DRAFT 3
6-27-05
Sun, Earth, seasons
Investigation 1: Daily cycle: day and night
Introduction
Discovery question: What are the patterns of the Earth’s daily cycle?
cartoon-earth
In this investigation you will explore how the Earth’s daily rotation around its axis
causes day and night and explains the changing pattern of the motion of the Sun
in the sky.
Thinking about the question
What are the patterns of the Earth’s daily cycle?
Stand up and face the front of the room. Pretend this is the direction of the Sun,
that your head is the Earth, that the top of your head is the North Pole, and that a
person is standing on the mountain of your nose. What time of day is it for that
person when you face the Sun?
ESSAY
Slowly turn your whole body to the left (counterclockwise). Describe where you
are facing so that it is sunset, then midnight, then sunrise for the person on your
nose.
ESSAY
When it’s midnight for your nose, what time is it for the back of your head?
ESSAY-SHORT
The Earth rotates once every 24 hours. Pretend that one second is an hour. The
teacher will count out 24 seconds. Start at midnight, and turn your body very
slowly so that it ends up at midnight again, 24 seconds later. What time of day is
it after 6 seconds? After 12 seconds? After 18 seconds? Fill in the talbe.
Time
6 seconds
Time of day
1
12 seconds
18 seconds
24 seconds
[ans: 6AM, 12noon, 6PM, midnight]
The teacher will show you a globe, and point out some features of its geometry.
Observe these carefully and then draw arrows from the labels to the picture
below. These include:
 The Earth’s axis: the line around which it rotates
 The North Pole: one end of the axis
 The South Pole: the other end of the axis
 The Equator: the circle halfway between the North and South Poles.
 The Priime Meridian: the half-circle running from the North Pole to the
South Pole.
DRAWING TOOL WITH IMAGE BENEATH
earth+labels
There is a standard way of locating points on Earth, called latitude and
longitude. Latitude measures your distance north or south of the Equator. It
is measured in degrees (°). The Equator is 0° Latitude. The North Pole is 90°N,
and the South Pole is 90°S.
On the classroom globe, find the approximate latitude of where you live.
ESSAY-SHORT your latitude =
Longitude measures your distance around the Earth. It is measured east or
west of a line from the North Pole to the South Bole, through Greenwich,
England. It is called the Prime Meridian, and it is 0° Longitude. Longitude circles
run from the North Pole to the South Pole. America is west of England, so the
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longitudes of places in America have values like 90°W. India is east of England,
so the longitudes of places in India are values like 75°E.
On the globe, find the approximate longitude of where you live.
ESSAY-SHORT your longitude =
Materials
For each team:
 Light sensor
 10 cm (4 in) Plastic foam ball
 20 cm (8 in) piece of coat hanger wire for axis
 Lump of clay
 Junk CD
 Printed drawing of the continents
Shared by the class:
 Plain white paper [??]
 Scissors
 Markers
 Meter stick
 Map pins or a stapler that can open up
 Push pins
 White glue
 White card stock
 At least one classroom Earth globe, with latitude and longitude.
 One bare light bulb (60W to 100W)
 Ability to darken the room
 Temperature sensor (for Further Investigations)
Trial 1: Where on Earth?
1. Make a right-angle bend in your wire, about 5 cm (2 in) from one end.
PICTURE
bentaxis
2. Push the wire through the ball, exactly through the middle. This is your
axis. If it’s not right in the center, and the ball turns with a wobble, try
again.
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3. Add a little bit of white glue to the axis to stop the globe from sliding
around.
axis-w-glue.jpg
4. Put dots on the ball around the middle, exactly half-way between the two
ends of the wire. Using the dots as a guide, draw the Equator circle on
your foam ball with a marker.
5. Draw a longitude line for 0 degrees (the Prime Meridian), running from the
North Pole to the South Pole. The North Pole will be in the direction of the
wire with the bend.
6. Draw and label two arrows along the Equator, one pointing east and one
pointing west. Also draw two arrows along the Prime Meridian, one
pointing north and one pointing south.
axis-w-equator
7. Print and cut out the outlines of the Earth’s land masses. Cut along the
dashed lines. The cutouts can be rough, leaving some ocean showing.
8. Label the land masses: North America, South America, Eurasia, Africa,
Australia & Indonesia, and Antarctica.
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continents-cutout
NOTE LABELS. DON’T INCLUDE “ & GREENLAND”
9. Staple or pin the continents on the plastic foam ball, in their proper places.
Here are some hints to help you:
a. Start with Africa. Line up the Equator and the 0° longitude line on
Africa with the Equator and the Prime Meridian on your globe.
africa-pinned –DON’T USE
africa-stapled
b. Make some cuts in Eurasia (dashed lines, marked “c”) so that it
bends around the globe better. Line up the 0° longitude line on
Eurasia with the Prime Meridian on your globe.
africa-asia-pinned
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c. You will need to poke a hole in Antarctica with one end of your wire
axis because the South Pole is right in the middle of the continent.
d. Add other land masses so that their relationship to each other is the
same as on the classroom globe. Make sure the Equator on the
land masses always matches the Equator on your globe. Here is a
picture of North and South America.
americas-pinned
10. When you have finished, check your model with your teacher.
11. What was the hardest land mass for you to locate? Why?
ESSAY-SHORT [ans: answers will vary.]
12. Put a small lump of clay on your globe, as if it were a mountain. Put right
where your E-W-N-S arrows are, on the Equator.
13. The teacher will set up a light source, and darken the room. This will
represent the Sun.
14. Rotate your globe so that it is noon for someone at the lump of clay. How
can you tell that it’s noon at that place?
ESSAY [ans: the Sun would appear directly overhead for a person at that place.]
15. Turn your globe counterclockwise so that it is sunset for a person on your
clay mountain. How can you tell when it’s sunset?
ESSAY [The mountain is right at the edge between light and dark.]
16. Continue turning your globe so that it’s midnight, then sunrise, and then
noon again for a person on your clay mountain. How can you tell when it’s
midnight?
ESSAY [The Sun is on the opposite side of the Earth.]
17. In the morning, which direction does your shadow point? Check this by
observing your clay mountain.
MULTIPLECHOICE: east
west
[ans: west]
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18. In the afternoon, which direction does your shadow point? Check this by
observing your clay mountain.
MULTIPLECHOICE: east
west
[ans: east]
19. Look at your E-W arrows. Note that the Earth rotates toward the east. But
the Sun rises in the East and sets in the West, so it moves toward the
west across the sky. How can you explain this?
ESSAY[ ans: One way describes looking at the Earth from the Sun, and the other
describes looking at the Sun from the Earth. The motions are the reverse of each
other. ]
Trial 2: Sunlight at different times of day
1. Make a clock face using a junk CD and marker. On this clock, 12 hours is
halfway around, from midnight (12AM) to noon (12PM). The other half is
from noon to midnight. It also goes counterclockwise (the opposite of
regular clocks) because that’s the direction the Earth rotates. Divide the
circle in half and mark noon and midnight. Divide each 12-hour period into
four parts and put marks at 3 o’clock, 6 o’clock, and 9 o’clock.
cdclockface
DO AS PDF AS WELL, REAL CD SIZE, TEACHER HANDS IT
OUT.
2. Use a lump of clay in the hole of the CD to hold the clock horizontally, and
stick the axis of your globe vertically into the middle.
globe-in-clay.jpg
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3. Cut out a piece of stiff white paper about 8 cm (3 in) square. Label it as a
compass card, showing north, east, south, and west. Put a pushpin in the
center.
compasscard-w-pin
BLUE PIN, NO SHADOWS, TIP TO MAKE PIN MORE
VISIBLE
4. Remove your clay mountain. And pin the compass card to your globe at
that place. This represents the surface of the Earth at that point. Make
sure that N points north.
globe-w-compasscard
USE GLOBE-IN-CLAY IMAGE, ADD COMPASS CARD.
NOON IN SAME DIRECTION AS COMPASS CARD AND
LIGHT SOURCE. SHOW MASKING TAPE HOLDING DOWN
COMPASS CARD.
5. The teacher will set up a bare light bulb and darken the room. Set up your
globe about 150 cm from the light bulb. Make sure that your CD clock face
has noon pointing toward the light bulb. Tape down the CD to keep it
pointing the right way.
6. Connect the light sensor to the computer. Refer to Technical Hints to
connect the light sensor.
7. Test the light sensor by pointing it toward the light bulb and then covering
it with your hand. The reading should go up and down.
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8. Now you will measure the brightness (also called intensity) of the light that
hits the compass card at different times of day. To measure this, point the
light sensor at the compass card and not at the light bulb. To avoid
making a shadow with the light sensor and your hand, hold the light
sensor at an angle to the compass card, to one side of it. Your light sensor
may look different from this picture. Refer to Technical Hints to see your
light sensor.
measure-refl-light
9. Have one student rotate the Earth through the day-night cycle, from noon
to midnight again, while another student points the light sensor toward the
compass card. Measure the light level on the compass card at different
times of day. See Technical Hints for taking a single measurement. Fill out
the table.
LIGHT DATA COLLECTOR in column 2
Time of day
brightness
12PM (noon)
3PM
6PM
9PM
Midnight
3AM
6AM
9AM
10. At what time of day is the brightness the highest?
ESSAY-SHORT highest brightness =
9
[ans noon]
11. What is your nighttime light measurement? On Earth, this measurement
would be very small. There would just be starlight and moonlight.
ESSAY-SHORT night-time measurement =
[Answers will vary, depending on the brightness of the room.]
SOME WAY TO DISPLAY THIS DATA AS A GRAPH, and then ask them to
describe the shape of the graph?
12. Your globe and light bulb system is meant to be a model of the Earth and
the Sun. Do you think the model can give you accurate results for real light
levels on Earth at different times of day? How could it be made more like
the real Earth- Sun system?
ESSAY [ans: Answers will vary. They may include: it isn’t the same, because
the distances are very different; the changes in light level could be accurate,
because the geometry is similar; the room would need to be very dark, like
the night sky; the bulb would need to be as bright as the Sun; the model Earth
has no atmosphere or clouds. ]
Trial 3: Sunlight at different latitudes
1. You have observed, using a bare light bulb as a model of the Sun, that as
a point on the Earth rotates away from the Sun, the intensity (brightness)
of light falling on the ground decreases.
Explain why you think this is true.
ESSAY [ans: As a location on the Earth rotates away noon, the Sun is less
directly overhead, from the point of view of that location. It is at a lower angle with
respect to the ground. It is spread out more, and the brightness on the ground
decreases.]
2. As one moves away from the Equator and toward the Poles, this also
changes the angle of the Sun. What do you think will happen to the
sunlight as you move your location toward the Poles?
a. It will become more intense.
b. It will become less intense.
c. It will stay the same.
[ans: it will become less intense]
3. Test your prediction. The teacher will set up a bare light bulb and darken
the room. Set up your globe vertically about 150 cm from the light bulb, as
in Trial 2.
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4. Connect the light sensor to the computer. Refer to Technical Hints to
connect the light sensor.
5. Pin the compass card on the globe at the Equator, and turn the globe so
that it’s noon from the point of view of the compass card. Even though the
real Earth’s axis is tilted, it should be straight up and down for this
measurement,
6. Have one student hold the Earth while another student points the light
sensor toward the compass. Your light sensor may look different from this
picture. Refer to Technical Hints to see your light sensor.
7. Record the brightness of the light reflected from the compass card. See
Technical Hints to record a single measurement.
measure-refl-light
8. Move the compass card to different latitudes and repeat this
measurement. Make sure that it’s always noon from the point of view of
the compass card. Keep the axis straight up and down. The different
latitudes are labeled on the land masses. Fill in the table.
latitudes-list
LIGHT DATA COLLECTOR in column 2
Latitude
brightness
Equator (0°)
Tropic of Cancer
(23.5°N)
11
45°N
Arctic Circle (66.5 °
N)
Tropic of Capricorn
(23.5°S)
45°S
Antarctic Circle
(66.5 ° S)
9. At which latitudes is the Sun’s brightness the greatest?
ESSAY-SHORT
[The Sun’s brightness is greatest near the Equator]
10. At which latitudes is the Sun’s brightness the least?
ESSAY-SHORT
[The Sun’s brightness is least near the Poles]
11. Would this change how fast the Sun would heat up the ground?
ESSAY-SHORT
[Yes, it would. More intense sunlight would heat up the ground faster.]
12. Based on your measurements, can you explain why the North Pole and
the South Pole are cold, and the Equator is hot?
ESSAY
Analysis
1. Using the classroom globe, find two cities, states, or countries that are far
apart but have about the same latitude.
Your cities:
ESSAY-SHORT
2. Using the classroom globe, find two cities, states, or countries that are far
apart but have about the same longitude.
Your cities:
ESSAY-SHORT
3. “Solar noon” is defined as the time each day when the Sun is highest in
the sky. It is noon at the same moment for places on the Earth with the
same
a. Latitude
b. longitude
[ans: longitude]
Explain how you decided on the answer.
ESSAY
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[ans: Along a longitude line, the Sun is highest in the sky at the same moment.
Along a latitude line, the Sun is highest in the sky for only one place, and it will be
highest in the sky at other moments for other places.]
4. You have been looking at your Earth model as if you were in outer space,
and making night and day by rotating your model. On the real Earth, it
appears that the Earth is not rotating, and the Sun is moving across the
sky.
a. Explain why it appears to us that the Earth is not rotating.
ESSAY
b. OMIT – TOO HARD? NEED ANOTHER QUESTION. If someone
insisted that the Earth was not rotating, and it was the Sun that
circled around the Earth, what argument would you use to convince
them that the Earth is rotating and the Sun is not moving?
ESSAY
5. Now imagine you are standing on the real Earth. Suppose it was solar
noon and you were asked to point to toward the Sun. Which way would
you point?
east west up
down
[ans: up]
6. Suppose it was midnight and you were asked to point to toward the Sun.
Which way would you point?
east west up
down
[ans: down]
7. What if the Earth rotated much more slowly, like Mercury, which rotates
three times in two years?
Describe what you think day and night would be like.
ESSAY
[ans: Day and night would be very long, and it would get very hot during the day
and very cold during the night.]
8. When the Sun is not as high in the sky, sunlight is not as bright and the
Earth’s surface is not heated as much by the Sun. Because of this, where
on the Earth would it be the coldest, and where would it be the hottest?
ESSAY
[ans: The Sun is lower as the latitude increases. Latitudes near the Poles would
be coldest, and latitudes near the Equator would be hottest.]
Further investigations
Measure the intensity of reflected sunlight as a function of its angle. On a piece
of stiff white card, at least 20 x 20 cm (8 x 8 in), mount a vertical screw or
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toothpick about 2 cm (3/4 in) long. Pick a day with a clear sky and bright sunlight.
Measure the surface brightness with the light sensor compared to the length of
the shadow. If the light is too bright for the range of your sensor, cover the sensor
with layers of masking tape until it does not reach its maximum even when
pointed at the brightest reflecting surface.
LIGHT DATA COLLECTOR
Measure heat gain of a dark surface as a function of the angle of sunlight. To do
this, tape a temperature sensor to the back of a dark piece of paper. Move the
paper from shadow into full sunlight, each time holding the paper at a different
angle to the sun. Note the temperature rise in the first 30 seconds.
TEMPERATURE DATA COLLECTOR
Measure light and temperature over a 24-hour period. Place a white card flat on
the ground, and mount the light sensor pointing at it but not casting a shadow.
Next to it, place a dark card flat on the ground, with a temperature sensor taped
to the underside. Run both data collectors for a full day, and compare them.
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