1. REASONS FOR THE SEASONS

2. How Earth Moves
Earth moves through space in two major ways: rotation and revolution

3. Rotation
The imaginary line that passes through Earth’s center and the North and South poles is Earth’s axis.
The spinning of Earth on its axis is called rotation.

4. Revolution
In addition to rotating on its axis, Earth travels around the sun.
Revolution is the movement of one object around another.

5. Earth follows a path, or orbit, as it revolves around the sun.
Earth’s orbit is not circular. It is a slightly elongated circle, or ellipse.

6. How Sunlight Hits Earth
Sunlight hits Earth most directly at the equator. Near the poles, sunlight arrives at a steep angle. As a result, it is spread out over a greater area.
This is why it is warmer at the equator than near the poles.
EQUATOR

7. Earth’s Tilted Axis
Earth has seasons because its axis is tilted as it revolves around the sun.
Earth’s axis is always tilted at an angle of 23.5 degrees from the vertical.

8. As Earth orbits the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part of the year.
Summer and winter are caused by Earth’s tilt as it revolves around the sun.
THE CHANGE OF SEASONS IS NOT CAUSED BY CHANGES IN THE EARTH’S DISTANCE FROM THE SUN!

9. Earth in June
The northern end of Earth’s axis is tilted toward the sun, making it warmer. It is summer in the northern hemisphere.
The southern end of Earth’s axis is tilted away from the sun, making it colder. It is winter in the southern hemisphere.
See… the north tilts towards the sun!

10. Earth in December
The northern end of Earth’s axis is tilted away from the sun, making it colder. It is winter in the northern hemisphere.
The southern end of Earth’s axis is tilted towards the sun, making it warmer. It is summer in the southern hemisphere.
See… the north tilts away from the sun!

11. Solstices
The sun reaches its greatest distance north or south of the equator twice a year.

12. June 21
Summer solstice (and longest day of the year) in the northern hemisphere.
Winter solstice (and shortest day of the year) in the southern hemisphere.

13. December 21
Summer solstice (longest day of the year) in the southern hemisphere.
Winter solstice (shortest day of the year) in the northern hemisphere.

14. Equinoxes
Halfway between the solstices, neither hemisphere is tilted toward or away from the sun. This occurs twice a year.
Each of these days are known as an “equinox.”

15. The word “equinox” means “equal night.”
During an equinox, day and night are both about 12 hours long everywhere on Earth.

16. March 21
The vernal (or spring) equinox marks the beginning of spring in the northern hemisphere and fall in the southern hemisphere.

17. September 22
The autumnal equinox marks the beginning of fall in the northern hemisphere and spring in the southern hemisphere.