The Reason for the Seasons

Rotation Earth spins on axis (imaginary line through center of Earth from North to South Pole) Rotation is counter clockwise as seen from North Pole Earth spins on its axis like a top

Rotation - Axis Earth tilt = 23.5° Polaris Rotation - Axis Earth tilt = 23.5° Axis currently pointed at Polaris (the North Star) NOTE: As the Earth orbits The Sun, the axis is always pointed at Polaris. This is why we have seasons. NOTE: Although our axis is currently pointed at Polaris, the axis “wobbles” (moves back and forth) and will eventually point to another star in the night sky (This is called “precession”.)

Rotation Earth rotates once every 24 hrs The rotation causes day & night Earth’s rotation causes celestial objects to appear to move from east to west in Northern Hemisphere The background image was taken from the space station astronauts showing the day/night terminus as Europe and Africa are passing from Day to Night. The Sun is on the left; therefore, the lighted side of the Earth is on the left.

Night and Day Map This is actually a map that tracks day and night across the globe. The * at about 50°W represents the location of The Sun’s most direct rays hitting the Earth. The darkest areas are night, the lightest area is day. The grayer areas is the twilight. Note: There are many versions of this map that can be bought to hang on your wall. As will be pointed out later in this PPT, you can also use this to determine the approximate season. In this case, it is either late spring or late summer due to the Sun’s proximity to the equator.

Just Checking… What does it mean when we say the Earth rotates? How would you define or explain the concept of Earth’s axis? Have students answer these in a think, pair share format. Answer: Earth spins on its axis once every 24 hrs causing night and day. The imaginary line passing through the Earth from North Pole to South Pole that always points to the north star, Polaris, upon which the Earth spins.

Revolution Movement of one object around another Earth revolves around Sun counterclockwise Earth’s orbit is almost circular rather than oval shaped but is still referred to as an elliptical orbit 365 ¼ days to revolve once around the Sun (~ a year) It is a GREAT time to discuss, “What is wrong with this model?” Note that the orbit path is “very” elliptical. This is an optical illusion. It happens because you are not looking straight down on the Earth and Sun. Since you are looking at it more from the edge, it makes the orbit appear more elongated. * You can show this illusion to students using an embroidery hoop or other small (about 6-8” in diameter) circular object and your data projector. Hold the loop in the light project so that it makes a shadow on the screen that is as close to circular as possible. Slowly tilt the top edge towards the screen and ask the student to tell you what is happening (although the hoop is circular, the shadow will change to elliptical.)

The imaginary line that runs through the north and south poles of the Earth is called the Equator Axis Longitude Latitude ANSWER: B, Axis

Decide weather each example is of A. Rotation or B. Revolution The Earth spinning on it’s axis. A baseball player running around the infield. Takes 1 year A top spinning Causes day and night. Takes 24 hours The moon circling the Earth B. Revolution B. Revolution A. Rotation Note, as you click, the question will pop up and the next click will bring up the answer. The next click will remove the answer and bring up the next question. A. Rotation A. Rotation B. Revolution

Location and Distance in Orbit Aphelion- the time when the distance between the Earth and the Sun is the farthest/greatest Perihelion- the time when the distance between the Earth and Sun is the least/smallest For distance measure we use the average of the aphelion and perihelion NOTE: This image is more of a top down view of our orbit around the Sun, but the orbit has been exaggerated to show the difference between “nearer to” and “farther from” the sun. NOTE: A term for PreAP students to hear is “Eccentricity” which is a measure of deviation from circular. A measurement of 0 would mean a perfect circle. Earth’s eccentricity is 0.0167, very near circular.

Two Primary Reasons for the Seasons (on Earth) angle of Sun in the sky at a given time and latitude number of hours of daylight at a given time and latitude. Handout available at http://www.uwm.edu/~kahl/CoVis/Seasons/

Seasons

Seasons Northern hemisphere tilts towards Sun = warmer (Summer) Remind students of the activity with the box and how much light was shining in a certain amount of area (light is less spread out in the summer months giving more direct energy to the Northern Hemisphere (warmer weather despite the Earth and Sun distance being the greatest during the warmer months of the year).

Seasons Northern hemisphere tilts away from Sun = cooler (winter) Remind students of the activity with the box and how much light was shining in a certain amount of area (light is more spread out in the winter months giving less direct energy to the Northern Hemisphere (cooler weather despite the Earth and Sun distance being the smallest during the colder months of the year).

Seasons In the Spring and the Fall, the tilt of the Earth is such that the spread of sunlight shines on the Earth approximately equally Remind students of the activity with the box and how much light was shining in a certain amount of area Ask the students: Why then if the amount of light (not spread out over a larger area or more condensed in one area) is approximately equal, why are the fall and spring months not the exact same temperatures (spring is usually moving from cooler to warmer temperatures and the fall is moving from warmer to cooler temperatures)?

Seasons Southern hemisphere seasons opposite Northern hemisphere seasons If its Winter here, then Australia is having _?_. Summer….that’s right, Australians go swimming during Christmas and snow skiing on the 4th of July…Yes, Australia HAS a 4th of July, they just don’t celebrate it. Please note “ELLIPTICAL ILLUSION” of Earth’s orbit again.

Solstices Occur twice yearly when the Sun is directly over “Tropics” (23.5°N and 23.5°S Latitude) winter solstice = shortest day of the year for Northern Hemisphere (Sun directly over Tropic of Capricorn-23.5°S-at noon) Summer Solstice is the longest day of the year in the Northern Hemisphere (Sun directly over Tropic of Cancer-23.5°N-at noon) This is for the U.S. and the Northern Hemisphere. * Ask students: Do you notice anything about the location of the noon Sun in relation to the tilt of Earth’s axis? The tilt of the axis and the location of the tropics are the same degree. Does not change. Please note “ELLIPTICAL ILLUSION” of Earth’s orbit again.

Equinox equal night & day Occurs twice each year Half-way between solstices noon sun directly over equator Sun will be at its highest point in our sky at noon but never directly overhead in Texas… why do you think this is true? Godley is ~32.45°N of the equator. Therefore, we will NEVER see the Sun DIRECTLY overhead. Be sure to emphasize that it is the sun’s energy that strikes perpendicular to the Earth’s surface where the most energy is absorbed. Note that the top and bottom “Sun rays” are hitting the Earth at an angle to the surface. This causes some of the energy to “bounce” off the atmosphere back into space rather than being absorbed at the surface.

Your answer should refer to Godley, TX. This occurs because the Earth’s axis is tilted as it moves around the sun. This occurs when the Northern hemisphere is tilted towards the sun. This is when the length of day and night are equal. The shortest and longest days of the year when the noon sun is directly overhead at 23.5 degrees. This occurs when the Northern hemisphere is tilted away from the sun. Winter Summer Solstice Equinox All of the above

Based on this Night and Day Map… What season do you think it is in Godley? What time do you think it is in Godley? Allow students to answer and explain their reasoning….allow for debate until class settles on an answer. Season: Since north pole is always in sunlight, it must be summer in Godley; Time: 5 PM (shadow moves from east to west) and since it is summer, sun sets about 8 in summer.