LESSON 3 - THE SEASONS, THE TILT OF THE EARTH, DAY AND NIGHT

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Objectives
Materials
Background Information
Key Questions
Vocabulary
Procedure
Evaluation

Objectives

• Students will explain how the tilt of the earth causes the seasons.
• Students will explain how the curvature of the earth affects the amount of sunlight an area receives.
• Students will explain how the Earth's rotation causes day and night.

Materials

• graph paper
• cardboard
• masking tape
• flashlight
• colored markers
• two sheets of black construction paper
• two pieces of cardboard
• books to prop up cardboard
• masking tape
• two thermometers
• Slide projector or bright flashlight
• large, single colored ball (such as a kickball)
• books to set light source on to achieve proper height
• globe of the earth
• sign to mark North
• a student to play the role of the Sun
• a student to play the role of the earth

Background Information

The Earth rotates once every 24 hours about its axis, and it revolves around the Sun once every 365 days. The four seasons occur because the Earth is tilted.

Image of Earth's tilt. NASA

The Earth has an axial tilt of 23.5°. The axis is tilted in the same direction throughout a year; however, as the Earth travels in its orbit around the Sun, the hemisphere tilted away from the Sun will gradually come to be tilted towards the Sun, and vice versa. This effect is the main cause of the seasons. Whichever hemisphere is currently tilted toward the Sun experiences more hours of sunlight each day, and the sunlight at midday also strikes the ground at an angle nearer the vertical and thus delivers more heat.

Image of Earth's tilt in December and June NASA

Image of Earth's tilt throughout the year UWM

It is important to understand that even if the Earth was not tilted, there would still be temperature differences from one location to another because the Earth is curved. Locations close to the equator would be warmer than locations close to the poles. Heat and light from the Sun would still hit the areas near the poles at a greater angle than near the Equator. The angle at which the light hits the Earth spreads out the heat energy over a larger surface area, which decreases the intensity of the heat brought to the Earth.

Daytime and nighttime is a direct result of the Earth's rotation upon its axis. The Earth rotates once every 24 hours. As the Earth rotates, only one side of the Earth faces the Sun. This side will be experiencing daylight. The opposite side of the Earth, the side away from the sun, will be experiencing night.

Key Questions

1. How does the curvature of the earth affect the amount of sunlight a specific area will receive?
2. How does the tilt of the earth create seasons?
3. How does the rotation of the Earth create night and day?

Vocabulary

• axis
• rotation
• revolution
• orbit
• curvature
• equinox
• Polaris
• Northern Hemisphere
• Southern Hemisphere
• equator

Materials:

• graph paper
• cardboard
• masking tape
• flashlight
• colored markers

Procedure

1. Tape the graph paper to the piece of cardboard. Hold the cardboard straight up and down and shine the flashlight at an angle onto the graph paper from a distance of about two feet. Make sure to keep the flashlight parallel to the floor. It might be easier to place the flashlight on a stack of books to keep it steady.

Trace around the area illuminated by the flashlight.

Next, tilt the cardboard towards and away from the flashlight. Is more or less of the graph paper covered in light? Tilt the cardboard at extreme angles, like 45° or 60°. Trace around each of the new areas that are illuminated using different colored markers for each.

Count the squares that are completely covered or partially covered by the circle of light. Do this for each color. You will notice that more surface area is covered by the light when the cardboard is tilted at larger angles.

Here's what happened. It doesn't matter at what angle the cardboard is tilted, the amount of light produced by the flashlight does not change. When the cardboard is tilted, the flashlight will illuminate a larger area on the graph paper than if it is not tilted, but the light energy is spread over a larger area.

Materials:

• two sheets of black construction paper
• two pieces of cardboard
• books to prop up cardboard
• masking tape
• two thermometers

Procedure:

2. This activity continues the concept that light striking a tilted surface will be less intense than a direct strike. Light generates heat. This activity measures how quickly two sheets of paper will warm up if one is tilted towards a light source and the other one is not.

Cut a one inch slit in the center of each sheet of construction paper. Tape the construction paper to the cardboard. Do this for both pieces of construction paper. Insert a thermometer into each slit making sure that the bulbs of the thermometers are between the construction paper and cardboard. You want to be able to read the thermometers without removing them. Secure the thermometers in place with tape. Place your assemblies out of direct sunlight until both thermometers level off and the temperatures read the same.

Place your assemblies in an area that will receive sunlight for approximately 30 minutes. You may wish to conduct this experiment outside. It is best to conduct this experiment in the morning or late afternoon if possible. Tilt one of the assemblies so that the thermometer directly faces the Sun. Place the other flat on the ground making sure it is not in the shadow of the other assembly. Experiment with the angle of the second assembly until you find an angle of indirect sunlight especially if the Sun is nearly or directly overhead.

Record the temperatures on each thermometer every two minutes until the temperatures stop climbing. Record their final temperatures.

Here's what happened. When the cardboard is tilted toward the sun, the sunlight strikes more directly on the paper, and more heat energy is concentrated in a particular spot. This is what happens when the sun's rays strike the surface of the earth near the equator. When the cardboard is tilted away from the sun (as the Poles are on Earth), it receives less intense light and does not heat the cardboard as quickly.

Materials:

• Slide projector or bright flashlight
• large, single colored ball (such as a kickball)
• books to set light source on to achieve proper height

Procedure:

3. Because the earth is a globe, we would expect that there should be areas that receive more direct sunlight than others and that these same areas will be warmer. This experiment will demonstrate that the equator receives more direct sunlight than do the Poles.

Set the ball on a table and secure it so that it does not roll. Aim the flashlight at the ball so that it is shining directly on the center, but make sure the beam illuminates the entire ball.

Where is the flashlight shining the brightest?. Students should be able to recognize that the light is shining brightest at the center of the ball and be able to relate this to how the sun strikes the equator.

Here's what happened. Since the ball is a sphere, the highest concentration of light rays will strike the ball in the middle of the ball, or along the equator of the ball, because it is closer to the light source.

Materials:

• globe of the earth that is tilted on its axis
• flashlight
• sign to mark North
• masking tape
• a student to play the role of the Sun
• a student to play the role of the earth

Procedure:

4. Now we are ready to tie everything together, the tilt of the earth, how light strikes a curved surface at different intensities, the rotation of the earth, and the revolution of the earth around the sun. This activity will explain how all of these components work together to create the four seasons we experience on Earth. This activity will also be used to demonstrate the concepts of night and day.

Place the "Sun" student in the center of the room with a flashlight. Have the "Earth" student stand about six feet away with the globe. Place the "North" sign in a corner of the room near the ceiling. Explain to your students that this represents Polaris, the North star, which lies nearly in a direct line with the axis of the Earth's rotation above the North Pole.

Have the "Earth" student tilt the North Pole towards the "North" sign. It's important that the student understand that he or she must keep the globe pointing in this direction.

Have the "Earth" student walk around the Sun keeping the North Pole pointed towards the "North" sign. The "Sun" student will pivot to keep the light shining on the Earth. When the North Pole is pointing away from the Sun, the Northern Hemisphere is experiencing winter. When it is pointing towards the Sun, it will be summer in the Northern Hemisphere.

Image of Earth's tilt in December and June NASA

Have the "Earth" student continue to walk around the Sun. Ask your students at varying points which season a particular location is experiencing. For example, When it is Summer in the Northern Hemisphere, what would it be in the Southern?

Once the students understand that the tilt of the Earth plays a role in creating the seasons, explain and demonstrate what would happen if there were no tilt to the Earth. If the Earth had no tilt, there would be no seasons.

Also while performing this activity, take the opportunity to explain to your students that because the Earth rotates, there will be times when one side of the Earth does not receive any light from the sun. There are alternate periods of darkness and light which are night and day.

Here's what happened. As the Earth travels in its orbit around the Sun, different areas of the Earth are tilted closer to the Sun than others. This, along with the curvature of the Earth, affects the amount of heat and light these areas receive. When the North Pole is tilted away from the sun, the Northern Hemisphere will be experiencing Winter and the Southern Hemisphere will be experiencing Summer.

Six months later, the Earth has moved in its orbit to the opposite side of the sun. The North Pole is now tilted toward the sun. It is Summer in the Northern Hemisphere, Winter in the Southern.

We live in the United States which is in the Northern Hemisphere where Winter begins around December 21. The North Pole is tilted the farthest from the sun. At the same time, the Southern Hemisphere is entering into Summer.

Spring begins for us around March 21. Earth has moved along its orbit and the sun is directly over the equator. This is the Spring Equinox. On this day, day and night are the same length everywhere on Earth. The sun rises directly in the east and sets directly in the west.

The days soon become longer because as the Earth continues in its orbit, the Northern Hemisphere becomes more and more tilted towards the sun. At this time, more direct sunlight will reach the Earth for longer time periods.

Summer begins in the Northern Hemisphere around June 21. This is the Summer Solstice. The North Pole is tilted toward the sun more so than any time of the year. Summer is the warmest of the seasons. At the same time, the Southern Hemisphere is entering into Winter.

Fall begins in the Northern Hemisphere around September 21. The first day of Fall is called the Fall Equinox. On the Fall Equinox, the Earth has traveled along its orbit and the sun is once again opposite the equator. On the Fall Equinox, day and night are the same length everywhere on Earth.

As the Earth continues in its orbit, the days become shorter and the nights are longer due to the North Pole being tilted farther and farther away from the sun.

Locations near the equator experience little temperature change throughout the year. This is because the equator is not as influenced by the tilt of the Earth. The seasons here are usually rainy or dry. Periods of daylight and darkness are nearly the same length each day.

The Poles are always cold. When one of the poles is tilted away from the sun, that area is experiencing winter. During this time, the sun does not appear over the horizon. When the Pole is tilted towards the sun, it is experiencing Summer. During this time, the Sun does not sink beneath the horizon and the day and nights are both light.

Evaluation

• The student will be able to explain how the Earth's rotation around the sun and the tilt of the Earth results in different area of the Earth receiving varying degrees of light and heat.
• The student will be able to explain the cause for Seasons.
• The student will be able to explain the causes of night and day.

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