LESSON THREE:
CAUSE OF DAY AND NIGHT THANKS TO THE ROTATING EARTH

Curriculum Expectations

• Identify cycles in nature (e.g., cycle of day and night, cycle of seasons) and describe the changes within the cycles (e.g., observe the phases of the Moon over several months to determine the pattern of change, and record these observations);

• Describe, using models or simulations, how the Earth’s rotation causes the cycle of day and night, and how the Earth’s revolution causes the cycle of the seasons;

Common Misconceptions

A problem with trying to understand the concept of day and night (as well as some other basic concepts of astronomy) has to do with "frames of reference". Most textbooks show the cause of day and night (the rotation of the Earth), as would be seen by an outside observer, far from the Earth. The students need to also see it from their own perspective as a person standing on Earth. Activity 1 has been designed to help clear up this misconception.

Background Information

• It is the rotation of the Earth that causes day and night

• The Earth rotates counterclockwise; thus a person standing on Earth sees the Sun rise in the East and set in the West. The Earth rotates toward the East and so, to the people on Earth, it looks like the Sun is moving towards the West. In fact, is the Earth that is moving. A good metaphor for this is the merry-go-round. When you’re riding on it and look out at the people standing on the ground, it looks as if they are moving, but in actual fact, you’re the one who is moving.

• The Earth rotates on its axis once every 24 hours and takes about one year (365 days) to revolve around the Sun.

Activities

1. Day And Night Thanks To The Rotating Earth developed by John Percy

Time: 40 minutes

N.B.: Take the time to test out this activity beforehand, so you know how it works.

Teacher Demonstration/Student Pairs

• For your demo: 2 large index cards - write EAST on one and WEST on the other in writing large enough for your audience to see
  
• orange card with the word SUN on it
  
• 2 regular-sized index cards per pair of students
  
• Follow-up activity requires a globe and a light source, toothpick and plasticine

You can tell students that this activity has been designed to help students and adults learn the about the difficult concepts of day and night, sunrise and sunset.
1. Ask for two student volunteers to help you demonstrate the activity. Student #1 is the Sun (this student can hold a large orange cloth or construction paper, or a sign that says SUN). Student #2 is the Earth. The Earth holds WEST card in right hand, EAST card in left hand. This is the correct orientation if you are facing south.

2. You can tape a sign on the front of the student to represent the city you’re in. On the student’s back, tape the name of a city whose time difference is about 12 hours from your city, but in the Northern Hemisphere. For example, Toronto on the front and Delhi, India on the back, or vice versa. At this point, don’t use a city from the Southern Hemisphere, as it tends to complicate issues.

3. Have Student #2 face the Sun and turn in slow motion counterclockwise because that's the direction that the Earth rotates, when seen from the northern hemisphere.

4. Optional: you can say this chant as the Earth turns:

   Ickly, pickley, paxis
   The Earth spins on its axis
   Ishkey, pishkey, pight
   It turns from day to night.

   Note: the Sun student can also rotate: the Sun rotates in the same sense as the Earth, but in about 30 days.

5. Ask Student #2 (Earth) to call out when the Sun is setting, when it is night and when it's rising on that city (e.g., Toronto). The student will see the Sun set in the West; then she/he will be facing away from the Sun (it is night). When is it day and night for the opposite country/city (e.g., Delhi, India)?

6. If you want precision, stop the Earth student at 90° intervals and ask what time of day it is in the chosen city at each of these stops.

7. Students in pairs: Partner students up so that the whole class gets a chance at the activity. Let students know they will each get a chance being the Earth and Sun. N.B.: It is crucial that each student has an opportunity to be Earth because it is from the Earth's view, that this activity seems to work the best. If it is just viewed from observers in class, kids get confused about East and West, and it will look as though the Sun rises in British Columbia before Ontario. The card-holding child is in the centre of the Earth looking out into space.

8. Instructions to whole group:

   Wave your hands in the air if you are a Sun. Clap your hands if you are Earth. OK. Sun stand still, Earth face Sun, ready and in slow motion Earth turn counterclockwise. Everyone freeze. Sun, ask Earth: "Approximately what time of day or night is it in this position?" Ready, Earth rotates another 90°. Freeze. "Approximately what time of the day or night is it, Earth?"

9. After everyone has had a turn, gather the group and ask them what they found surprising or puzzling during the time they were playing the role of the Earth.

10. Post-discussion/Follow-up Activity:

    Teacher Demonstration/Whole group

    Use the globe and rotate it slowly counterclockwise in front of a light source so that children get an even more concrete view of the Sun hitting different areas of the globe (representing sunrise) and the dark side of the globe (night). You can stick a toothpick on the city/area you are in by using a piece of plasticine. It’s a good idea to point out the difference between two cities on opposite sides of the globe but in the Northern Hemisphere (e.g., Delhi, India and Toronto, Canada).

    Refer students back to introductory lessons: Sunrise is the time that the center of the Sun’s disk is on the horizon and sunset is when the center of the Sun’s disk first touches down on the horizon.

• "Mount Nose"

  In this clever activity, the student’s head becomes Earth and the location on Earth is "Mount Nose" (the student’s nose). This activity is wonderfully set out with clear instructions and has obviously been tested on many students. It can be found in: Earth, Moon & Stars (1986) by Cary I. Sneider. Great Explorations in Math & Science (GEMS), Lawrence Hall of Science, University of California, Berkeley.

• "Earth’s Orbit Relay" by James & Joy Humphrey, (1991). Developing elementary school science concepts through active games. Charles C Thomas Publisher: Springfield, Illinois

• http://www.fourmilab.to/earthview/

  This is a great site that will allow your students (and you!) to view Earth from the Sun or the Moon either as day or night.

• Time Zones: The story of Sir Sandford Fleming and his "invention" of dividing the Earth into 24 time zones. (The time in each zone corresponds to one of the 24 hours in the day.)

• Math extensions: Dealing with the time in different parts of the Earth, setting watches ahead or back when travelling, jet lag, and all that other practical stuff! 24 X 15° = 360°. The number of degrees in a circle or the spherical Earth! Each time zone covers 15° latitude? 24 time zones how many degrees wide is each time zone? 15° If it's 7 p.m. in Boston where is it midnight? 5 zones away - Britain.

• Geography: a nice lead-in to latitude and longitude.

• Sunrise/sunset times around the world can be found at:
  http://www.argonet.co.uk/location/works/sunrise/ssr.html