4. earth movements-student

7/25/2019 4. Earth Movements-student

1/23

Motions of the Earth

Our goals for learning:

What are the main motions of the Earth in space?

How do we see these motions on the ground? Howdoes it affect our lives?

How does the orientation of Earth's axis changewith time?


2/23

What are the Main Motions of

the Earth in Space?

There are 3 main movements:

Rotation (daily)

Orbit or revolution (annual)

Precession (millennia)


3/23

1. Daily Motion

Earth moves counterclockwise, so Sun

appears to move clockwise

Sun appears to rise in the East and set

in the west.

Speed average 15/hr for 360 in 24 hrs


4/23

View of the Sun With LatitudeThanks to Earths rotation, the Sun appears to move in the

sky. But, its location depends on the viewer's latitude.

Here is the equinox,

when sunlight falls

evenly everywhereon the Earth.


5/23

View of Daily motion from Ground

Sun path & latitude Hand out


6/23

Suns Path in the Local Sky
http://path_of_the_sun.htm/


7/23

View of Daily motion

from Ground

Earth rotates west to east

(counterclockwise), so, like the

Sun, stars appear to circle fromeast to west. Stars will rise and

set, depending on your location


8/23


You will always see !of the celestial sphere at one time.

However which part you see depends on your location onthe Earth.

This affects whichstars you see and

the angle at which

you see them.

Some stars will

rotate in and out of

view.


9/23


If you are in the northern hemisphere then:

" Stars near the north celestial pole never set.

" We cannot see stars near the south celestial pole.

" All other stars (and Sun, Moon, planets) rise in east and set

in west.

Your Location Your View


10/23

2. Orbital Motion


11/23

Thought Question

TRUE OR FALSE? Earth is closer to the Sun in summer

and farther from the Sun in winter.

A. True

B. False


12/23

How do we mark the seasons?We define four special points:

summer solstice (longest day)winter solstice

spring (vernal) equinox

fall (autumnal) equinox


13/23

Summary: The Real Reason for Seasons

Earths axis points in the same direction(to Polaris) all year round, so itsorientation relative to the Sun changes asEarth orbits the Sun.

Seasons are caused by.

1) Constant Axial Tilt

2) Orbit around the Sun


14/23

Why doesntdistance matter?

Variation of Earth-Sun distance is small about 3%; this small variation is overwhelmed

by the effects of axis tilt.


15/23

View of Orbital Motion From Ground

Summer solstice: Highestpath, rise and set at most

extreme north of due east.

Equinoxes: Sun rises

precisely due east and sets

precisely due west.

Winter solstice: Lowest

path, rise and set at most

extreme south of due east.


16/23

When the sun is high in the sky,

its light is more concentrated &

more effective in heating the Earth.


17/23

Why does Temperature Change Seasonally?

Summer

High sun angle+ longer time in the sky

=> increased surface heating=> WARM

Winter

Low sun angle+ shorter time in the sky

=> decreased surface heating => COLD!


18/23

Seasonal changes are more

extreme at high latitudes

summer solstice at the Arctic Circle-no sunset


19/23

Thought Question

It is December 31 and you see 24 hours ofdaylight. Where are you standing?

A. North Pole

B. Equator

C. South Pole

D. San Mateo


20/23

The sky varies as Earth orbits the Sun

As the Earth orbits the Sun, the Sun appears to move eastward

along the ecliptic. The result is that the sun blots out different stars in different

sesaons, while constellations opposite the Sun are visible


21/23

The sky varies as Earth orbits the Sun

Different constellations swing into view at different seasons.


22/23

How does the orientation

of Earths axis change

with time?

!Although the axis seems fixed on

human time scales, it actuallyprecesses over about 26,000 years.

!Earths axis precesses like the axis

of a spinning top


23/23

The View of Precession

from the Ground

Polaris wont always be the

North Star.

Positions of equinoxes shift

around orbit; e.g., spring

equinox, once inAries, is now

in Pisces!

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