8: Orbits: Your World is Tilted

Group Size: Whole class and small group recording teams

Summary: Having established how the Earth's spinning causes the alternation of day and night, and more importantly having learned the use of the model to understand the effects of motions of the Earth on our experience on its surface, in this lesson we will introduce the fact that the Earth's axis is not perpendicular to the Sun's direction. This means the Sun does not lie always above the equator as we found in the last Activity. In fact, it lies sometimes to the North and sometimes to the South of the equator, and the annual alternation of these configurations creates the seasons on Earth. In this activity students will, as a first step, investigate the effects a tilt in the axis will have on the length of days and nights, as well as on climate, at various latitudes in the two hemispheres.

Standards:

Science:

Objective 3.01 Observe that light travels in a straight line until it strikes and object and is reflected and or/absorbed.
Objective 3.02 Observe that objects in the sky have patterns of movement including: Sun, Moon, and Stars.
Objective 3.03 Using shadows, follow and record the apparent movement of the Sun in the sky during the day.

Math:

Objective 3.01 Use the appropriate vocabulary to compare, describe, classify two and three-dimensional figures.

Pre-assessment:

Learning Objectives

After this activity, students should be able to:

Use modeling to demonstrate and describe how the Earth is tilted.
Explore the effects on the Earth if we were tilted differently.
Describe the effects of the tilt on each hemisphere, the poles and the equator: differing lengths of day and weather.
Connect how when the North Pole is pointed toward the Sun, the Northern Hemisphere receives more sunlight and has higher temperatures and when the South Pole is pointing toward the Sun the Southern Hemisphere receives more sunlight and has higher temperatures.
Explore how the Earth's tilt causes only slight changes in the temperature and amounts of sunlight on the area near the Equator but the North and South Poles experience extreme changes in temperature and the amounts of sunlight.

Pre-Req Knowledge

The students explored in the previous activity how sunlight, the Earth's spin and its round shape cause the day/night cycle. Additionally, they learned that because the Earth is round what you see when you look overhead into space depends on your location on Earth. So where up is for someone in Australia is different from up in NC and therefore what we see in our day and nighttime sky is different. These principles will be combined with this lesson's principle of the Earth's tilt to help the students discover why and how sunlight falls differently on Earth depending on where you are located on Earth.

Pre-Activity Assessment

In the science notebooks and recorded on your chart paper the students' responses to the opening Motivation/Challenge.

Assessment:

Activity Assessment

The students will answer several questions from the steps of the activities in their science notebook:

Of the countries we already have on our map Brazil, USA, China and Australia, what hemisphere is each country located? 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 someone see if they looked up from the Equator at midday?
Demonstrate the idea that the globe's axis could be oriented differently. Show an exaggerated tilt of 90° with the North pole facing the Sun. Have everyone place their North Pole pointing directly at the Sun and spin their globe. Ask, "What would happen if this was the direction of Earth's axis?" Help students note that the entire Northern hemisphere is now in perpetual daytime, while the entire Southern hemisphere is in perpetual night. The Sun is always overhead at the North pole.
"Something new is going to happen to your countries and to both hemispheres as you spin the "Earth" 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 Pole's toward different directions." Allow the class 10-15 minutes to explore, draw and explain. Focus the kids to think about where and how much sunlight is hitting the Earth. Help them to notice that whereas without a tilt the golf-tee "people" at the same longitude in the two hemispheres (e.g., in the US and Brazil) 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!
What is happening at the Equator where there is equal distance to each pole? How does the 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? Allow your class to experiment by moving their golf tee people onto the North and South Pole. Ask them to draw and write explanations to how living on the equator or the poles affects the amount of sunlight and the temperatures.

Post-Activity Assessment

Have each science partner group test out teaching the lesson they have planned for your teacher friend's class. It's best if one pair teaches it to another pair and they switch so everyone gets a turn.

Introduction:

Background

In previous lessons students understood how the Earth's daily rotation governs the cycle of day and night, light and darkness. In modeling this, we kept the axis of our "Earth" vertical (in the classroom) so that the North pole was close to the room's ceiling. (We repeated to students, in Activity 6, "What's Up, Earth" that the existence of a particular direction that is "up" in the classroom was not an accurate model of the real situation. On Earth, up is defined as "away from Earth's center" and signifies different directions at different points on Earth). With the "Sun" at about the same height above the floor as we held the "Earth," this means the Earth's axis and the line from Earth to Sun are perpendicular (create a 90° angle). 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 (day) while the other half is dark (night) because it lies in the Earth's own shadow. In our model configuration, the line separating the light from the dark half passes through both the North and the South poles. As the 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 right on the horizon, drifting to the right (at the North pole) or left (at the South pole) as we saw in Activity 7, "Spinning into Darkness, Spinning into Light." As the Earth spins, any other location on its surface spends precisely half of the time in the illuminated half and half in the dark half. In this model, day and night are 12 hours long everywhere, and perpetual twilight reign 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 length changes with the seasons. In summer, days are longer and nights shorter; the opposite is true in winter (a day and a night continue, always, to comprise one full spin of the Earth, and since the rate at which the Earth spins does not change, this is constant). The magnitude of seasonal changes depends on latitude. Near the equator, days and nights are in fact of equal length always, while near the poles darkness can be entirely absent in summer, and light 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, in fact, perpendicular to the line from Earth to Sun. Rather, it tilts away from perpendicular by 23.5°. In this Activity, 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° tilt), so that the Sun is directly overhead for someone standing at the North pole. As the Earth spins now, the illuminated half of its surface is always the Northern hemisphere, and the dark half is always the Southern hemisphere!

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 (in the Northern hemisphere; it is tricky to talk about seasons because "summer" can mean June in the North or December in the South. In these lessons we will be referring to the Northern hemisphere seasons unless we explicitly mention that we are talking about the Southern hemisphere - hence "summer" means June). 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. 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 the Earth, and the South pole into the dark part. Since the poles do not move as the 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, which is another "great circle." 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 which never enters the dark half of the Earth' surface as the planet spins. In this region, we find at this tilt 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° to make the effects of tilting the globe easier to see. Because of this, these regions will appear larger in our model than they are in fact.

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 (Northern 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 the "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 it 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 the 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 (this is not completely correct - see the Background section for Activity 9, "Seasons and the Orbit"). But because the 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. We will take this up in the next Activity. We also defer for then the discussion of the relation between length of the day and climate.

The actual tilt of the axis, 23.5°, is quite small. In this Activity, we will exaggerate the tilt so that the effect will be more noticeable. At first, of course, we use a 90° tilt, but for the subsequent investigations we recommend using a tilt of around 45° so that students can see the effects more readily.

Classroom Time Required:

60 minutes

Materials Need:

Teacher needs:

Modeling clay (not needed if you use golf tees or other object that sticks into Styrofoam)
4 Golf tees or small object to place in clay
One plate with straws from the "Tilted and Spinning Plates" activity
Globe
Lamp or light source in the middle of the room with 250-watt bulb

Each pair needs:

6" Styrofoam Earth model and 4 golf tee people
Markers

Preparation:

Place the lamp/light source in the middle of the room or some location that everyone has a direct sight line to its light.
Have 3" Styrofoam Earth models ready from last activity
Place your Globe for easy access.
Have a plate with straw attached ready for modeling the 23 ½ degree tilt.
Place a North Star cutout in the Northern section of your room.

Activities:

Procedure

Introduce the activity with the following Motivation / 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 who in February 2006 set off in winter to reach the North Pole traveling 600 miles. They traveled for 50 days and everyday was in twenty-four hours of complete darkness. Check out more info at http://www.nationalgeographic.com/adventure/0602/features/north-pole-exp... if you or your kids want more info on the expedition. My students discovered for the first time that the North Pole in spring and summer has sunlight twenty-four hours a day called Polar Day or a six-month day and in the fall and winter has twenty-four hours of darkness or a six-month night 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. Allow the class to brainstorm with either a table group or a partner to list what they already know that will help them explain why the North Pole can have 24 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 to help teach a lesson about why places on Earth receive different amounts of light. As they discuss students should record their ideas in their science notebook for 5-7 minutes. As a whole class, list their responses on chart paper to be referred to during the activity.
Return each student's Earth model and instruct him or her to use a marker to re- draw his or her equator line. Explain that the equator is the imaginary line that is like a belt around the middle of the Earth separating the top half or the Northern Hemisphere from the bottom half 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 and South Poles. Ask, "Of the countries we already have on our map Brazil, USA, China and Australia, what hemisphere is each country located? How can you prove it? Have each student write "Northern" just on the side of the Equator line with the North Pole at its top and "Southern" on the side of the equator line with the South Pole at its top. Be sure North and South Pole are labeled as well.
Turn off the lights and have the class imagine they are out in space. Remind them that like last lesson everything in the class that is off their model Earth is in outer space. Explain that they are "Space Giants" again like in the Earth is Round activity. If they think how big they are compared to their Earth, they are huge! Students should hold their Earth model 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 they have to look to see the Sun?" Have them place a golf tee person on 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 but always 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 50 days through winter never seeing the Sun. 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° with the North pole facing the Sun. Have everyone place their North Pole pointing directly at the Sun and spin their globe. Ask, "What would happen if this was the direction of Earth's axis?" Help students note that the entire Northern hemisphere is now in perpetual daytime, while the entire Southern hemisphere is in perpetual night. The Sun is always overhead at the North pole.
Have your students switch it around and have the South Pole pointed directly at the Sun and listen to your students' description as to 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° 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 the "Earth" 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 Pole's toward different directions." Allow the class 10-15 minutes to explore, draw and explain. Focus the kids to think about where and how much sunlight is hitting the Earth. Help them to notice that whereas without a tilt the golf-tee "people" at the same longitude in the two hemispheres (e.g., in the US and Brazil) 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 "Earth" the Sun is directly overhead (at the zenith), where the Sun never sets as the Earth spins, and where it never rises. The circles they find will be the tropic of Cancer, the Arctic and Antarctic circles, respectively.
Have students reverse the tilt so that the South pole tilts toward the Sun at 45° and repeat their observations. 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 (tilting to "right" or "left" when facing the Sun) corresponding to fall and spring (in the Northern hemisphere) respectively. When everyone is close to finished have the class listen to 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 notebook and then add demonstrations, drawings on the board and The Earth's Tilt Causes More or Less Light handout attached to explain further. 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° tilt.
Ask your students, if you haven't already discussed it, what is happening at the equator where there is equal distance to each Pole? Help them realize that the various tilt directions make no difference whatsoever at the equator, one-half of which is always illuminated. 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 Pole. Ask them to draw and write explanations to how living on the equator or the poles affects the amount of sunlight. Remind them that these drawings and explanations will be used as they design their lesson to help my fellow teacher teach a lesson to explain how it can be true that the North Pole has days of complete darkness called Polar Night.
Safety Issues
- Warn students never to look at the lamp i.e. light source directly as it could cause eye damage.

Supplemental Information:

Activity Extensions

A great NASA site for kids that helps them understand space exploration, missions, operations as well as answer some of their space science questions at http://ksnn.larc.nasa.gov/35newsbreaks.cfm
Additional activities include Daily Temperature Recording and Mapping and Tracking Sunset and Sunrise Times found at http://hea-www.harvard.edu/ECT/the_book/Chap2/Chapter2.html
Your students can learn all of the other 8 planets tilts in our solar system at http://mars.sgi.com/worlds/CyberMarz/planetStats/planetSizes.html
Amazing images from off Earth to help your students gain the perspective of being Space Giants who look down on earth to learn about our planet at http://www.awesomelibrary.org/Classroom/Science/Astronomy/Earth.html
Learn about the Analemma or the Sun's figure 8 path in the sky at http://www.analemma.com/Pages/framesPage.html

References

Exploring Light (Print-Non-Fiction). Ed Catherall. (Exploring Science Series). Wayland, East Sussex, England (NEL), 1989. 48 p. ISBN 1-85210-703-0 ($21.95 hdc.)
The Science Book of Light by Neil Ardley, Harcourt Brace Jovanovich, 1991. The book contains simple light experiments demonstrating the principles of light with colorful photographs.

Critical Vocabulary:

Axis: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

Spin:Turn on its axis. One complete spin for the Earth's takes close to 24 hours or 1 day

Tilt:(n) a slant; (v) 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

Equator:a great circle of the earth or a celestial body that is everywhere equally distant from the two poles and divides the surface into the northern and southern hemisphere

Arctic/Antarctic Circle: The line of latitude 66.5° N/S near the North/South pole. This is the Southernmost/Northernmost location 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.

Zenith: The point in the sky directly overhead.

Tropic of Cancer/Capricorn: The lines of latitude 23.5° N/S. 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 US the Sun is always South of the zenith at noon, though it is higher in the sky (closer to the zenith) in the summer.

Supplemental Files:

Assessment Activity for Lesson 8

Author:	J. Heffernan
Level:	Elementary School
Content Area:	Science, Math