1. Investigation 3 Part 2
Day Length

3. Hey there sunshine
I’ve heard it said that December 21 is the shortest day of the year. Why would someone say that? Does earth rotate in fewer than 24 hours on December 21?
If it’s true that there is a shortest day, is there also a longest day of the year?
Update Moon Log
Discuss sunshine
Remind students that in Part 1, they concluded that heat and hours of daylight were indications of summer. Now that they’ve explored summer heat, ask students to think about day length.
Ask questions on slide.
Answers:
No, we get fewer hours of sunlight in winter, more hours of sunlight in summer, but the sum of light and dark hours always equals 24 hours.
Yes, June 21.

4. Focus question
Why are there more hours of sunlight leading up to summer?
3. Focus question: Why are there more hours of sunlight in the summer?
- Why are there more hours of sunlight in the summer?

5. Calculate hours of sunlight
How many hours of sunlight will we have today?
4. Calculate hours of sunlight
Ask question on slide.
You may need to provide some guidance to help students attack this problem. Write the day’s sunrise and sunset times on the board. Work with students to describe a procedure for calculating hours of sunlight.
Find out the exact time of sunrise and sunset.
Calculate the hours and minutes between sunrise and noon.
Calculate the hours and minutes between noon and sunset.
Add these two figures to get the total hours of sunlight.

6. example Sunrise: 6:24 a.m. Sunset: 6:44 p.m.
Between sunrise at 6:24 a.m. and 11:24 a.m., there are 5 hours. It takes 36 more minutes to get to noon. The time from sunrise to noon is 5 hours + 36 minutes, or 5:36.
Between noon and sunset at 6:44 p.m., there are 6 hours and 44 minutes, or 6:44.
Add the two spans of time: 5:36 + 6:44 = 11:80. Because 60 minutes equals 1 hours, the total elapsed time from sunrise to sunset is 12 hours and 20 minutes or 12:20.
5. Provide an example
Work through an example together.

7. 6. Introduce sunlight for a year
Distribute a copy of notebook sheet 11, Local Sunrise/Sunset Times, which you have prepared with local sunrise/sunset times, to each student. Project notebook master 11, filled in with the same data.
Tell students,
“This table shows the sunrise and sunset times on the 21st of each month this year in our city.”
If you were unable to prepare a table of local sunrise and sunset times, use the generalized data for Berkeley, CA, on notebook sheet 10, Sunrise/Sunset Times for Project notebook master 10, and tell students,
“This table shows the sunrise and sunset times on the 21st of each month during the year 202 in Berkeley, CA.”
TEACHING NOTE: The focus of this activity is on the concept of sunlight variation, not on students’ ability to calculate minutes and hours correctly. If students are frustrated with the math, you might choose to model the math for each item. The goal of the activity is for students to confirm that in the Northern Hemisphere, daylight hours are longest in the summer and shortest in the winter. If you choose to focus on the math as well, converting to military time is sometimes helpful.

8. Calculate and graph 7. Calculate and graph the information
Have students calculate the hours and minutes of daylight on the 21st of each month. Ask them to create a graph to show the amount of daylight, month by month. Discuss what kind of graph would be appropriate for displaying these data. In this case, a bar graph works well. Distribute a copy of notebook sheet 12, Graph of Daylight Hours, to each student, and allow minutes for students to calculate and graph the daylight hours.

9. Review graphs
What does the graph reveal about the amount of daylight in the northern hemisphere over 1 year?
When do people in the northern hemisphere experience the longest daylight?
When do people experience the shortest daylight?
8. Review graphs
Ask questions on slide.
Answers:
Daylight gets longer from January to June, and shorter from June to December.
June
December.

10. Review graphs continued
When do people experience about 12 hours of daylight?
How many hours of daylight do people have on their longest and shortest days?
In what season do people have their longest and shortest days?
8. Continued
Answers:
March and September.
In Berkeley, the longest is 14+ hours; the shortest is 9+ hours.
Longest in summer, shortest in winter

11. Discuss the questions with your group.
9. Discuss global daylight
Project notebook master 13, Day-Length Questions. Ask students to discuss the questions in their groups for about 5 minutes.
10. Review longitude and latitude (optional)
Students learned about longitude and latitude in the first part of Investigation 2, but you might want to review it here. If so, go to “Latitude and Longitude” through FOSSweb, and review the concepts of longitude, latitude, and equator.

12. 11. Introduce “Worldwide Sunrise/Sunset Data”
Help students understand the data in the Science Resources table “Worldwide Sunrise/Sunset Data” in the Images and Data section of the book. Students should conclude that in the summer, locations farthest north received the most sunlight hours and that in the winter, it was the opposite Distribute copies of notebook sheet 13, Day-Length Questions, and project notebook master 13 to guide a class discussion. Students can record ideas on their notebook sheets and affix the sheets into their notebook.
Answers:
No, daylight length varies around the world, depending on latitude.
No, it depends on the latitude of the observer.
No, the longest day in the Northern Hemisphere is the summer solstice on June 21, but the longest day in the Southern Hemisphere is their summer solstice on December 21.
Yes, although summer occurs in different months north and south of the equator.
---POSSIBLE BREAKPOINT---

13. Day length
There are more hours of daylight in summer than in winter. What ideas do you have to explain why daylight hours change over 1 year?
12. Gather ideas about day length
Ask question on slide.
Give students some time to record ideas in their notebooks before answering orally. Some students may have ideas about Earth traveling around the Sun, that winter happens when Earth is farthest from the Sun and summer when it is closest, or that it has something to do with the tilt of Earth’s axis. Few students are likely to have complete ideas.

14. Large Globe 13. Set up large globe
Point out the North Star you positioned on the north wall of the classroom, and explain to students that this paper star represents the actual location of the North Star as viewed from Earth. Remind students that Earth’s North Pole points toward the North Celestial Pole, which is near the North Star.
Clip the lamp into position in the middle of the classroom, and turn it on. Tell students that this lamp represents the Sun in this model. Dim the classroom lights and show students the large globe.
Have students explain how you should move with the globe (in a counterclockwise revolution around the lamp), how the globe should be tilted (always toward the North Star at 23.5°), and what the direction of the globe’s rotation on its axis should be (counterclockwise) in order to make the model accurate.

15. 14. Investigate hours of daylight
Place a dot on the globe to represent your town. Tell students,
“We know that it takes 1 year – 12 months – for Earth to make one trip around the Sun. I’m going to place Earth in four different locations around the Sun, and let’s observe how long the day is in each location. Try to find out where Earth is in its orbit when days are longest during the year.”
Start with the globe positioned as show in this illustration, with the North Pole pointed toward the North Star. Allow students to come close to observe the dot as you rotate the globe slowly in a counterclockwise direction.
Move the globe one-quarter of the way around the lamp in a counterclockwise direction (one-quarter of the way in its orbit around the Sun). Again, allow students to observe the dot as you rotate the globe. Students may need to get up and move around in order to see the globe and dot in each position.
Move the globe a third time, and repeat the observations.
Finally, advance the globe one more quarter, and observe the dot.
TEACHING NOTE: Make sure the North Pole of your globe stays oriented toward your North Star throughout as you reposition it around the lamp.

16. Which way was Earth’s axis tilted?
Where was the globe positioned when our town experienced the longest daylight?
Which way was Earth’s axis tilted?
Where was the globe positioned when our town experienced the shortest daylight?
What way was earth’s axis tilted?
15.Discuss globe observations
Ask students to recall the duration of light at each position of the globe. Move the globe to each position indicated by students.
Answers:
The fourth position in the illustrated example.
The North Pole was tilted toward the Sun.
The second position.
The North Pole was tilted away from the Sun

17. Which position represented winter? Which position represented spring?
The four positions of the globe represent the four seasons. Which position was summer?
Which position represented winter?
Which position represented spring?
Which position represented fall?
15. Continued
The fourth position
The second position.
The third position.
The first position.
TEACHING NOTE:
Students might benefit from some additional work in small groups to reinforce the mechanics of seasons. You could ask students to mark their town on the small globes with a piece of a self-stick note, then move to one of the four positions around the Sun. Keeping the globe oriented toward the North Star, students should rotate it on its axis and discuss what season it is on the globe.

18. Seasons and equinoxes When do you think the autumnal equinox occurs?
16. Introduce the four seasons
Tell students,
“The amount of sunshine falling on our town changes throughout the year. The time when daylight is longest we call summer. The time when daylight is shortest we call winter. Halfway between summer and winter, when daylight hours are about equal to nighttime hours, we call spring and fall. Summer, fall, winter, and spring are the hour seasons, and the length of daylight in each season is predictable.”
17. Introduce equinox
Tell students, “There are 2 days during the year that day and night hours are equal. These days are called equinoxes. Equinox comes from Latin, meaning equal night. One equinox, the spring, or vernal, equinox, occurs around March 21, and it marks the first day of spring. The other equinox is the autumnal equinox, and it marks the first day of fall, or autumn.”
Ask question on slide.
Answer: Around September 22 or 23.

19. solstice When do you thin think winter solstice occurs?
18. Introduce solstice
Tell students,
“The day of the year with the longest daylight is called the summer solstice. The summer solstice occurs on about June 21 and marks the first day of summer. The day of the year with the shortest daylight is called the winter solstice.”
Ask question on slide.
Answer: Around December 21.

20. variables
What variables affect daylight length? Why are there more hours of daylight in summer and fewer in winter?
19. Record vocabulary
20. Review seasons variables
Ask students to think about the long daylight during the summer and the short daylight during the winter. Then ask them to think about why this is the case.
Ask question on slide.
Ask students to talk in their groups to identify the important factors in determining day length Give them about 4 minutes. Call on groups to suggest variables. Write their ideas on the board. Here are the three important variables that students should identify:
The tilt of Earth ‘s axis is always 23.5°
The axis tilt points in the same direction, toward Polaris, the North Star.
The position of Earth changes as it orbits around the Sun.

21. Focus question Why are there more hours of sunlight in the summer?
Record three variables in journal.
21. Answer the focus question
Have students record the three variables in their notebooks beneath the focus question.
- Why are there more hours of sunlight in the summer?

22. explaining
Work with your group to explain how those three variables work together to produce longer summer daylight and shorter winter daylight.
22. Practice explaining day length
Ask students to talk in their groups to explain how these variables work together to produce longer summer daylight and shorter winter daylight. Give them 5 minutes to develop a concise description of the reason for daylight length.

23. The tilt of Earth’s axis means that Earth leans.
Since the tilt is always in the same direction, the lean of the Northern Hemisphere varies from toward the Sun at 23.5° to away from the Sun at 23.5°.
When the Northern Hemisphere is leaning toward the Sun, more of the northern part of the planet gets sunshine.
The position of Earth in its orbit determines how much the lean is toward (or away from) the Sun.
The more the North Pole leans toward the Sun, the longer the days in the Northern Hemisphere.
23. Summarize day length
Call on a student from several groups to explain day length in terms of the three variables. Listen for key ideas that contribute to a comprehensive explanation.
The tilt of Earth’s axis means that Earth leans.
Since the tilt is always in the same direction, the lean of the Northern Hemisphere varies from toward the Sun at 23.5° to away from the Sun at 23.5°.
When the Northern Hemisphere is leaning toward the Sun, more of the northern part of the planet gets sunshine.
The position of Earth in its orbit determines how much the lean is toward (or away from) the Sun.
The more the North Pole leans toward the Sun, the longer the days in the Northern Hemisphere.
After students have provided as many ideas as they can, project teacher master E, Earth/Sun System. Use this graphic representation of the Earth/Sun relationship to summarize the description of the seasons.

24. 23. Continued
“This illustration shows Earth in the four key seasonal positions – the two equinoxes and the two solstices. Earth’s orbit around the Sun creates a plane – a two-dimensional disk-shaped surface – with the Sun in the center. As you can see, Earth is tilted on its axis with respect to this orbital plane. Notice that the tilt makes the North Pole lean. Earth’s North Pole always points in the same direction – toward the North Star. When Earth is here in its orbit (summer solstice), the Northern Hemisphere is leaning toward the Sun. This time of year is summer, and we have our most hours of daylight. Six months later (winter solstice), when Earth is over here in its orbit, the Northern Hemisphere is leaning away from the Sun. This is winter, and we have our fewest hours of daylight. Halfway between summer and winter, we have the two equinoxes, marking the onset of spring and fall. Since the Northern Hemisphere is not leaning toward or away from the Sun, daytime and nighttime are about the same length.”

25. Important factors
Earth is tilted on its axis, and the axis always points in the same direction, toward the North Star.
Earth revolves around the Sun. As a result, the amount of sunlight falling on the Northern Hemisphere changes throughout the year.
When the Northern Hemisphere is leaning toward the Sun, daylight is longer.
Write in your journal (Observation section)
23. Continued
“The important factors that produce our seasons on Earth are:
Earth is tilted on its axis, and the axis always points in the same direction – toward the North Star.
Earth revolves around the Sun. As a result, the amount of sunlight falling on the Northern Hemisphere changes throughout the year.
When the Northern Hemisphere is leaning toward the Sun, daylight is longer.
Point out that all these factors explain seasons regardless of which hemisphere you are in and that none of them have to do with Earth’s distance from the Sun. That is not a factor for seasons on Earth.

26. Review and edit
number your ideas, draw lines of learning, and then write critical reviews of how your thinking about seasons changed.
24. Assess progress: quick write
Return the quick writes that students wrote in Part 1. Ask students to review their ideas and to confirm, correct, or complete their thinking about the reasons for the seasons. They should number their ideas, draw lines of learning, and then write critical reviews of how their thinking about seasons changed. If students need more space than on the original index card or quarter sheet of paper, they can tape their cards on a half sheet of paper and keep writing, but don’t have them affix their quick writes into their notebooks just yet. Collect and review students’ ideas about seasons.
WHAT TO LOOK FOR:
Earth’s orbit around the Sun takes about 365 days and is nearly circular.
Earth’s axis is tilted 23.5° and always points toward the North Star as Earth revolves around the Sun.
In summer, the axis is tipped toward the Sun; and in winter, the axis is tipped away from the Sun.
When neither end of the axis is tipped toward the Sun, it is either spring or fall.
Energy is more concentrated in areas that receive sunlight at a high solar angle and less concentrated in areas that receive sunlight at a low solar angle.
In summer, daylight is longer, and the area receives sunlight at a high solar angle, so it is warmer; in winter, daylight is shorter, and the area receives sunlight at a low solar angle, so it is cooler.
Plan to spend 15 minutes reviewing a selected sample of student responses.
---POSSIBLE BREAKPOINT---

27. Investigate earth’s revolution
Distribute copies of notebook sheet 14, Seasonal Changes, to students. Preview the sheet briefly. Explain that students should use the interactive “Seasons” program through FOSSweb to answer their questions.
The questions on the sheet are difficult. Careful observation and concentration are required in order to understand the geometry. The feature of the simulation that will be very useful is the ability to control Earth’s rotation hour by hour (click by click) at various points in Earth's revolution. In this mode, students will be able to study the movement of a city through daylight and night and to count the hours of light and dark.
The value of this simulation is enhanced when students work in pairs at computers. Show how to get to the simulation, and let them work at individual computers.
If you have only one computer, call a series of students to control the simulation as you work though the questions on the sheet as a class.
Ideally, students should have most of a class period to work with this interactive simulation. If you have extra time for them to use computers, they can start working on the homework assignment or some of the extensions listed at the end of the investigation. Students can also access the program at home by visiting FOSSweb.

28. Return notebook sheet 9 26. Assess progress: response sheet
Return students’ notebook sheet 9, Response Sheet – Investigation 3, from Part 1 and have students write their self-assessments. This is a complex task, as students will need to synthesize all the information they learned about day, night, Earth, the Sun, and seasons.
Scaffold students’ work by leading a class discussion of the key ideas that will help students answer this response sheet. Students can record new ideas beneath a line of learning or with a colored pencil. Give time at the end for students to write a coherent summary of what causes seasons on Earth. Collect students’ work, and consider students’ ideas about the seasons.
WHAT TO LOOK FOR:
Earth’s orbit around the Sun takes about 365 days and is nearly circular.
Earth’s axis is tilted 23.5° and always points toward the North Star as Earth revolves around the Sun.
In summer, the axis is tipped toward the Sun; in winter, the axis is tipped away from the Sun.
Summer is hotter because days are longer, and light is more concentrated due to less beam spreading.
The four changes of season are marked by summer solstice (the longest day of the year in the Northern Hemisphere – June 20/21), fall equinox (September 22/23), winter solstice (December 21/22), and spring equinox (March 20/21).

29. Possible homework
What is the relationship between latitude and day length?
27. Extend the Investigation with homework
One final challenge for students with the “Seasons” online activity can be assigned as homework. Tell students,
“Select the cities between the equator and the North Pole, one by one. Use the Earth View window to step through a day, hour by hour, on the summer solstice. Count and record the hours of daylight on a blank page in your notebook. Repeat the process for the winter solstice. Record the length of day for each city. Graph the results.”
What is the relationship between latitude and day length?
---POSSIBLE BREAKPOINT---

30. reading 28. Read “Seasons on Earth”
Plan time during class to read the article “Seasons on Earth,” as it will help students solidify their understanding of the complex reasons for seasons prior to the benchmark assessment.
Distribute the black and white copies of the reading (notebook sheets 15-18) to students, and have them affix the pages in their notebooks. Tell students,
“As we read this article together, have a pen, colored pencil, and highlighter ready. When we read an important point, highlight or underline that passage of the text on your copies. If you encounter a word you don’t know, circle the word. If you have a question about a part of the reading, or an interesting idea related to the reading, make notes in the margin of the pages.”
Use the guide on the following pages to lead an in-class reading of the article. You or student volunteers can read the article aloud as students follow along. At breakpoints indicated in the reading guide, stop the class for a minidiscussion. You may also want to distribute Science Resources to students so they can see the images in color as they read.

31. Review notebook entries
29. Review vocabulary
30. Review notebook entries
Give groups time to look at their notebook entries and come up with a list of points (including drawings or illustrations) on a piece of paper or chart paper. Each group should explain what they have written on their chart. The charts can be posted around the room for the rest of the class session. Students can add a list of key points to their own notebooks and you can record a class chart.
Here are the ideas that should come forward:
Earth’s round shape results in beam spreading when the Sun hits its surface. This amount of beam spreading changes for each place throughout the year, affecting the heat of that location.
As Earth revolves (orbits) around the Sun, it points toward the North Star. As a result, the amount of sunlight falling on the Northern Hemisphere changes throughout the year.
Summer is when we have more direct sunlight (less beam spreading) and more hours of daylight; winter is when we have less direct sunlight (more beam spreading) and fewer hours of daylight.
You might ask students to draw a line of learning under the last notebook entry and to compose a short statement summarizing what they know about the reason for seasons.