1. What causes the Seasons?
Ms. Bridgeland – 6th Grade

2. The Seasons happened by accident
The Seasons happened by accident! The Earth was supposed to have stable conditions that did not change, but something happened that changed our planet forever!

3. Long ago, a protoplanet called “Theia” hit the Earth, and knocked its axis to the side.

4. Theia hit the Earth’s surface, causing a big crater in the Earth and dust and rubble to fly off the Earth’s surface.
What are two important things that happened to Earth because of Theia?

5. Theia crashing into Earth caused…
1) The Earth to be forced to tilt to its side (–its axis is now tilted 23 degrees)
2) Caused the formation of the Moon!

6. “Theia” is a protoplanet that is the same size as Mars
“Theia” is a protoplanet that is the same size as Mars. Theia is named after the Greek goddess who gave birth to the Moon goddess, Selene.

7. Theia crashing into the Earth also caused axial tilt
Axial tilt: the tilt of the Earth’s axis 23.5 degrees

8. Why would axial tilt cause the seasons?

9. Because the Northern Hemisphere and Southern Hemisphere are getting different amounts of light

10. In this picture, which hemisphere is having summer?

11. Answer: The Northern Hemisphere
The Northern Hemisphere is getting direct sunlight from the Sun, whereas the Southern Hemisphere is getting indirect sunlight, so it is WINTER there

12. Now look at this picture
Now look at this picture. Why is it winter in the Northern Hemisphere now?

13. Answer: It is winter in the northern hemisphere because the southern hemisphere is exposed to direct sunlight

15. Important: Remember that the Earth’s axis is always tilted in the same direction

16. As it revolves around the Sun, the EARTH’S AXIAL TILT WILL NEVER CHANGE! It always points toward the North Star

17. What is the North Star? (Polaris)

18. North Star:
Important because the Earth’s axis is pointed almost directly at it
The North Star remains in the same spot over the northern horizon

19. North Star: Right above the North Pole

20. Remember that as the Earth revolves around the Sun, its axis points in the same direction. This is why during some seasons, the northern hemisphere gets more direct sunlight, and in others, the southern hemisphere gets more sunlight.

22. Why are days in summer considered “longer” than days in winter?
8 o’clock in the summer, it is still light outside
8 o’clock in the winter, it is dark outside

23. The answer is that during the summer, the Northern Hemisphere gets exposure to more hours of sunlight than in the winter

24. Solstice
Occurs twice per year
What IS it?

25. Solstice:
The longest day of the year (for Northern hemisphere = June 21)
The shortest day of the year (for Northern hemisphere = December 21)

26. Solstice:
Summer Solstice: “The longest day of the year” means when the Earth is getting the most direct sunlight for the longest period of time
- Means that the Northern hemisphere is facing as close to the sun’s direct rays as possible

27. Summer Solstice 2012: In Washington, D.C. Sunrise: at 5:43 am
Sunset: 8:37 pm
Length of daylight hours: 14 hours, 54 minutes

28. Solstice:
Winter Solstice: December 21st “the shortest day of the year” –this means that the Northern hemisphere is getting the least amount of direct sunlight during daylight hours
Meaning the Northern Hemisphere is tilted as far away from the sun’s direct rays as possible

29. Winter Solstice 2012: In Washington, D.C. Sunrise: 7:24 am
Sunset: 4:50 pm
Length of daylight hours: 9 hours, 26 minutes

30. What is an Equinox?

31. What is an Equinox?
An equinox is when the sun is directly hitting the equator, which means that the Northern and Southern hemispheres are getting about the same amount of sunlight
Both the Northern and Southern hemispheres have 12 hour days (equal amount of day and night)

32. Equinox
Fall Equinox: September 21 (in Northern Hemisphere) – first day of Fall
Spring Equinox: March 21 (in Northern Hemisphere) –first day of Spring

33. Note: A season cannot officially change without a solstice or an equinox!

34. Challenge Trivia: If during the winter, the Northern Hemisphere is facing away from the sun and does not get direct sunlight, why do we still have daylight during those months?

35. Answer: Rotation!
Remember that while the Earth is revolving around the sun every 365 days, it is rotating on its own axis every 24 hours!
So for about 12 hours, the planet still is exposed to daylight, just NOT at a direct angle.

36. Rotation

37. Rotation
Which direction does the Earth rotate?

38. Rotation
Counterclockwise!
THE OPPOSITE OF HOW A CLOCK’S HANDS MOVE

39. Did you know the seasons influenced how we count days, months, and years?
A year is 365 ¼ days because people noticed that there were 4 seasons
With each season change, the moon went through its full cycle 3 times
4 season x 3 moon cycle per season = 12 months

40. Bill Nye