here in Rochester, the Sun is high in the sky in summer, and low in winter. (The top of the sky is 90°)

If you live in Arctic or Antarctic regions the effect of the tilt is

even greater

Rochester Alaska

Ht. of Sun - Fairbanks, AK

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Ht. of Sun (°)

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In Alaska, days are very long in summer, and very short in winter. The same is true for Antarctica.

Ht. of Sun Buenos Aires, Arg.

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Ht. of Sun (°)

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For locations in the Southern Hemisphere, the seasons are reversed, so Dec. 21st is the longest day, and June 21st is the shortest.

The Earth’s PrecessionThe Earth’s Precession

The Earth slowly wobbles like a top…once every 26,000 years.

By an incredible coincidence, Polaris is almost directly over the North Pole. So it can be used to help us navigate.

This is a temporary situation, however. Since the Earth is wobbling like a top, the pole slowly moves. This is called precession.

The Precession Cycle

Now the North Celestial Pole is near Polaris (off by about 1 degree).In the days of the Egyptian pharaohs, it was near Thuban in Draco.In around 12,000 years the wandering pole will point toward Vega, but there will be centuries when there is no North Star.

Quick Quiz…• How do we know the earth is round?• What proof do we have that the Earth rotates

once every 24 hours?• If we lost all our calendars, how could we

determine the length of a year?• Why are seasons reversed in the southern

hemisphere from the northern hemisphere?• What is meant by precession? What does

that have to do with Polaris?