Earth's Rotation, Magnetic Field, and Causes of the Seasons

1. Chapter 23: The Sun-Earth-Moon System Section 1

2. Properties of the Earth • The Earth is a three-dimensional sphere. • The Earth bulges slightly at the equator and is flattened at the poles, so it is not a perfect sphere.

3. The Earth has an axis- an imaginary line vertical line around which the Earth spins. The line cuts directly through the center of the Earth.

4. Rotation • The spinning of the Earth on its axis is called rotation. • Rotation causes day and night to occur.

5. Day occurs when you are facing the sun. As the Earth continues to rotate, the sun moves through the sky until it sets. Night occurs when you are facing away from the Sun.

6. Rotation • One complete rotation takes 24 hours. • The Earth rotates counterclockwise, moving west to east.

7. The Magnetic Field • The movement of the material inside earth’s core along with Earth’s rotation generates a magnetic field.

8. This field is like a bar magnet – Earth has a north and south magnetic pole. • The magnetic axis does not align with the rotational axis. It is inclined at an angle of 11.5°.

9. The Magnetic Field • If you followed a compass needle, you would end up at the magnetic north pole instead of the true north pole.

10. The location of the magnetic poles changes slowly over time and can reverse completely. The position of the magnetic field over Earth’s history has been recorded as rocks form on the seafloor.

11. Causes of the Seasons • The Earth’s yearly orbit around the sun is called a revolution. One complete revolution takes 365.25 days – one year.

12. The Earth’s orbit is an ellipse – an elongated circle. The Sun is not at the center of the ellipse, instead it sits a little closer to one end.

13. Causes of the Seasons • We are the closest to the sun on January 3rd and the farthest away on July 4th. • Earth’s tilt on its axis causes the seasons. Earth is tilted at a 23.5° angle.

14. The hemisphere tilted toward the sun gets more daylight hours than the hemisphere tilted away from the sun.

15. Causes of the Seasons • The hemisphere tilted toward the sun also gets more solar radiation than the hemisphere tilted away from the sun.

16. Summer occurs in the hemisphere tilted toward the sun when its radiation strikes the Earth at a higher angle and for longer periods of time.

17. Winter occurs in the hemisphere receiving less radiation and shorter periods of daylight

18. Solstices • The solstice is the day when the Sun reaches its greatest distance North or South of the equator. • In the Northern Hemisphere, the summer solstice occurs on June 21 or 22. This is the longest period of daylight of the year. After this, the number of daylight hours become less.

19. The winter solstice occurs on December 21 or 22. This is the shortest period of daylight of the year, then the hours of daylight start to increase again

20. Equinoxes • An equinox occurs when the sun is directly above Earth’s equator. This only happens twice a year.

21. During an equinox, the number of daylight and night hours are equal all over the world. Neither the northern or southern hemispheres are tilted towards the sun.

22. Equinoxes • In the Northern Hemisphere, the Sun reaches the spring equinox on March 20 or 21 and the fall equinox occurs on September 22 or 23.

23. In the Southern Hemisphere, both the solstices and equinoxes are reversed. Spring occurs in September and fall occurs in March.