Earth’s Motions

Earth’s Motions – Big Questions:

How can we make a model to help us understand our

universe?

What are the motions of the Earth?

How do we know?

How do the motions of the Earth affect our observations?

Yes, the Earth really is a SPHERE!

http://testtube.com/dnews/how-we-know-the-earth-isn-t-

flat/

Earth’s Motions

There are three major motions of Earth

Rotation

Revolution

Precession

ROTATION

Rotation

Earth turning or spinning on an axis

How does the sun appear to

move in our sky? WHY?

Because the Earth turns on its axis from

the West to the East (counter-clockwise)!

Effects of Earth’s Rotation…

We experience day as our location on Earth turns to the sun and night as it turns away from the sun

When we look south, the sun, stars, moon, planets, and & constellations all appear to rise in the East and set in the West

Why do the star trails in the

southern sky look different

than the ones in the

northern sky?? Because the Earth’s axis is tilted toward Polaris, so stars in the north appear to circle around that axis of rotation.

Effects of Earth’s Rotation: Coriolis effect

Because the Earth is wider at the equator, Earth rotates faster

at the equator than it does at the poles

In the Northern Hemisphere winds, weather patterns, water

currents, airplanes, etc. all appear to deflect to the right

because the Earth is moving underneath them.

https://www.youtube.com/watch?v=mPsLanVS1Q8

Effects of Earth’s rotation…

Plane flights

pilots and navigators

must compensate for

objects veering to the

right

Space flights

Creates special issues for

astronauts in space

because they move at a

different rate than Earth

Because of Earth’s Rotation

Winds

Uneven heating of the atmosphere causes winds blow from areas of high pressure to areas of low pressure

Distribution of Rain

If not for the deflection caused by the rotating Earth, the equator would get constant rain and north & south would be deserts.

Evidence the Earth Rotates:

The Foucault Pendulum

A ball swinging back and

forth on a string is called

a pendulum.

In 1851, French scientist

Léon Foucault observed

that his pendulum

seemed to change

direction over time.

What force was causing

this change in direction?

http://www.youtube.com/watch?v=C3-uFao4dxQ

The Foucault Pendulum

Forces acting on a pendulum: Inertia causes it to swing straight out (Newton’s First Law of Motion)

Gravity pulls it back down (Newton’s Law of Gravity)

Air resistance makes it swing in shorter arcs over time (friction is a force that acts against inertia)

So what makes the pendulum change directions?

Pendulum as Evidence that Earth rotates:

There is no force acting on the pendulum to make its plane of

oscillation rotate

The pendulum seems to change its path during the day, but

actually it is the floor beneath it that changes, being twisted

around by the daily rotation of earth.

At the north pole the apparent rotation

would be a full circle of 360 degrees

each day, or about 15 degrees per hour

Evidence of Earth’s rotation

Astronauts have actually observed Earth’s rotation from

space!

https://www.youtube.com/watch?v=eqCyHeJzO4s

All Planets Rotate: Rotation times:

Mercury…………………58 Earth days

Venus………………….. 243 Earth days

Earth……………………23 hours 56 minutes

Mars…………………….24 hours 37 minutes

Jupiter…………………..9 hours 50 minutes

Saturn…………………..10 hours 14 minutes

Uranus………………….18 hours

Neptune…………………18 hours 10 minutes

Which two planets have the longest periods of rotation?

Which planet has the shortest?

Which planet’s period of rotation is almost the same as Earth’s?

Very similar

REVOLUTION

Revolution

Earth revolves around the Sun at an average speed of

about 27 km/s or about 66,000 mph!

One trip around the sun on Earth’s elliptical orbit equals

one year.

Earth’s Revolution in action…

http://www.classzone.com/books/earth_science/terc/cont

ent/visualizations/es0408/es0408page01.cfm?chapter_no=

04

We are closest to the Sun at perihelion on January 3rd

(147 million kilometers)

We’re a little further from the Sun at aphelion on July 4th

(152 million kilometers)

Earth’s Orbit

Observational Evidence of Earth’s

Revolution: Changing Constellations

As earth moves along its orbit, we face the sun from

a different perspective and we view different areas of

space at night.

Most of the constellations we can see in the summer

are not visible in the night sky during the winter.

This is evidence that Earth has changed locations in

space!

All planets revolve around the sun

All Planets revolve around the sun near the same ecliptic plane as Earth.

Remember Kepler’s third law… outer planets have longer orbital periods than inner planets do.

Mercury………………...……….88 days

Venus……………………………225 days

Earth…………………………….365.25 days

Mars…………………………….687 days

Jupiter…………………………..12 years

Saturn…………………………..29.5 years

Uranus………………………….84 years

Neptune………………………..165 years

Understanding Orbital Motion

The universal law of gravity allows us to understand

orbital motion of planets and moons:

Earth and moon attract each other through gravitation.

Since Earth is much more massive than the moon, the

moon’s effect on Earth is small.

The moon’s inertia would cause it to move in

a straight line (V), but…

Earth’s gravitational force constantly

accelerates the moon towards Earth (F)

Gravity is constantly changing

the moon’s direction of motion ,

holding it on its almost circular orbit.

Earth

Moon

v v’

Dv

F

(Dv)

Video : gravity and orbits

http://svtube.svsd.net/play?id=uzptg84xwc

Precession

Precession

Earth’s “wobble” on its

axis

Takes ~26,000 years to

complete one wobble

What causes Precession?

The Earth is slightly oblate

it bulges in the middle

~0.34% larger at equator due to its rotation and the fact that it

is not completely rigid.

The Sun and Moon have non-uniform gravitational forces

on the Earth, pulling harder on the near side of the bulge

than on the far side. (gravity weakens with distance)

Consequences of Precession

Polaris marks the north

pole today

Thuban was the pole

star in 3000 BC

Vega was in 12000 BC

Polaris will become the

north star again around

27800 AD

Earth’s motions and TIME

A year, a day, a month…

A year is linked to Earth’s revolution (one orbit)

A day is linked to Earth’s rotation (one spin)

A month is linked to the cycle of the moon

BUT… each of these things can be defined in different

ways. How do you know when it’s been one full rotation?

One full orbit? One full moon cycle?

For example: Sidereal Day vs. Solar Day

sidereal (or star) day:

It takes the earth 23 hours and 56 minutes to rotate once with

respect to the stars. This is one sidereal (or star) day.

solar day:

During that time, the earth has moved a bit along its orbit

around the Sun. So…

It takes about 4 minutes longer for the Sun to return to the

same position in the sky

Because we use the solar day, the stars rise and set 4 minutes

earlier each day.

Orbital Motion in One Solar Day

Earth’s Motions – Big Questions:

How can we make a model to help us understand our

universe?

What are the motions of the Earth?

How do we know?

How do the motions of the Earth affect our observations?