The Reason for Seasons

1. The Reason for Seasons It’s the Tilt, Not the Distance created by Mrs. Bodine-Donahue 12/2006

2. What will you learn? • How is Earth’s tilt on its axis related to the seasons? • Why is summer in the Northern Hemisphere warmer than winter? • How are solstices and equinoxes related to the position of Earth in its orbit? created by Mrs. Bodine-Donahue 12/2006

3. Our Four Seasons • Earth’s revolution around the Sun takes 365.25 days. During that one-year period, Earth’s weather patterns change in a regular, predictable cycle. Spring, summer, autumn and winter are the four divisions of the year that we call seasons. created by Mrs. Bodine-Donahue 12/2006

4. You might think that the seasons are caused by Earth’s changing distance from the Sun due to Earth’s elliptical orbit. In other words, some people believe it is warmer in the summer because we are closer to the Sun, and colder in the winter because we are farther away. But this is not true!The difference in the distance to the Sun at different points on the ellipse is very small, not enough to cause a major change in the weather. created by Mrs. Bodine-Donahue 12/2006

5. Polaris; the North Star We have different seasons because Earth’s axis is tilted 23.5 degrees. This tilt is called Earth’s axial tilt. As Earth travels in its orbit around the Sun, Earth’s tilt stays the same. This makes Earth’s North or South Poles lean towards or away from the Sun at different times of the year. The North Pole is always pointed towards the star Polaris, also known as the North Star. created by Mrs. Bodine-Donahue 12/2006

6. Did you notice that when the northern [top] half of the Earth is pointing away from the Sun, the southern [bottom] half is pointing toward it? As the Earth revolves around the Sun, the northern and southern hemispheres have opposite seasons. created by Mrs. Bodine-Donahue 12/2006

7. As Earth moves around the Sun, its northern half, or hemisphere, receives more or less sunlight than the southern one. These changes in direct or indirect sunlight create the seasons. In the diagram above, the Sun’s rays hit the southern hemisphere almost directly, so days are warm and bright. It is experiencing summer. In the northern hemisphere the Sun’s rays hit are hitting at an angle, so the sunlight is indirect and not as strong. Days are cooler and dimmer. The northern part is having winter. created by Mrs. Bodine-Donahue 12/2006

8. The Sun’s rays strike less directly on the part of the Earth that tilts away from the Sun. That part gets less energy. A place that gets less energy from the sun becomes colder. The Sun’s rays strike more directly on the part of Earth that tilts toward the Sun. That part of Earth gets more energy. A place that gets more energy is warmer. created by Mrs. Bodine-Donahue 12/2006

9. Try it at home DIRECT AND INDIRECT ENERGY: Imagine the flashlights to be the sun. The energy coming from each flashlight is the same, but the way the light is striking the ground is different. The two flashlights on the left are allowing their energy to strike the ground DIRECTLY in a concentrated manner. The flashlight on the right is tilted so that when its energy strikes the ground, the energy is spread over a much larger area. The energy from the tilted flashlight is striking the ground INDIRECTLY, and its energy is less concentrated. created by Mrs. Bodine-Donahue 12/2006

10. Near Earth’s equator, the Sun is never far from directly overhead at noon. Temperatures stay warm all year at the equator because the Sun’s rays always hit it almost directly. Earth has two solstices a year. The solstices are the two times of the year when the Sun’s direct rays strike earth the farthest north or south of the equator. created by Mrs. Bodine-Donahue 12/2006

11. Around June 20, Earth passes through a point in its orbit called the summer solstice. At this time, the Northern Hemisphere is tilted directly towards the Sun. It is the first day of summer. It is the longest period of daylight. The Southern Hemisphere is tilted directly away from the Sun at this time, so it is the first day of winter. Around December 21, Earth reaches the winter solstice. At this time, the Northern Hemisphere is tilted directly away from the Sun. It is the Northern Hemisphere’s first day of winter. It is also the day with the shortest period of daylight. But in the Southern Hemisphere, it is the day with the longest period of daylight and the start of summer. created by Mrs. Bodine-Donahue 12/2006

12. Twice a year, between the solstices, the Sun’s rays strike the equator directly. When this happens, day and night are equal in length everywhere on Earth. Both hemisphere have 12 hours of daylight and 12 hours of darkness. These two days are called equinoxes, from the Latin word for “equal night.” The autumnal equinox occurs around September 23, marking the beginning of autumn in the Northern Hemisphere. The vernal equinox occurs around March 21, marking the beginning of spring in the Northern Hemisphere. created by Mrs. Bodine-Donahue 12/2006

13. It’s the Tilt, Not the Distance! The Sun is not quite at the center of Earth’s elliptical orbit. So, the distance between Earth and the Sun changes as Earth moves throughout the year. In some parts of its orbit, Earth is a little closer to the sun. In other parts of its orbit, it is a little farther away. Earth’s distance from the Sun does not affect the seasons. We know this to be true because if it were the distance, every place on Earth would have the same season at the same time! created by Mrs. Bodine-Donahue 12/2006

14. http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/earth_sun_relations_seasons.htmlhttp://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/earth_sun_relations_seasons.html created by Mrs. Bodine-Donahue 12/2006