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Chapter 4 SkyGazer CD-Rom Activities

Motions of the Sun

INTRODUCTION In this activity you will learn about the motions of the Sun that can be observed from your location and other locations on Earth. You will see how the motions are similar in some ways to those of the stars but are different in other ways. You also will learn how the Sun’s motions define time measurements and how they cause seasonal changes and weather differences between different locations on Earth.

PART 1: THE SOLAR CLOCK File | Open Settings | Basics | Linked Charts

1. Close all the windows except the one titled Chart 1. Enlarge your screen and bring up the Time Panel.

2. Bring up the Location Panel, extend it, and move the location marker to a place near your location.

3. Set the date to March 21 of this year at noon. You can set the time of day by using the cursor to drag the hands of the clock on the Time Panel.

4. Use the Planet Panel to deselect all objects except the Sun. You can select/deselect objects on the Planet Panel by clicking on their buttons. When the buttons are selected they appear darker gray than when they are deselected. You can also turn all the objects on the Planet Panel on and off at once with the “Hide All Planets/Show all Planets” button. Before you continue, make sure the Sun is selected.

5. Change your horizon to Translucent (if necessary refer to Activity 2 to review how this is done). Also, use Reference Markers under the Display menu to turn on the Horizon line and make sure the Cardinal Points are on. Turn off any other markers.

6. Use the left and right scroll bar or arrow keys to face south and zoom out until your Sky Chart looks similar to the one in Figure 4-1.

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Figure 4-1 Sky Chart facing south at noon on March 21

7. Click on the Sun to bring up its Info Panel. Under the General tab, see Figure 4-2, note the time the sun Transits. Reset your clock (to within the nearest minute) to this time. This is the time of day that the Sun has its highest altitude in the sky. Record the Sun’s transit time, altitude and its rise and set times for March 21 on the RESULTS sheet.

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Figure 4-2 The Sun’s Info Panel

8. Now advance the time with a 1-minute time step and watch the motion of the Sun. Observe the setting point on the western horizon and note whether it is north of west, south of west, or due west. Note: You will have to use your scroll bars or arrow keys to keep the Sun in your Sky Chart view. Record the position of sunset on the RESULTS sheet.

9. Advance time to the next morning, then record the rising position of the Sun on the eastern horizon on the RESULTS sheet. (Again, you will have to use your scroll bars or arrow keys to keep the Sun in your Sky Chart view.)

10. Once the Sun has returned to transit, one solar day has passed. Now change the time step to 1 day and manually advance time. Note the change(s) you observe in the Sun’s midday altitude as you move through the year.

11. Stop advancing time when the date is June 21 and record on the RESULTS sheet the same information you recorded for March 21.

12. Continue advancing time with 1 day step, but stop on September 21 and December 21 and record the information requested on the RESULTS sheet. What change(s) do you notice between June 21 and December 21? Once you have returned to March 21, you have observed the cycle known as a tropical year.

PART 2: THE ANALEMMA Before beginning this section, read about the analemma in Basic Concepts: Date and Time; The Equation of Time and the Analemma in the Sky Guide under the Explore menu in Date and time. 1. Bring up the Planet Panel and click the Sun’s button off and on to center it in your

Sky Chart. Then click on the Sun’s Path. An “X” will appear in the Sun’s row in the Path column when its Path is on. This will display the Sun’s path when you advance the time. Now advance the time in 1-day steps, this time not stopping until a year has

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passed. The pattern you see, the Sun’s path observed once a day, at the same time of day, for a whole year, is called the analemma. Draw the pattern you see on Figure 4-3 on the RESULTS sheet.

2. Now open the settings below to observe another analemma.

File | Open Settings | Earth and Moon | Analemma

PART 3: AROUND THE WORLD 1. Reset your Sky Chart in the same way as you did for Part 1, but this time set the

date to June 21. Opening the setting in Part 2 will have wiped out everything you set up before, so you will need to redo all the settings. You may even want to quit and reopen SkyGazer before you reset.

2. Drag your location marker ⊕ on the Location Panel north and south and observe what happens to the Sun on the Sky Chart.

Observations from a high northern location: 3. Use the location marker to move your location to a high northern latitude, close to

90°N (the North Pole). 4. Center on the Sun by double-clicking on it or by clicking the Sun’s button on the

Planet Panel off and on. 5. Advance the time in 1-minute steps for a whole day and watch the Sun. You will

have to use the left and right scroll bars or arrow keys to keep it in your Sky Chart window. Record your observations on the RESULTS sheet.

Observations from an equatorial location: 6. Now use the Location Panel to change your latitude to a location near 0° latitude,

the equator. 7. Center on the Sun. 8. Once more, advance the time in 1-minute steps for a whole day and watch the Sun.

You will again have to use the left and right scroll bars or arrow keys to follow the Sun. Record your observations on the RESULTS sheet.

Observation from a southern location: 9. Now drag the location marker ⊕ on the Location Panel to a location in the Southern

Hemisphere that is comparable in latitude to your home location. Make your observations of the motion of the Sun and record them on the RESULTS sheet.

Observation from the South Pole: 10. Finally, make and record observations of the Sun from Antarctica, near 90°S, the

South Pole.

PART 4: AROUND THE WORLD AGAIN Return to your home-latitude and repeat Steps 2–10 in Part 3 for December 21 (instead of June 21); then record your observations on the RESULTS sheet. Use what you record to answer the questions on the RESULTS sheet.

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RESULTS SHEET 4 Motions of the Sun

NAME __________________________DATE ___________ SECTION __________________________

PART 1: THE SOLAR CLOCK In the table below, record the Sun’s transit altitude, rising and setting times, and rising and setting positions from its Info Panel for each date as seen from your location. Also, compute the number of hours that the Sun is visible in the sky. Your location (from Location Panel): Longitude: ______________ Latitude: ________________

Date Transit Altitude

Rise Time

Set Time

Hours in the Sky

Rise Position

Set Position

March 21 June 21 Sept. 21 Dec. 21

From inspection of your table, name three ways in which you can observe the passage of a tropical year.

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PART 2: THE ANALEMMA Draw the analemmas that you observed from your location in Figure 4-3.

Figure 4-3 The analemma observed from your location

From where did you observe the other analemma?

PARTS 3 AND 4: AROUND THE WORLD (TWICE) Answer the questions below based on your observations of the Sun’s daily motion at the different latitudes on the different dates. (You may want to redo the steps in Parts 3 and 4 that apply to the specific questions.) What happens to the Sun’s altitude when you travel toward the poles? Toward the equator? What do these observations suggest about the Sun’s effect on weather and climate?

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How long was the Sun up at the North Pole on June 21? On December 21? About how long was the Sun up at the equator on these two dates? Based on your answers to the last two questions, which is a more important cause of warm or cold weather, the amount of time the Sun is in the sky or its altitude? Compare the motion of the Sun you observed from your home latitude to that of the comparable southern latitude on the two dates. What difference did you see and what does this suggest about seasonal variations in the Southern Hemisphere compared to those in the Northern Hemisphere? (Hint: This could be answered in one word.)

CONCLUSION In the space below, write a conclusion for this activity. Briefly explain what you did and what you learned from it.

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CHECK YOUR UNDERSTANDING 4: MOTIONS OF THE SUN

MULTIPLE-CHOICE QUESTIONS 1. In which location would there be a day where the Sun is up for 24 hours straight? a. a middle-north latitude b. the equator c. the North Pole d. [That can never be observed.] 2. In which location would the day with the least sunlight be December 21? a. a middle-north latitude b. the equator c. the South Pole d. [All the locations have the least sunlight on that date.] 3. In which location could the Sun be observed at the zenith? a. a middle-north latitude b. the equator c. the North Pole d. [none of the above] 4. Which location has the least variation in the daily amount of sunlight throughout the

year? a. a middle-north latitude b. the equator c. the North Pole d. the South Pole 5. On which date would an observer at a location in Australia observe the midday Sun

at its highest altitude for the year? a. March 21 b. June 21 c. September 21 d. December 21 6. On which date would an observer on the North Pole not see the Sun at all? a. March 21 b. June 21 c. September 21 d. December 21

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7. What would an observer on the South Pole observe on March 21? a. 24 hours of sunlight b. 24 hours of darkness c. sunrise d. sunset 8. On which date would the Sun appear its lowest when observed from the equator? a. March 21 b. June 21 c. September 21 d. [more than one of the above] 9. On which date would the Sun appear its highest when observed from the South Pole? a. March 21 b. June 21 c. December 21 d. [none of the above] 10. At which location(s) do the most hours of daylight occur over the entire year? a. the poles b. the equator c. mid-latitudes d. [Throughout a whole year, all locations receive the same amount of sunlight.]

OPEN-ENDED ACTIVITY Use the skills you learned in this activity to gather the data to fill out the table below. Plot a graph of the Hours of Sunlight on all four dates. Plot a line on the graph for each location.

Hours of Sunlight March 21 June 21 September 21 December 21 North Pole Equator South Pole