A Year Viewed From Space

ANTICIPATORY SET:

Using the diagnostic assessment "My Ideas About a Day, Year, Seasons and Moon Phases: Before: discuss students’ current ideas about what causes a year.

Students:

• Look over My Ideas About a Day, Year, Seasons and Moon Phases: Before.
• Discuss written responses
• What causes a year?
• What causes seasons?
• Additional questions for discussion:
• What is a year?
• What happens to Earth in a year’s time?
• Even if we didn’t have calendars, how would we know that a year has passed?
• What did you learn in the last activity about what happens to the angles of the Sun and length of daylight over a year?

Record student responses on chart paper.

Students are likely to propose that Earth is closest to the Sun in summer than in winter.  The evidence they find in the first part of this activity should help them change this misconception.  A few students may know something about the role the Earth’s tilt in determining the seasons.  Leave this question open – return to it after students complete the activity.

Discuss how the passage of the year has always been notable to humans in many areas of the world in view of the significant impact of the seasons on climate and on the availability of food and water.  Early in history people in many cultures figured out when to plant crops by studying the changing time of the sunrise and sunset and the patterns of the stars.

MODELING:

Model Earth’s rotation, revolution and tilt.  Use a globe or an Earth beach ball to introduce/review:

• Equator
• Northern Hemisphere
• Southern Hemisphere
• North Pole
• South Pole
• Latitude

On globe point out:

• Anchorage, Alaska (latitude 61°N) is an example of a very northern city
• Chicago, Illinois (latitude 42°N) is a mid-latitude Northern Hemisphere city and similar in latitude to Buffalo, NY;
• Quito, Ecuador (latitude 0°) is near the equator; and Melbourne,
• Australia (latitude 38°S) and is a mid-latitude Southern Hemisphere city.

As students learned in the last activity, Earth rotates around its axis once during each day-night cycle.  Now introduce the concept that Earth also moves around, or revolves around, the Sun, and explain that one complete turn around the Sun is called a revolution.  Earth’s orbit is the path it follows as it revolves.  Model this by moving the globe around a light bulb or other object (a student) that represents the Sun.  Then model both rotation and revolution at once.  Throughout this activity encourage students to use the terms rotate and revolve as much as possible to describe the motions of Earth.

Raise the point that Earth’s axis is tilted.  The best way is to use a standard tilted globe that shows the correct orientation of the axis of Earth relative to the plane of its orbit (23.5° from a vertical line perpendicular to Earth’s orbit).  You can also use the beach ball globe to demonstrate Earth’s tilt.

GUIDED PRACTICE:

Let students know they will be using an interactive computer simulation to explore another planetary characteristic, the year length.  Beforehand use the screen-shot of the Seasons Interactive Simulation in the Student Book to orient them to what they will see. 

Move class to computer lab.  Distribute Computer Lab Activity Procedures Sheet.  Students log into computers and then use web browser to go to sepuplhs.org and go to Activity 76 A Year Viewed From Space, SEPUP Seasons Interactive (http://www.sepuplhs.org).

 Step 1 of the A Year Viewed From Space Computer Lab Procedure directs them to open an introductory page of the simulation.  This page reiterates for them the position of the equator and shows the locations of the four cities that appear in the interactive simulation.  It also defines the optional terms Tropic of Cancer and Tropic of Capricorn.  These are considered optional because there are so many terms in this activity and students can grasp the main ideas of the unit without them.

Be sure to tell students:

• That the size of the Sun and the Earth in this simulation are not to scale. 
• The Sun is much larger (its diameter is more than 100 times that of Earth). 
• Point out that the top view shows the orbit as nearly circular, while the side view shows it stretched out into a more eccentric ellipse. 
• The top view is much closer to the correct view. 
• The side view stretches out the orbit to make it easier to see Earth.  This kind of view contributes to the misconception that the distance from Earth to the Sun is the variable that determines the seasons.  Make sure that students understand that the top view is more accurate. 
• Note that students will explore size and distance of planets in the solar system in future lessons.

INDEPENDENT PRACTICE:

Students investigate the simulation (Examples of the simulation screens.) 

• Distribute Student Sheet 76.1, A Year Viewed From Space:  Top View. 
• Students complete Lab Procedure Steps 1-7 (Part A) of the activity.  (Note:  Due to space constraints, copies of the A Year Viewed From Space Computer Lab Procedures are printed for students to use instead of taking text book to lab. 

As they watch the simulation, circulate among the students:

• Ask what they are seeing. 
• Refer them back to their initial ideas in the activity, and ask if they have seen any evidence for or against those ideas. 
• Be sure they confront the observation that Earth is closer to the Sun during our (Northern Hemisphere) winter and that this refutes the idea that distance from the Sun determines the seasons.  This may be difficult for students to grasp.

Distribute 2 copies of Student Sheet 76.2, A Year Viewed From Space:  Side View:

• Explain to students how they will use sketches of Earth like those at the top of the page to show Earth as each season in the Northern Hemisphere. 
• Suggest that they look specifically at the tilt of Earth as they stop the simulation in each of the four months designated. 
• Indicate that they should label one worksheet A Year Viewed From Space: Side View asChicago and the other worksheet as Melbourne.  If any students have extra time, encourage them to explore additional months as well.  Then have students continue to Part B of the activity.

To check for their understanding of the effect of Earth’s tilt, stop before Lab Procedure Step 13 and have students vote on whether they think that changing the tilt to 0° will:

• a) have no effect
•  b) make the seasons less extreme, or
• c) make the seasons more extreme in Chicago/Buffalo.

For Lab Procedure Step 13, students should find that at 0° tilt, there is little or no seasonal variation for Chicago/Buffalo.

For Lab Procedure Step 11, they should observe:

• The daylight period in Melbourne in December is 14 hours and 46 minutes, while in June it is 9 hours and 33 minutes. 
• Find that there the average temperature in December is 63°F, 17°C, while in June it is 50°F, 10°C.  From this they should describe these seasons as reversed from those in Chicago/Buffalo.  This is the important point for them to notice now.  They may also notice that winter and summer are milder in Melbourne.  (Although there are other variables that affect weather, they may be able to reason that one factor is the greater distance of Chicago from the equator.  Another is Chicago’s distance from an ocean, while the ocean has a moderating effect on temperatures in coastal Melbourne.) 

Ask them to review their diagrams and speculate why Melbourne would have winter in June and summer in December. That will help to see if they can reason that the orientation of Earth’s axis causes the Southern Hemisphere to tilt away from the Sun in June and toward the Sun in December.

CLOSURE:

Discuss Earth’s revolution around the Sun and its role in determining the length of Earth’s year and seasons.  The purpose for this discussion is for clarification and to model for students the concept of scientists sharing and collaborating on information.  This sharing helps students (scientists) to help develop a deeper understanding.

Independently or in small group allow students time to discuss and answer Analysis Questions 1-3.  Circulate around the room and provide hints as needed.  Be sure they have observed the Northern Hemisphere’s tilt toward the Sun at the beginning of its summer in June and away from the Sun in the beginning of its winter in December.

When they have had a chance to think about the ideas on their own, hold a class discussion on the seasons before asking them to complete the remaining questions.  Have students discuss:

• How the tilt of the Earth leads to warm summers and cold winters in many places. 
• Review the idea that the seasons in the Southern Hemisphere are reversed from those in the Northern Hemisphere. 
• Explain that when one of these hemispheres of Earth is tilted toward the Sunk that half of Earth receives more direct sunlight (closer to vertical) and is in the Sun for a longer period of time, both of which leads to warmer temperatures.  Students will explore these concepts in further activities.

Remind students of the explanations for the seasons that they offered before doing the activity and ask them to describe how their ideas have changed.  The idea that seasons are determined by distance from the Sun is still logical based on our experience on Earth – the closer you get to a hot object, the warmer you get.  But the actual evidence shows that distance from the Sun as an explanation for the seasons is just not correct.  The distance factor also does not explain why it is summer in the Southern Hemisphere when it is winter in the Northern Hemisphere.  For these reasons, distance from the Sun as an explanation for the seasons is no longer plausible.  A good explanation for any natural phenomenon, such as the changes of the seasons, must make sense, and it must explain most, if not all, aspects of the phenomenon.