1. THE EARTH AND SEASONS PAHS SCIENCE 10

2. This is our home….the Earth as seen from space.

3. THE EARTH The Earth follows the path of an ellipse around the sun. This means that sometimes the earth is closer to the sun and sometimes it is farther away. Could this explain what we observe as the seasons?

4. WHAT CAUSES THE SEASONS? This is not an easy question to answer. A common misconception is that the seasons are caused by how far the earth is from the sun. In Canada, we are actually closer to the sun in the winter time by about 900 000 kilometers. So that cannot explain our weather being colder in winter and warmer in summer.

5. If distance were the answer, we would have winter all over the earth at the same time. This is not what happens…when we have winter in the Northern Hemisphere, the Southern Hemisphere has summer.

6. THE EARTH’S AXIS The earth’s axis is tilted from the vertical by 23 ½ degrees.

7. Winter In winter, the earth is tipped away from the sun. Let’s look at a simulation of how this works. The earth's orbit and tilt

8. Length of day and night Sunlight and the four seasons As you know from living, the length of a day is shorter in winter and longer in the summer. The days get shorter until the shortest day occurs Dec 21. That day is called the winter solstice

9. Length of day and night The days get longer now until we reach the maximum length of days. This occurs at the summer solstice June 21. The day and nights are of equal length on two days of the year…the spring equinox and the autumnal equinox. There are 12 hours of daylight and 12 hours of darkness on these days. Let’s look at the animation again: Hours of daylight and darkness

10. Summary…so far what have we learned so far about the earth?

12. In December, the earth’s axis is tipped AWAY from the sun. We get less sunlight and the rays are less direct.

13. In June, the earth’s axis is tipped TOWARDS the sun. We get more sunlight and more direct sunlight.

15. SOLAR HEATING What matters is how directly the sun’s rays hit the earth’s surface.

16. PRECESSION OF THE EQUINOXES The earth’s axis slowly wobbles or precesses.

17. It takes 26 000 years to complete one circuit. It is caused by tidal torques caused by the moon and sun.

18. In ancient times, the pole star was Thuban. Today it is Polaris. Polaris is closest to the North Celestial Pole in 2099.