#19 Learning Objective: Scholars’ will be able to explain the “reasons for the 4 seasons” by analyzing how Earth revolves on its axis. Standard:1.1h The apparent motions of the Sun, Moon, planets, and stars across the sky can be explained by Earths rotation and revolution. Earths rotation causes the length of one day to be approximately 24 hours. This rotation also causes the Sun and Moon to appearto rise along the eastern horizon and to set along the western horizon. Earths revolutionaround the Sun defines the length of the year as 365 1/4 days. 1.1i The tilt of Earths axis of rotation and the revolution of Earth around the Sun cause seasons on Earth. The length of daylight varies depending on latitude and season.

Recall the 2 Types of Earth Motions. They are….. 1- Rotation- spinning of Earth on its Axis.

(i.e. Movement of Earth around the sun) 2- Revolution- The movement of one body around another. All planets revolve around the sun (i.e. Movement of Earth around the sun)

Think-pair-Share There are several factors that cause seasons…. Using the nominal group technique, each group member think of one factor they think causes the seasons.

1- Tilt of Earth’s Axis The Earth is tilted 23.5o angle. This is the reason for the seasons that many parts of the Earth experience! The seasons are opposite in the Northern and Southern hemispheres. Between the Tropic of Cancer and the Tropic of Capricorn (23.5° latitude south) there really are no seasons as the sun is never very low in the sky so it stays warm and humid ("tropical") year-round. Only those people in the upper latitudes north and south of the tropics experience seasons.

Tropic of Cancer & Capricorn the tropics are based on the sun's position in relation to the Earth at two points of the year (winter & summer)

When Northern Hemisphere of the Earth leans towards the sun, it receives more light and heat and we enjoy Summer! At the same time, the Southern Hemisphere receives less light and heat, making it cooler. It is in the midst of Winter. As the Earth continues along its orbit around the Sun, the angle that the Earth’s axis tilts changes. Eventually the Southern Hemisphere faces the Sun, and the Northern Hemisphere leans away. During this time of the year, it is the Southern Hemisphere’s turn to enjoy Summer.

2- Revolution of Earth around the SUN As the Earth revolves around the sun, there are 4 points when the seasons begin! As the Earth moves around its orbit the portion leaning towards the Sun changes. This causes different parts of the Earth’s surface to receive a different amount of sunlight and heat.

Brain Pop Video Earth’s revolution, tilt and the Seasons http://www.brainpop.com/science/weather/seasons/ As you watch the video, please take notes.

The distance between the Earth and the Sun changes throughout the year.

3- Parallelism of Earth’s Axis Earth’s axis always points in the same direction in space.

Sun’s path in the sky The sun is at its highest path in the sky in the Summer and lowest in the Winter. In the Spring and Fall. In the Spring and Fall , the path is in between.

4- The angle of the Sun, which is called the Angle of Insolation Direct Rays= Hotter temperatures Indirect Rays= Cooler temperatures Think about what happens when you use a hair dryer!

5 - Duration of Insolation The length of TIME sunlight is received at a location (daytime hours) Ex: Summer in NY has 15 hrs day/9 hrs night Winter 9 hrs day/ 15hrs night Fall & Spring 12/12 If you live near the equator, day and night are nearly the same length (12 hours). But elsewhere on Earth, there is much more daylight in the summer than in the winter. This is because the sun’s rays are more direct.

Solstices At two point throughout the year, the tilt of the Earth’s axis reaches its maximum angle compared to the Sun, and begins to move back the other direction. This usually happens around June 21 st, and December 21 st. These days are known as solstices. On these solstices the rays of the Sun shine directly on one of the two tropics. During the Summer Solstice the rays of the Sun shine directly on the Tropic of Cancer. During the Winter Solstice the Sun’s rays shine on the Tropic of Capricorn.

Equinox An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun (its actually sideways), the center of the Sun being in the same plane as the Earth's equator. During these equinoxes the rays of the Sun shine directly on the equator. This happens on approximately March 20th, and September 22nd. Every part of Earth has 12 hours of daylight and twelve hours of night.

Brain Pop Video Solstices and Equinoxes http://www.brainpop.com/science/earthsystem/solsticeandequinox/ As you watch the video, please take notes.

Group Reflection In your own words, explain the “Reasons for the Seasons”.