1. Ch. 23 The Sun-Earth-Moon System
Revised: 5/2/12

2. Earth’s Shape How recent, was it thought that the Earth was flat?
Some people, as recently as the days of Christopher Columbus believed the Earth was flat.
However, this was mostly a MYTH that most people believed the Earth was flat.
Columbus did believe the Earth was round!!!
And, many believed the Earth was round prior to Columbus. This was NOT a new idea!!!

3. Earth’s Shape Earth is a sphere. It’s not a perfect sphere.
A round, three-dimensional object
It’s not a perfect sphere.
Equatorial radius = kilometers. Polar radius = kilometers
So the difference in circumference is 71.1 kilometers.
In other words, it bulges at the equator.
It is flattened at the poles.

4. When was the first evidence of Earth’s Shape?
In 350 B.C., Aristotle, A Greek Astronomer and philosopher, reasoned that Earth was spherical.
What evidence did he have?
Observations of the stars when traveling North and South. For example: on the equator, the North Star is low on the horizon; when further north it is higher up.
Behavior of ships leaving shore.
Eclipses
The edge of the Earth’s shadow
The only shape that always cast a circular shadow is a sphere.

6. Earth’s Rotation Axis—imaginary line around which Earth spins.
23.5 degrees
This is what causes the seasons!!
The Seasons
Rotation—spinning of Earth on it’s axis.
West to East
(Causes Day/Night)
Earth From Space
How long is one rotation?
23 hr. 56 min.
How many rotations does Earth complete during 1 year?
Earth’s Rotation

7. Earth’s Rotation
Remember: The rate of rotation is currently 23 hr 56 min.
Is this rate constant?
No……
10,000 years ago, the rotation was 23 hr 54 min.
That’s 2 minutes in 10,000 years
20 minutes in 100,000 years
200 minutes in 1 million years.
200,000 minutes in 1 billion years.

8. Earth’s Rotation How do we know Earth’s rotation is slowing down?
In 1820, the Earth’s rotation was 86,400 seconds.
Now, the rotation is 86, seconds.
The rotation has slowed approximately 2 milliseconds since 1820.

9. Earth’s Rotation Has the slowing rate affected Earth? Yes….
Since the slowing rate affects the angular momentum of Earth, the Earth alters it’s shape slightly.
Caused mountains to rise and earthquakes
Climate since ocean depths have changed at the poles and equator.
This has also affected Earth’s atmosphere.

10. Earth’s Revolution Revolution—yearly orbit around the sun.
Is Earth’s orbit round?
No, it’s an ellipse. (Elongated curve)
The Sun is offset from the center of the ellipse.
Important Dates:
“Perihelion” (January 3rd )
Earth is closest to the sun
147 million km or 91.4 million miles
“Aphelion” (July 4th)
Earth farthest from the sun
152 million km or 94.5 million miles

11. Revolution
Are the seasons caused by the distance of Earth from the Sun?
NO!!!!
If so, it would be warmest during perihelion (January 3rd)

12. So, what causes the seasons?
Earth’s tilted axis
Earth has a tilt of 23.5 degrees
THIS CAUSES THE SEASONS
Hours of daylight
Electromagnetic Radiation

13. The sun’s position relative to the equator constantly changes.
When is the sun directly over the equator?
TWO TIMES A YEAR
Equinox—sun is directly over the Earth’s equator.
Number of daylight hours at equator= 12 Hrs
Number of nighttime hours at equator = 12 Hrs
When does this occur in the Northern Hemisphere?
Spring (Vernal) Equinox – March 20 or 21
Fall (Autumnal) Equinox – September 22 or 23
When does this occur for the Southern Hemisphere?
The dates are reversed.
What else are these dates known as?
First day of spring and fall
Equinoxes & Solstices

14. Equinoxes & Solstices
Solstice—point at which the sun reaches the greatest distance north or south of the equator.
When does this occur in the North?
June 21 or 22 (Summer solstice)
December 21 or 22 (Winter solstice)
What day of the year has the most daylight hours in the northern hemisphere?
June 21 or 22
When is the shortest day of the year?
December 21 or 22

15. Earth’s Magnetic Field
What creates Earth’s Magnetic Field?
Scientist Hypothesize: movement of molten metal core inside Earth
Magnetic field is like a bar magnet.
And, every magnet has a North & South Pole.
The magnetic field is not aligned with Earth’s spin axis, but at 11.5 degrees.
Magnetic Field Animation
Magnetosphere
THE CORE (>4 Min)

16. Earth’s Magnetic Field
Earth’s magnetic field would be similar to that of a bar magnet if no other forces were involved.
The solar wind distorts Earth’s magnetic field.

17. Earth’s Magnetic Field
Earth’s magnetic field extends thousands of miles into space.
Magnetic poles are not static.
They wander. As much as 15 kilometers/year.
Occasionally the poles reverse.
The last reversal was 780,000 years ago.
It is estimated the next reversal will be 200,000 years from now.

18. Motions of the Moon Lunar Phases The moon rotates on an axis.
The Moons revolution causes changes in appearance.
Lunar Phases
Why don’t we see the moon rotate?
The moon rotates on it’s axis every 27.3 days & the time for the moon to revolve around Earth is 27.3 days (if the Earth were not moving.)

19. Why the Moon Shines Reflecting sunlight from it’s surface.
What causes the phases?
The positions of the moon, Earth, and sun, changing showing different portions of the lit side.

20. Phases of the Moon Lunar Phases
New Moon—lit half of the moon facing the sun and therefore we can’t see the side facing the Earth.
We cannot see the moon.

21. Waxing Phases Waxing—more of the moon becomes visible.
Waxing Crescent—24 hours after new moon.
First Quarter—1 week after new moon, half of lighted side (quarter of moon’s surface).
Waxing Gibbous—more than one quarter is visible.
Full Moon—all of the side facing Earth is lit up.

22. Waning Phases Third Quarter—only half of the lighted side seen.
Waning—amount of the lit side becomes smaller.
Waning Gibbous—after full moon, almost all of the side of the moon facing Earth is seen.
Third Quarter—only half of the lighted side seen.
Waning Crescent—just before new moon. (slice)

23. Moon’s Orbit The complete cycle takes: 29.5 days.
Exactly: 29 days 12 hours 44 minutes
Time for Moon to revolve around Earth: days (if Earth wasn’t moving)
Why the difference?
It takes about 2 days for the moon to “catch up” with Earths advancement around the sun.

24. Eclipses Revolution of the moon. When do they occur?
The Earth blocks light to the moon.
OR the moon blocks light to the Earth.
When do they occur?
Only when the sun, Earth, and moon are perfectly lined up.

25. Solar Eclipses Moon casts a shadow on Earth.
The Moon is between the sun and Earth.
2 Parts of the Moons shadow.
Umbra—darkest portion (person sees total eclipse). Only see part of the suns atmosphere.
Penumbra—lighter shadow. Person sees partial eclipse.
March 29th 2006 Total Solar Eclipse "Turkey"

26. Lunar Eclipses Earth’s shadow falls on the moon. 2 Parts
Moon moves into Earths penumbra.
Then into Earths umbra. (Moon appears red) Why?
Light from the sun is refracted by Earth’s atmosphere onto the moon.

27. When and Where to view Solar Eclipses
Total Solar Eclipse:
up to 2 times a year, you have to be in the right position on the Earth at the right time.
A person can only see if in the umbra.
A person in the penumbra can see a partial solar eclipse.
WARNING: never look directly at either.
Total Lunar Eclipse:
anyone on the nighttime side of Earth can see.

28. How to Safely View a Solar Eclipse
Pinhole Projection Method
“Pinhole Camera”
Solar Filters
Welder’s Goggles
Must be a rating of
14 or higher.
Do not use behind a pair of binoculars or telescope.

29. How to Safely View a Solar Eclipse
Camera and Solar Filters
Fully Exposed and Developed Black & White Film
Binocular Projection
Telescope Projection

30. Next Solar Eclipses Total Solar Eclipse (top) November 13, 2012
Annular Solar Eclipse (bottom)
May 20, 2012

31. Next Solar Eclipses

32. Mr. Aprill’s Eclipse Annular Solar Eclipse: Interactive Eclipses
May 30th, 1984
Very close to total in Wisconsin….was a total eclipse along the red line.
From Wisconsin it was a 63.8% Eclipse
My First Eclipse
Interactive Eclipses

33. When and Where to View Lunar Eclipses
Total Lunar Eclipse:
anyone on the nighttime side of Earth can see.

34. Next Lunar Eclipses Total Lunar Eclipses: June 4, 2012
June 4th, 2012 Eclipse
Total Lunar Eclipses:
June 4, 2012
November 28, 2012

35. The Moon’s Surface Maria Basins Dark colored flat areas on the moon.
3-4 byo lava flows
Basins
Low areas on the moon.

36. Craters What caused the craters?
Meteorites, asteroids, comets striking the surface.
Impacts may have caused cracks, causing lava to rise.

37. Inside of the Moon
Scientists have been able to study the inside of the moon in the same way they studied the inside of the Earth.
They used moonquakes
60 km crust (on side facing Earth)
150 km crust (side away)
Solid mantle—depth of 1000 km
Below is a partly molten zone
Iron-rich solid core

39. Theories of Moon Formation
Before the lunar landings there were three theories.
The Capture Theory
The moon was captured by Earth’s gravity. Video
The Condensation Theory. (AKA Accretion Theory)
Moon and Earth condensed from the same cloud of dust and gas.
The Ejection Ring Theory.
Earth ejected molten material that became the moon.

40. New Theories After going to the Moon, there is now a new theory.
Moon formed billions of years ago.
Impact Theory
Mars-sized object collided with Earth.
Condensing gas and debris thrown off into orbit.
Material condensed, forming the moon.