11 The spinning Earth: Your world is tilted
Provided by Kenan Fellows Program.
Having established how Earth’s spinning causes the alternation of day and night, this lesson will introduce the fact that Earth’s axis is not perpendicular to the Sun. This means the Sun does not lay always directly above the equator as students may have found in the lesson Spinning into Darkness and Light. In fact, it is sometimes to the north and sometimes to the south of the equator. The alternation of these configurations creates the seasons on Earth. Students will investigate the effects the tilt in the axis has on the length of days and nights at various latitudes in the two hemispheres.
After this activity, students should be able to:
- Use modeling to demonstrate and describe how Earth is tilted
- Explore the effects on Earth if it was tilted differently
- Describe the effects of the tilt on each hemisphere, the poles, and the equator
- Realize that when the North Pole is pointed toward the Sun, the Northern Hemisphere receives more sunlight
- Realize that when the South Pole is pointing toward the Sun, the Southern Hemisphere receives more sunlight
- Realize that Earth’s tilt causes only slight changes in the amounts of sunlight on the area near the equator but the North and South Poles experience extreme changes in amounts of sunlight
The teacher will need:
- 1 plate with straws from the lesson As the Plate Tilts
- Lamp or light source in the middle of the room with 250-watt bulb
- Cut-out of the North Star
Each pair of students needs:
- Styrofoam Earth model and 4 golf tee “people”
Teacher background information
In previous lessons students learned how Earth’s daily rotation governs the cycle of day and night — light and darkness. While modeling this, students probably kept the axis of the Earth model vertical so that the North Pole pointed at the ceiling. With the “Sun” at about the same height above the floor as students held their Earth models, this means the Earth’s axis and the line from Earth to Sun were perpendicular. Note that this is a real fact, meaningful in space. Light from the Sun hitting a round Earth will at any moment illuminate precisely one-half of the Earth’s surface while the other half is dark because it lies in the Earth’s own shadow.
In our model configuration, the line separating the light half from the dark half passes through both the North and the South Poles. As Earth spins, the poles do not move so this remains true, the result being that at the poles we observe at all times a “twilight” with the Sun sitting on the horizon drifting to the right (at the North Pole) or left (at the South Pole). As Earth spins, any other location on its surface spends precisely half of the time in light and half in dark. In this model, day and night are twelve hours long everywhere, and perpetual twilight reigns at the poles. This is close, but it is not precisely what happens.
In fact, day and night are not always of equal length. Rather, their lengths change with the seasons. In summer, days are longer and nights shorter; the opposite is true in winter. The magnitude of seasonal changes depends on latitude. Near the equator, days and nights are always of equal length. Near the poles darkness can be entirely absent in summer, and sunlight can be scarce in winter.
Seasons in the Southern Hemisphere are the reverse of those in the Northern Hemisphere. The reason for this variation is the fact that the Earth’s axis is not actually perpendicular to the line from Earth to Sun. Rather, it tilts away from perpendicular by 23.5 degrees. In this lesson, we will investigate the effects of this tilt, and see how it can explain these variations. To clarify this, we will first imagine a world in which the Earth’s axis actually points right at the Sun (the equivalent of a 90 degree tilt), so that the Sun is directly overhead for someone standing at the North Pole. As the Earth spins, the Northern Hemisphere would always be illuminated while the Southern Hemisphere would remain in darkness all the time.
Clearly this is not true on our Earth, though it does occur some of the time on Uranus. But it does suggest a way to realize the situation we observe in the summer. Tilting the axis toward the Sun somewhat, we see that the illuminated half of the Earth’s surface includes more of the Northern Hemisphere than of the Southern Hemisphere. As the Earth spins about this tilted axis, days in the Northern hemisphere will be longer than nights, while the opposite will be true in the Southern Hemisphere. At the poles, the effect is extreme. As soon as we tilt the Earth, the North Pole moves into the illuminated part of Earth, and the South Pole into the dark part. Since the poles do not move as Earth spins, this means, on the tilted Earth, it is never dark at the North Pole, nor ever light at the South Pole. Near the poles, days and nights have very different lengths.
The equator, however, because it is a “great circle,” will always be divided in half by the line between light and dark. So as the Earth spins, every location on the equator spends half of the time in the light and half in the dark, even with the axis tilted.
Looking closely, we see that when we tilt the globe in this way, there is a region around the North Pole that never enters the dark half of Earth’s surface. In this region, we find that the Sun never sets. The circle bounding this region is the Arctic Circle. Similarly, there is a region around the South Pole where the Sun never rises, surrounded by the Antarctic Circle. Note that in our class model, we will use an exaggerated tilt of 45 degrees to make it easier to see the effects. Because of this, these regions will appear larger in the Earth models than they actually are.
Notice that on the tilted globe, the Sun will not be overhead at noon at the equator. To a person at the equator, the Sun will appear at noon to be north of the zenith. The Sun will appear directly overhead to a person somewhat North of the equator, at a point where the tilt of the axis is precisely balanced by the tilting of the local vertical by the Earth’s roundness. The line of latitude along which this occurs is the Tropic of Cancer.
If we reverse the tilt so that the North Pole faces away from the Sun and the South Pole towards the Sun, we will find the opposite effect, of course. Days will be shorter in the Northern Hemisphere and longer in the Southern Hemisphere, as is the situation during winter. The Arctic Circle will be in perpetual darkness and the Antarctic Circle in perpetual light. The Sun will be directly overhead at the Tropic of Capricorn, south of the equator.
Is it possible for the axis to be tilted without creating these differences between North and South? The answer is “yes.” If the tilt of the axis is neither toward the Sun nor away from it, but sideways, (imagine holding Earth while facing the Sun, then tilt the axis to your right or left) then we see that the line separating light from darkness still passes through both poles and, in fact, divides every line of latitude in half, not just the equator. In this configuration, day and night are of equal length everywhere on Earth, and the poles are in perpetual twilight, as was the case with no tilt whatever. This represents the situation in spring and in fall.
By tilting the axis in various directions, keeping the magnitude of the tilt constant, we can continuously shift from summer through fall to winter to spring and back again.
We could thus explain the annual cycle of seasons by discovering that Earth’s axis is tilted in such a way that the direction of the tilt varies over the course of a year, so that in summer the North Pole is tilted towards the Sun, in winter tilted away from the Sun, and in the intermediate seasons we have the sideways tilt described above. Of course, this is not what happens. The Earth’s axis points in a fixed direction and does not change noticeably throughout the course of a year. But because Earth orbits the Sun, the line to the Sun does not always point in the same direction! The relation between the axis and the direction to the Sun thus changes over the course of a year.
The actual tilt of the axis, 23.5 degrees, is quite small. In this lesson we will exaggerate the tilt so that the effect will be more noticeable. At first, of course, we use a 90 degree tilt, but for the subsequent investigations use a tilt of around 45 degrees so that students can see the effects more clearly.
In the lesson Spinning into Darkness and Light, students explored how sunlight, Earth’s spin, and Earth’s round shape cause the cycle of day and night. Additionally, they learned that because Earth is round what is seen overhead in space depends on the location on Earth. So “up” for someone in Australia is different from “up” for someone in the United States and, therefore, what each sees in the day and night sky is different. These principles will be used in this lesson about Earth’s tilt to help students discover why and how sunlight falls differently on different locations on Earth.
- Place the lamp in the middle of the room or some location that everyone has a direct line of sight to its light.
- Have the Styrofoam Earth models ready from the previous lesson.
- Place your globe for easy access.
- Have a plate with straw attached ready for modeling the 23.5-degree tilt.
- Place a North Star cutout in the northern section of your room.
- Introduce the activity with the following challenge.
A fellow teacher came to me the other day and explained, “My students have been studying about two explorers, Mike Horn from South Africa and Borge Ousland from Norway. They traveled in winter to the North Pole for fifty days and every day was in twenty-four hours of complete darkness.”
Tell the class that they will be helping this teacher prepare a lesson about how it is possible to have entire days without sunlight. Allow the class to brainstorm with their table or a partner and list what they already know that will help them explain why the North Pole can have twenty-four hours or full days of complete sunlight or complete darkness. Have the class also add some things they don’t know but they will need to find out understand why places on Earth receive different amounts of light. As they discuss, students should record their ideas in their science notebooks for 5–7 minutes. As a class, list their responses on the board to be referred to during the activity.
- Return the Earth model to each pair of students and instruct them to use a marker to draw the equator. Explain that the equator is the imaginary line that is like a belt around the middle of Earth separating the top half or the Northern Hemisphere from the bottom half or the Southern Hemisphere. But, unlike a belt on a person that doesn’t separate the person perfectly in half, the equator does cut the Earth in half because from anywhere on the equator you are an equal distance away from both the North Pole and the South Pole. Ask, “In which hemisphere is each of the countries we already have on our Earth models (United States, Brazil, China, and Australia)? How can you prove it?” Have students label the Northern Hemisphere, Southern Hemisphere, North Pole, and South Pole on their Earth models with a marker.
- Turn off the lights and have the class imagine they are out in space. Remind them that, just like the last lesson, everything in the classroom that isn’t on their model Earth is in outer space. Explain that they are space giants. If they think how big they are compared to their Earth, they are huge!
Students should hold their Earth models so the North Pole is facing directly toward the ceiling. Have them spin their globe in this position modeling day and night. Ask them, “If you were standing on the North or South Pole and you looked out into space directly overhead would you see the Sun? Where would you have to look to see the Sun?” Have them place a golf tee person on each of the poles. They should realize that if the person looks directly overhead from either pole they will never see the Sun, as it will always be in view on the horizon.
Ask, “Where on Earth would the Sun be exactly overhead?” In our current model the answer is the “equator.” Describe and demonstrate how the path of the Sun overhead gets lower and closer to the horizon as you move toward the poles.
Explain that this isn’t really how the Earth is positioned because we know for a fact the explorers traveled fifty days through winter never seeing the Sun. Ask students, “How can this be? What has to happen to the Earth for the North Pole or the South Pole to have days of total darkness or days of total light?”
- Demonstrate the idea that the globe’s axis could be oriented differently. Show an exaggerated tilt of 90 degrees with the North Pole facing the Sun. Have everyone place their North Pole pointing directly at the Sun and spin their Earth model. Ask, “What would happen if this was the direction of Earth’s axis?” Help students notice that the entire Northern Hemisphere would be in perpetual daytime, while the entire Southern Hemisphere would be in perpetual night. The Sun would always be overhead at the North Pole.
- Have your students switch their models around and have the South Pole point directly at the Sun and listen to your students’ descriptions of what would happen if this were the case.
- Now ask them to try something intermediate between the two cases they have seen so far. Demonstrate a 45-degree tilt with the North Pole facing the “Sun.” Ask the class, “Something new is going to happen to your countries and to both hemispheres as you spin your Earth models now. Do both hemispheres receive the same amount of light all the time? How is the amount of light changing for each hemisphere and their poles? You can move your golf tee people around to different locations and compare the amount of sunlight falling on them during one spin cycle or day. Look carefully, and then draw and explain what you believe is happening in your science notebook. Additionally, experiment and discover what happens if you keep the tilt the same but point your North Pole in different directions not just towards the North Star. Draw how different amounts of sunlight fall on different locations as you point your North Poles toward different directions.”
Allow the class 10–15 minutes to explore, draw, and explain. Focus students to think about where and how much sunlight is hitting the Earth. Help them to notice that, without a tilt, the golf-tee people at the same longitude in the two hemispheres (in the United States and Brazil, for example) entered and exited the illuminated half of Earth at the same time. With a tilt, one can be in the light while the other is still (or already) in darkness!
- Ask students to find out where on their Earth models:
- the Sun is directly overhead (at the zenith)
- the Sun never sets
- the Sun never rises
The circles they find will be the Tropic of Cancer, the Arctic Circle, and the Antarctic Circles, respectively.
- Have students reverse the tilt so that the South Pole tilts toward the Sun at 45 degrees and repeat their observations. Ask, “Where are days longer now? Where are nights longer? Where does the Sun never set? Never rise? Where does it reach the zenith?”
- Challenge students to find a way to tilt the globe at the same angle to the vertical, in such a way that day and night will be of equal lengths everywhere on Earth. There are two ways to do this, either by tilting to the right or tilting to the left when facing the “Sun.” Tilting to the right corresponds to fall in the Northern Hemisphere. Tilting to the left corresponds to spring in the Northern hemisphere. When everyone is close to finished, have the students volunteer several explanations.
- Now it’s time to lead the class in understanding how and why the tilt causes changes in the amount of sunlight shining on different places on Earth. First, listen to your students’ explanations from their science notebooks and discuss while demonstrating with the globe and drawings on the board. Students need to understand that when the North Pole points toward the Sun longer amounts of sunlight heat the Northern Hemisphere and shorter amounts of sunlight shine on the Southern Hemisphere. Don’t yet explain the Earth’s orbit around the Sun and the progression of the seasons, only allow your students to tilt their North Pole in different directions while staying at the 45-degree tilt.
- Ask your students, if you haven’t already discussed it, “What is happening at the equator?” Help them realize that the various tilt directions make no difference whatsoever at the equator, one-half of which is always illuminated.
Ask, “What happens at the poles to the length of daylight hours and darkness?” Allow your class to experiment by moving their golf-tee people onto the North and South Poles. Ask them to draw and write explanations to how living on the equator or the poles affects the amount of sunlight. Tell them that these drawings and explanations will be used to help your fellow teacher teach a lesson to explain how it can be true that the North Pole has days of complete darkness called Polar Night.
Pre-activity assessment consists of your students’ responses to the opening challenge.
The students will answer several questions from the steps of the activities in their science notebooks. These questions are also included in attached worksheet.
- Activity assessment
- Post-activity assessment
- Open as PDF (16 KB, 4 pages; also available as Microsoft Word document)
- In which hemisphere is each of the countries we have on our Earth models (United States, Brazil, China, and Australia)? How can you prove it?
- If you were standing on the North or South Pole looking directly overhead, would you see the Sun? Where would you have to look to see the Sun? What would you see if you looked up from the equator at midday?
- Point the North Pole on your Earth model directly at the “Sun” and spin your model. What would happen if this was the direction of Earth’s axis?
- Demonstrate the idea that the Earth’s axis could be oriented differently. Show an exaggerated tilt of 90 degrees with the North Pole facing the Sun. Draw a picture of what this looks like and describe what would happen if this was the direction of Earth’s axis.
- As you spin your Earth model, do both hemispheres receive the same amount of light all the time? How is the amount of light changing for each hemisphere and their poles?
- Move your golf-tee people around to different locations and compare the amount of sunlight falling on them during one spin cycle or day. Look carefully, and then draw and explain what you believe is happening.
- Experiment and discover what happens if you keep the tilt the same but point your North Pole in different directions — not just towards the North Star. Draw how different amounts of sunlight fall on different locations as you point your North Pole toward different directions. What happens at the equator?
- How does Earth’s shape affect how much light shines near the equator? How do you think this affects the temperature?
- What happens at the poles to the length of daylight hours and darkness? Experiment by moving your golf-tee people onto the North and South Pole. Draw and write explanations about the difference between the sunlight and temperature at the equator and at the poles.
Have each pair of students test out teaching the lesson they have planned for your fellow teacher’s class. It’s best if one pair teaches it to another pair and they switch so everyone gets a turn.
Warn students never to look at the lamp or light source directly as it could cause eye damage.
- an imaginary line connecting the North Pole to the South Pole through the center of the Earth
- east, west, north, south
- the four cardinal directions on the compass
- turn on its axis
One complete spin for the Earth’s takes close to twenty-four hours or one day.
- as a noun: a slant; as a verb: to move an object and cause it to lean or incline
- Northern Hemisphere
- half of the Earth north of the Equator
- Southern Hemisphere
- half of the Earth south of the equator
- a great circle of the Earth that is everywhere equally distant from the two poles and divides the surface into the Northern Hemisphere and Southern Hemisphere
- Arctic and Antarctic Circles
- the lines of latitude 66.5° North and South near the North and South Poles
These are the southernmost and northernmost locations at which the Sun does not set for a full 24-hour day in the summer, and does not rise above the horizon for a full 24 hours in the winter.
- the point in the sky directly overhead
- Tropics of Cancer and Capricorn
- the lines of latitude 23.5° North and South
Anywhere between the two tropics, there is one day a year during which the Sun is directly overhead at noon. Farther away from the equator, this does not happen, for example in the United States the Sun is always south of the zenith at noon, though it is higher in the sky (closer to the zenith) in the summer.
Check out National Geographic’s feature “Dark Alliance: Two Explorers Trek to the North Pole in Complete Darkness,” if you or your students want more information on the expedition mentioned in the challenge. My students discovered for the first time that the North Pole in spring and summer has sunlight that lasts for twenty-four hours a day and is called Polar Day (a six-month day!). They also found out that in the fall and winter there are twenty-four hours of darkness for six months, also known as Polar Night. They couldn’t believe there was a place on Earth that had nighttime straight through for six months or sunlight for six months straight.
I tried and tried to come up with a way to explain this to my students but everything I came up with was confusing. Explain to your students their job will be to come up with an activity to explain why the North Pole can have long periods of complete darkness or sunlight.
- 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...
- 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.