The REASONS for

1. The REASONS for The SEASONS

2. The seasons are NOT due to the Earth’s varying distance from the sun. The Earth is closest to the Sun in the Northern Hemisphere winter and farthest away in summer. The variation is only 1.5% and is exaggerated in the diagram.

3. The REASONS for The SEASONS

4. 4.5 billion years ago… …when the Earth was still new, a collision occurred between the Earth and a celestial body – a proto planet called Theia (estimated to be about ¼ the size of Earth).

5. This collision… …tilted the Earth’s axis (and created our moon). The Earth’s tilt varies from 22 to 24.5 degrees (average tilt is 23.5 degrees).

6. Without a tilt… …Earth would not experience seasons. Other planets with a tilt, and therefore seasons are Mars, Saturn, Neptune and Uranus. Comets also have seasons.

7. Why does tilt matter? It causes some places to receive direct sunlight while other places receive indirect sunlight.

8. Direct sunlight is more concentrated than indirect sunlight. 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. What months would this picture be in the south?

9. These geographical variations in the sun's angle above the horizon also account for the major geographical variations in earth's climates. The arctic and antarctic regions are almost always cold—even in the summer when they get 24 hours of sunlight a day—because the sun's angle above the horizon is never very high. And the tropics are almost always warm—even though they never get much more than 12 hours of sunlight in a day—because the mid-day sun is always so high in the sky. The intermediate latitudes, which generally have hot summers and cool or cold winters, are called the temperate zones. The north temperate zone lies between the Tropic of Cancer and the Arctic Circle, while the south temperate zone (where the seasons are reversed) lies between the Tropic of Capricorn and the Antarctic Circle.

10. Seasons = Day Length + Light Angles Summer – longer days, sun arcs higher in the sky. Winter – shorter days, sun arcs lower in the sky.

11. Copy and complete this diagram FROM THE POINT OF VIEW OF THE SOUTHERN HEMISPHERE.

12. The Four Seasons

13. Equinox = Autumn/Spring Equinox sounds like equal– equal day and night.– equally direct sunlight on both hemispheres. Which date is the autumnal equinox and which is the vernal equinox in the Southern Hemisphere?

14. Latitude The latitude of the tropics match the angle of the Earth’s tilt.

15. June 21 The Sun’s rays are directly overhead at the tropic of Cancer. Is it summer or winter in the Southern Hemisphere? December 21 The Sun’s rays are directly overhead at the Tropic of Capricorn. Is it summer or winter in the Southern Hemisphere? Solstice = Summer/Winter

18. Which event, Solstice or Equinox, is occurring at each of Earth’s four positions below? In which case, what season is it in the Southern Hemisphere?

19. Test Yourself! For each of the following (Slides A-C) State: • whether it is an equinox or a solstice • what season it is in the Southern Hemisphere Discuss: • why it is cold, warm or moderate in New Zealand

20. Slide A

21. Slide B

22. Slide C