10 The spinning Earth: Spinning into darkness and light
Provided by Kenan Fellows Program.
Why do we have day and night? What makes it so we aren’t in darkness all the time? What causes us to have a sunset and sunrise? How can it be midnight in Beijing, China when it is noon in North Carolina?
To find out the answers to these questions students will manipulate their model Earths from the lesson What’s Up, Earth? Specifically, they will study the effects of the fact that the Earth spins about its axis from west to east, completing a rotation once a day. They will understand that this causes celestial objects, as viewed by observers on Earth, to appear to be moving around Earth from east to west. When applied to the Sun, this explains not only the apparent motion of the Sun in the sky, but also the daily cycle of light and darkness — day and night.
After this activity, students should be able to:
- Describe and display the spinning or rotating movement of the Earth on its vertical axis
- Describe how day and night are a result of Earth’s spinning movement on its axis and a person’s position or location on Earth
- Understand that, due to Earth being a sphere, one-half of the Earth is always in darkness and one-half is always in light
- Explain where sunset, sunrise, noon, and midnight are occurring on the globe at a given time
- Demonstrate how light traveling in a straight line or path from the Sun causes day and night on Earth
- Explain how the daily pattern of sunrise, noon, sunset, and night at a particular location on Earth corresponds to the appropriate parts of the 24-hour rotation cycle
- Explain how stars appear to move along arced paths in the sky, while their relative positions are unchanged so that the patterns we see are constant
The teacher will need the following:
- Modeling clay
- 4 golf tees
- Lamp or light source in the middle of the room with 250-watt bulb
- Time of your city’s sunrise
- Optional: cubes and cones
Each pair of students will need the following:
- Styrofoam Earth model from the lesson What’s Up, Earth?
- 4 golf tees
Teacher background information
Much of the background for this lesson has already been laid out in previous ones. At any given time, exactly one-half of Earth’s surface is illuminated by the Sun, simply because the other half of the sphere is inside the Earth’s shadow. People on the dark half cannot see the Sun — they would need to look down and Earth lies in the way. Thus, sunlight cannot reach them. As Earth rotates about its axis, a fixed point on Earth is swept into and out of light.
In this lesson, students will hold their Styrofoam models of Earth so that the axis is vertical, while the direction from Earth to Sun is horizontal. They are thus perpendicular to each other. (This is not quite accurate, but it is a reasonable approximation and makes the relevant discussion for this activity a bit simpler.) This means that, other than the poles, each point on Earth is in light precisely one-half of the time. Given our previous experience, it should come as no surprise that our world is dark about half of the time — when we are in the Earth’s shadow. That is what we call night. The other half of the time, we are out of the shadow, which means we can see the Sun (remember that being in the shadow is the same as not having a line of sight to the light source). That the entire world then seems filled with light is because we are surrounded by objects that reflect or scatter the sunlight. Even the atmosphere, because of impurities like dust, ice crystals, or tiny drops of water, scatters enough light to appear bright blue rather than dark.
These are the essential points. To complete the discussion, students should make as many connections as possible between the model they manipulate and the experiences of people living on Earth.
If we follow a fixed point on Earth as it goes through a full rotation, we can reproduce the experience of a person living at that point through a full 24-hour period including a day and a night. Let’s begin with the moment in the rotation when our observer is just entering the light. This is the transition from night to day — morning. The Sun is just becoming visible to our observer. On closer inspection, we notice that when the Sun first becomes visible, the line of sight from the observer to the Sun just grazes the Earth. The observer emerging from the shadow will see the Sun emerging from being obscured by the Earth and visible just above his or her eastern horizon.
As the Earth rotates, the fixed point containing our observer moves deeper into light. From the point of view of the observer, the Sun is moving higher in the sky, reaching its highest point when the observer is along the line of longitude closest to the Sun. At one point on this line, at the equator, the Sun is directly overhead, while north of the equator it appears to the south and south of the equator to the north. The farther from the equator our observer is, the lower the Sun will appear at midday.
As the Earth continues to spin, our observer will be carried toward the eastern edge of the illuminated half of Earth, and the Sun will approach the horizon in the west, finally disappearing when the observer crosses the line into the dark half.
Following through a rotation we see that the observer spends precisely half of the time (twelve hours out of each day) in the dark and the other half in the light, except at the poles where a perennial twilight occurs, with the Sun on the horizon always. Of course this is not the true state of things on Earth; the tilt of Earth’s axis will be discussed later. But it is the state of things in our current model.
In terms of our study of the apparent motion of stars in various parts of the sky, the Sun is just one such star (object outside Earth) which lies above the Earth’s equator. Once more, this is not precisely true, we will correct that later.
This activity will synthesize understandings from previous lessons. The concepts of sunrise, sunset, midday, and midnight and their relation to the location of Earth, Sun, and observer were introduced in the lesson As the Plate Tilts. Here, those concepts will be combined with the effects of Earth’s shape to understand the apparent motion of the Sun. Combined with our knowledge of the motion of light and of shadows, this will also explain the progression of light and darkness on Earth. Because this activity brings together the insights collected thus far, it is a natural place to stop, assess students’ comprehension, and help them to work through surviving misconceptions and confusions.
- Place the lamp or light source in the middle of the room or some location where everyone has a direct view of its light.
- Make whatever arrangements you have settled on to darken the classroom.
- Have the Styrofoam models ready from the last activity.
- Place your globe for easy access.
- Introduce the activity with the following challenge.
A second grader came to one of my third graders last year and asked, “On Saturday at noon, right around lunchtime, I was watching the news and they said they were ‘live’ from Beijing, China and it was midnight there. Then the TV news went to a story in Perth, Australia and in Australia it was midnight too. Now, how can it be the middle of the day with the Sun overhead here in North Carolina, United States and it is midnight with the Sun nowhere in the sky in China and Australia? I think the TV news was trying to trick us. Do you?” With your table mates or partner, draw and write what you know to answer this question. Describe what the second grader needs to know to figure out why it is midnight in China and Australia where people are sleeping, while here in North Carolina it is noon and people are eating lunch.
Allow the class to brainstorm and record their ideas in their science notebooks for 5–7 minutes and then list their responses on the board to be referred to during the lesson.
- Pass out the Styrofoam Earth models to each pair of students and instruct them to use a marker to draw China in the Northern Hemisphere above the equator and Australia directly below China and the equator in the Southern Hemisphere. Place a golf tee on each country. Use your large globe and board to demonstrate where students should place China and Australia on their models. Their drawings don’t have to be perfect but should be in the correct hemisphere. On the opposite half of the world, have the students draw the United States in the Northern Hemisphere and Brazil directly below in the Southern Hemisphere. Have them place a golf tee in each country. Explain to students that if they were to dig from China through the center of Earth they would come up in the United States and if they dig from Australia through Earth they would come up in Brazil.
- Turn off the classroom lights and turn on the light source in the middle of the room. Explain to students, “In the last couple of lessons you’ve learned about the shape of Earth, the way light travels in space, and how we see objects in space. I want you now to work with your partner and answer the following question. ‘Can everyone on Earth be in sunlight at the same time?’ Experiment with your Earth models to discover an answer by imagining the light in the center of the room is the Sun. Keep in mind what the shape of the Earth has to do with who on Earth is in light or darkness. After you are confident about an answer, write and draw an explanation in your science notebook.”
Once everyone is finished, review the responses with the class that prove or disprove the responses from Step 1. Add any new responses that are helpful and correct. To develop further understandings ask, “How would your answer be different if the Earth was a cube?” If you have several cubes in the class hand them out and let the students experiment. Ask, “How would your answer be different if the Earth was a cone?” Again, if you have geometric solid cones in the class, pass them out for experimentation to help answer the question.
- Ask students to imagine it’s the middle of the day in North Carolina. When their golf tee people look up out into space what do they see? They see the Sun overhead. Have them move their Earth models to the midday position for North Carolina. Ask how they know this position is midday or noon? See if anyone can make the connection to the midday mark on the Sun’s Path Map in the room or the midday shadow tracing of his or her body (from previous lessons in the unit). Next have them imagine they call someone on the phone in China or Australia. Ask, “What time of day is it for the person on your Earth model in Beijing, China’s capital city, if it is still midday or noon in North Carolina? How do you know? Now pretend you look up from China or Australia. What do you see now?” Take some time to allow students to demonstrate and discuss their answers. After discussion and demonstrating their answers with their models, have each student draw the North Carolina midday or noontime position and the Beijing midnight position of the Earth and Sun in their science notebooks. Additionally, given what they know about the way light travels and the shape of the Earth, ask them to explain why Beijing is at midnight and North Carolina is at noon.
- Next instruct the class, “Imagine that twelve hours have passed and people in China and Australia will be eating lunch at noon. What will your golf tee person back in North Carolina be doing? Move your Earth model and explain to your partner what people in North Carolina will be doing and prove why you believe this to be true. Also, work with your partner to come up with a description of how the Earth moved so that now China and Australia are in light and the United States and Brazil are in darkness. Write and draw your explanation in your science notebooks.”
Once most everyone is finished, allow students to read, discuss, and demonstrate their explanations.
- Ask the class to look down on their Earth’s North Pole and imagine their Earth is spinning counterclockwise or opposite the direction of the hands of a clock. Explain this is the direction the Earth is spinning in space and have them spin their Earths in this direction. Tell students, “Start with North Carolina at midnight in total darkness. Spin your Earth very slowly and figure out where your North Carolina golf tee people are first touched by the Sun’s light. What is the time of day called when we first see the Sun? (Sunrise) Now, work with your partner and see if you can both come to an agreement about where sunrise is for your North Carolina golf tee person.”
Once everyone finds sunrise explain, “Today the sunrise time for our town was ____ a.m. If you place your Brazil and United States people at sunrise, what position are your people in China and Australia at? Are they moving into the Sun’s light or into the Earth’s shadow or darkness? What is that time of day called? Now slowly move your United States and Brazil golf tee people to the sunset position where the golf tee people are getting hit by the last bits of sunlight for the day and moving into nighttime or darkness.”
- Question the class, “How are the positions of your Earth model at sunset and sunrise similar to your experiments with the plate’s sunset and sunrise positions?” If you have the plates on hand nearby pass them out and allow students to compare and demonstrate the similarity between the sunset and sunrise positions with the plates and with their model Earths. Next, make the connection to the Sun’s position near the horizons at sunrise and sunset on the Sun’s Path map in the class.
- Ask students to draw a picture of Earth’s position when the United States and Brazil are at sunset and draw another picture where the United States and Brazil are at sunrise. Ask them to describe what the differences are between the two times and explain what causes these differences.
- Instruct the class to place their United States and Brazil golf tee people in midnight and ask, “Instead of looking up and seeing the star we call our Sun, what does the person see out in space?” Their responses should include items such as stars, planets, airplanes, another galaxy of stars, and constellations. Ask, “Has anyone looked at the stars throughout the night and noticed something about the way they move throughout the night?” Hopefully, someone responds that the stars move across the sky like the Sun and Moon and if not make the connection for them. Explain that just as our closest star, the Sun, rises and sets each day, so too do all of the stars at night. Because we are spinning counterclockwise, the stars appear to moving clockwise or east to west during the night just like the Sun appears to be during the day. Ask, “If the Sun, our closest star, has a path across our sky that is shaped like an arc — or half circle — what path do you think the stars have across the night sky? Explain your answer.”
- You can have students draw the appropriate constellations that will be up in the night sky near their birthdays. A helpful resource is at The Night Sky at Different Times of Year from the Department of Astronomy at the University of Massachusetts.
- Ask, “Everyone place the United States in darkness or midnight. Have your United State golf tee person look out into space. What do they see? Are there any playground toy or amusement park rides you have been on or any sport or activity you have played where you have spun around and it felt like Earth was moving and not you?” Answers could include merry-go-rounds, carousels, and vortex rides. Explain how we on Earth are spinning as well and believe the stars are moving — just like when the students were on spinning ride and their friends or family waiting in the park even though they aren’t. The same is true for the Earth and the stars.
- Ask, “Are there other stars in the sky besides the Sun during the day?”
The pre-activity assessment consists of students’ responses to the opening challenge.
Students should be assessed on their ability to represent and locate the United States, China, Australia, and Brazil on their Earth models. Through this lesson, they should develop an understanding of the term counterclockwise. Students also should be assessed based on the following tasks. These questions and tasks are included the attached worksheet.
- Assessment activity
- Activity assessment
- Open as PDF (12 KB, 2 pages; also available as Microsoft Word document)
- Can everyone on Earth be in sunlight at the same time? What does the shape of Earth have to do with your answer? Imagine the light in the middle of the room is the Sun and use your model Earth to figure out an answer to the question. After you are confident about an answer write and draw an explanation in your science notebook or on the worksheet.
- Position your model Earth so that it is midnight in China and Australia and noon in the United States and Brazil. Draw the position of Earth and the Sun in your science notebook or on the worksheet. Given what you know about the way light travels and the shape of Earth, explain why and how this happens. Next, imagine that twelve hours have passed and people in China and Australia will be eating lunch at noon and people in the United States and Brazil will be asleep at midnight. How is the Earth moving now that China and Australia are in light and the United States and Brazil are in darkness? Write and draw your explanation.
For the post-activity assessment students should once again draw a model of the Sun’s path either by hand or using a computer graphics software (i.e. KidPix) to illustrate and answer the following objectives and questions. The student worksheet is also provided below. In this version, most of the questions are the same. However, in the final questions student will decide if Earth revolves around the Sun or if the Sun revolves around Earth. The model should reflect the students’ new learning.
- Post-activity assessment
- Post-activity assessment
- Open as PDF (13 KB, 2 pages; also available as Microsoft Word document)
- Draw the southern horizon. Label east, south, west and at least four landmarks.
- Illustrate the Sun’s path as it changes position each hour.
- Describe the shape of the Sun’s path. What is the shape similar to?
- What do you notice about the pattern of the Sun’s path each hour?
- What happens to the Sun after it sets? Why can’t we see it?
- Is Earth revolving around the Sun or is the Sun revolving around Earth? How does your data support your answer?
Track the Sun’s movement by tracking a stick’s shadow each hour. Much like the Sun tracking groups, each hour have a group track the stick’s size, shape, and direction. Check out the This is a Stickup! lesson in which students record the smooth-arced motion of the Sun across the southern horizon.
- Warn students never to look at the light source directly — this could temporarily blind or daze them.
- Use caution when setting up the wiring for the central light source. Ensure that students do not trip over the wires in the darkened classroom.
- the imaginary line through the Earth’s center about which Earth rotates once a day
The Earth’s axis “points” in a fixed direction in space.
- east, west, north, south
- the four cardinal directions on the compass
- a rotation of an object about its axis
Earth spins about its axis from west to east, completing a rotation in about twenty four hours or one day.
- the path taken by an object as it moves in space
Earth moves in an orbit around the Sun that is shaped almost exactly like a circle. It takes Earth one year to complete an orbit and returns to its starting point.
- the imaginary line along which sky and earth appear to meet as seen by an observer on Earth
At sea or in flat, featureless terrain, this is a horizontal circle. The line is imaginary in that there are no points at which sky and Earth meet, but it is very real as a collection of directions in space.
- the time of day when the Sun first appears above the horizon in the east
At any instant the Sun is rising along one line of longitude on Earth.
- time of day when the Sun disappears below the horizon in the west
- noon or midday
- the time when p.m. begins and the Sun is near its highest altitude or height in the sky
- the time in the night when a.m. begins
- movement in the same direction as the hands of a clock
- movement in the opposite direction as the hands of a clock
- North Carolina Essential Standards
- Science (2010)
- 3.E.1 Recognize the major components and patterns observed in the earth/moon/sun system. 3.E.1.1 Recognize that the earth is part of a system called the solar system that includes the sun (a star), planets, and many moons and the earth is the third planet...
- 4.E.1 Explain the causes of day and night and phases of the moon. 4.E.1.1 Explain the cause of day and night based on the rotation of Earth on its axis. 4.E.1.2 Explain the monthly changes in the appearance of the moon, based on the moon’s orbit around the...
- Science (2010)
North Carolina curriculum alignment
- Goal 3: Geometry - The learner will recognize and use basic geometric properties of two- and three-dimensional figures.
- Objective 3.01: Use appropriate vocabulary to compare, describe, and classify two- and three-dimensional figures.
- Goal 3: The learner will make observations and use appropriate technology to build an understanding of the earth/moon/sun system.
- Objective 3.01: Observe that light travels in a straight line until it strikes an object and is reflected and/or absorbed.
- Objective 3.02: Observe that objects in the sky have patterns of movement including:
- Objective 3.03: Using shadows, follow and record the apparent movement of the sun in the sky during the day.
- Objective 3.06: Observe that patterns of stars in the sky stay the same, although they appear to move across the sky nightly.