1. Why Do We Have Seasons?

2. Let’s review some terms:
Rotation:
a spin about an axis
Revolution:
the motion of one body orbiting another body

3. Earth’s rotation gives us day and night
One day is one complete spin about the axis. A day on Earth is 23hrs 56min.
Earth revolves once around the sun every days. This is called a year.

4. Earth’s rotation and revolution

5. Some common misconceptions:
What are some things you have heard about the reason for the seasons?
Caused by the distance to the sun
Caused by the weather
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6. What about the shape of Earth’s orbit?
The Earth’s orbit around the sun is NOT a perfect circle. It is an ellipse.
Seasons are not caused by how close the Earth is to the sun!
Ellipse

7. In fact, the Earth is closest to the sun in early January….
and
farthest away from the sun in early July.

8. Why do we have seasons?
The seasons are a result of the tilt of the Earth’s axis.
Earth’s axis is tilted 23.5°.
The sun’s rays more directly hit that part of Earth that is tilted toward the sun.
The number of daylight hours is greater for the hemisphere that is tilted toward the Sun.

9. Look at this diagram. Notice how the Earth is always tilted in the same direction. As the Earth revolves around the Sun, the part of Earth tilted toward the sun changes.

10. During part of the year the North Pole tilts towards the sun
During part of the year the North Pole tilts towards the sun. This season is ________ in the Northern Hemisphere.

11. During another part of the year, the North Pole points away from the sun. This season is ______ in the Northern Hemisphere.

12. Why do we have seasons?
Summer is warmer than winter because the Sun's rays hit the Earth at a more direct angle during summer than during winter.

13. Energy distribution from Sun

14. Energy distribution from Sun
slanted hit
large area
(cooler) U
small area
(warmer)
direct hit N

15. The intensity of sunlight striking the ground depends on the sun's angle in the sky. When the sun is at a lower angle, the same amount of energy is spread over a larger area of ground, so the ground is heated less. The angles shown here are for the noon sun at latitude 41° north.

16. http://www. windows2universe. org/earth/climate/sun_radiation_at_earth
When the Sun is directly overhead, its rays strike Earth perpendicular to the ground and so deliver the maximum amount of energy. When the Sun is lower in the sky, a sunbeam strikes the ground at an angle (in the example above, 45°) and so its energy is "spread out" over a larger area... thus "diluting" its energy. In this example, the energy is spread over an area of 1.41 square meters (instead of 1 square meter when the Sun is directly overhead), so the energy per unit area is reduced from 342 W/m2 to 242 W/m2 (342 ÷ 1.41 = 242). Credit: Artwork by Randy Russell.

17. Why do we have seasons? In summer, days are longer than nights.
In winter, days are shorter than nights.
In winter, the sun's rays hit the Earth at an extreme angle, so the days are very short.
These effects are due to the tilt of the Earth's axis.

18. Seasons Simulator Animation

19. Significant Events: Solstice: Equinox:
Time at which the sun is as far north or as far south of the equator as possible
June/ Dec
Equinox:
Time when the sun directly crosses the Earth’s equator. Earth is neither tilted toward or away from the sun.
March/Sept
“Equal Night”

21. Summer Solstice

22. Winter Solstice

24. During the equinoxes, the axis of the Earth is not tilted toward or away from the Sun. This situation does not suggest that the 23.5 degree tilt of the Earth no longer exists. The vantage point of this graphic shows that the Earth's axis is inclined 23.5 degrees toward the viewer for both dates.

25. http://solar. physics. montana

26. Bedford, Coordinates
° N, ° W
Variations in solar altitude at solar noon for 50 degrees North during the June solstice, equinox, and December solstice.

27. Where are we? What’s the latitude?
Yes….The equator (0º)!

28. What does that mean in terms of daylight hours?

30. The Earth's seasons are not caused by the differences in the distance from the Sun throughout the year!
….. It’s a result of the tilt of Earth’s axis!

31. This tilt impacts: the angle of sun’s rays hitting Earth and the length of daylight hours

32. A private Universe