1. Earth in Space
Seasons

2. Key Terms
Axis is an imaginary line around which Earth spins, causing day and night, and that is drawn from the north geographic pole through Earth to the south geographic pole.

3. Key Terms
Rotation is the spinning of Earth on its axis, which causes day and night; it takes 24 hours for Earth to complete one rotation.

4. Key Terms
Revolution is the motion of Earth around the Sun; it takes about 365 1/4 days, or one year, to complete.

5. Key Terms
Orbit is the curved path followed by Earth as it moves around the Sun.

6. Earth's axis is tilted by 23.45°
The Earth’s Tilt
Earth's axis is tilted by 23.45°

8. As Earth moves around the sun, its axis stays pointing in the same direction—all the time.
What does this mean?

9. This means that, during part of the year, the northern part of the Earth will lean more directly to the sun, and during other parts of the year the southern part of the Earth will.

10. When the northern hemisphere is leaning away from the sun, the rays coming from it hit this part of Earth at a smaller angle than on other parts of the world.

11. This means that the same amount of light is distributed over a larger surface, and so these places receive less heat than the others.

12. The southern hemisphere is experiencing Summer, the northern hemisphere has Winter.

13. In half a year, the situation reverses, and it is now Winter in the southern hemisphere since that part of the earth is now leaning away from the sun.

14. The sun’s light that hits Earth changes over of the year. 
The angle the sun is above the horizon determines how much heat and light strike each square meter of ground.

15. In the winter the Sun's energy is weakened because the Sun's ray strike the ground rather indirectly as compared to the summer months when the Sun's rays strike the ground more directly.

16. This means that the ground receives more energy (more heat) per square meter in the summer than in the winter. 
More energy is received by the ground during the summer (high temperatures) and less energy during the winter (lower temperatures).

17. A regular change in temperature that repeats itself every year.
Seasons
A regular change in temperature that repeats itself every year.

18. So, seasons are caused the tilt of the Earth’s axis

19. Remember: The seasons are the result of this tilt of the Earth's axis.
If the tilt of the Earth's axis was 0° there would be no difference in how the rays from the sun hit its different regions, and there would be no seasons.

20. The Earth's seasons are not caused by the differences in the distance from the Sun throughout the year.

21. The seasons are the result of the tilt of the Earth's axis.
I know this is a repeat, but it is important that you understand this idea. Many Americans, including Harvard graduates, do not know what causes seasons!

22. Review Look closely at where the Sun is hitting the Earth during each season:

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