Lab 2 Earth Sun Geometry and Seasons PDF

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Second Lab on Earth and the Sun...

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Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society Lab 2: Earth Sun Geometry and Seasons

About the Exercise(s) Readings & Viewings for Lab Exercise: • This Readings sheet • Earth-Sun Geometry • Textbook Chapter 2 • Solar Declination (video) • Video of Solar Declinations and Sun Angles Materials needed for Lab • Textbook • Pencil(s)

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This lab meeting will allow students to strengthen their understanding of Earth-Sun relationships and how they affect seasons on Earth Students will also familiarize themselves with the orbit of the Earth around the Sun.

Learning Goals •

Develop an understanding of: o Solstices and Equinoxes o Solar Angles o Annual cycle of seasons

Key Concepts • •

The Sun provides most of the energy powering the Earth’s weather The Sun has a predictable annual cycle which determines energy input and results in the Seasons

• Key Terms • • • • • • • •

Solstice Equinox Axial tilt Rotation Solar Declination Solar Altitude Angle Beam Spreading Atmospheric Beam Depletion

Page 1 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society The Earth’s Seasons The Sun emits a nearly constant amount of radiation; however on Earth we experience significant changes in the amount of radiation received during the course of a year. These variations in solar radiation are known as our seasons. The seasons are caused by the properties of Earth’s orbit around the Sun, and the consequential change in the receipt of incoming solar radiation, also known as insolation. Earth’s Revolution and Rotation The Earth orbits the Sun once every 365.25 days as if it were riding along a flat plane. We refer to this imaginary surface as the ecliptic plane and the annual trip around the plane is the Earth’s revolution. As Figure 1 shows, the Earth’s orbit is not quite circular but instead sweeps out an elliptical path, so the distance between the Earth and Sun varies over the course of the year. Earth is nearest the Sun at the point called the perihelion, on or near January 4th. In contrast, the Earth is farthest from the Sun at the point called the aphelion, on or near July 4th. However this plays no role in causing the seasons experienced in the higher latitudes.

! Figure 1. Illustration of the Earth's orbit around the Sun, with perihelion and aphelion labeled.

In addition to its revolution, Earth also undergoes a spinning motion called rotation. Rotation occurs every 24 hours around an imaginary line called the Earth’s axis. The axis connects the North and South Poles; the Earth does not orbit the Sun perpendicular to the ecliptic plane, but its axis is instead is tilted at a 23.5° angle. No matter what time of the year it is, the axis is always titled in the same direction and at the same angle. The constant direction and angle of Earth’s tilt means that for half of the year the Northern Hemisphere is tilted somewhat toward the Sun, and for the other half of the year the Northern Hemisphere is tilted away from the Sun. The changing orientation of hemispheres to the Sun’s radiation is the cause of the seasons, not the varying distance between the Earth and Sun. Solstices and Equinoxes During 6 months of the year, the Northern Hemisphere receives more insolation than does the Southern Hemisphere; during the other 6 months, the Southern Hemisphere receives a greater amount of insolation. Figure 2 shows the Earth’s orbit around the Sun, with four particularly important points marked.

Page 2 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society

Figure 2. The solstices and equinoxes of the Earth's orbit around the Sun. The Earth's axial tilt combined with its rotation around the Sun causes the seasons we have on Earth. Note how the orientation of that axis never changes – it is always pointing to the star Polaris.

In the farthest left position in Figure 2, the Northern Hemisphere has its maximum tilt toward the Sun. This occurs on or about June 21, which we refer to as the June solstice or summer solstice. We designate this day as the first day of summer; however it actually represents the day on which the Northern Hemisphere experiences the most insolation. Six months later, on or around December 21, the Northern Hemisphere experiences the least insolation on the December solstice or winter solstice. This is the first day of winter here in the Northern Hemisphere. In between the solstices are the March equinox (spring equinox) on or around March 21, and the September equinox (fall equinox) on or around September 21. On the equinoxes every place on Earth has 12 hours of day and night, and both hemispheres receive equal amounts of energy. Axis Tilt and Solar Altitude Angle The annual change in the relative position of the Earth’s axis in relationship to the Sun causes the height of the Sun to vary in our skies. The latitude on the Earth where the Sun is directly overhead at solar noon is known as the solar declination. This location is at the equator during the two equinoxes. On these dates, neither hemisphere is pointing toward the sun (fig 2). From the March equinox to the June solstice, the solar declination moves northward to 23.5°N latitude (the Tropic of Cancer). Then, the declination begins moving south again until it reaches 23.5°S latitude (the Tropic of Capricorn) at the winter solstice. The area between the two latitudes is known as the Tropics, and these are the only areas where the sun’s altitude angle ever reaches 90 degrees. Those boundaries are due to the 23.5° tilt of the Earth’s axis.

Page 3 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society Part 1: To fill in Table 1 use the Solar Declination video for Lab 2 by clicking the link here or on D2L. 37 points Start and stop the animation as needed as you fill out the first three columns. You will find the solar declination in the big frame on the left, the sun’s altitude in the lower frame on the right. For the last two columns, the stick figure is at 0.0 latitude – the equator. Play the animation again, and fill in the last two columns. Table 1 (20) Date

Solar Declination

Sun’s Altitude in Carbondale 37.7oN

Sun’s Altitude on Equator 0.0 Latitude

What direction do the Sun’s rays come from?

What direction do the Sun’s rays come from??

December 21

23.5 oS

28.9o

S

66.5o

S

January 1

22.9 S

29.4

S

67.2

S

February 1

17 S

35.2

S

73

S

March 1

7.7 S

44.6

S

82.4

S

March 21

0o

52.3o

S

90o

-- (straight up)

April 1

4.1 N

56.4

S

85.8

N

May 1

14.9 N

67.2

S

75

N

June 1

22 N

74.2

S

68

N

June 21

23.5oN

76.5o

S

66.5o

N

July 1

23 N

75.3

S

67

N

August 1

17.5 N

69.8

S

72.6

N

September 1

7.3 N

59.6

S

82.9

N

September 21

0o

52.3o

S

90o

-- (straight up)

October 1

4.3 S

48

S

85.5

S

November 1

15.3 S

36.9

S

74.8

S

December 1

22.2 S

30.1

S

67.9

S

December 21

23.5 oS

28.9o

S

66.5o

S

Note that this animation uses 23.4 rather than 23.5 for the tilt – to keep things consistent with the course lecture and readings, I have filled in certain values using 23.5 (which will be different than what the animation will show if you stopped it on those dates).

Page 4 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society Questions about the animation and your entries in Table 1: 1. Looking at column 2, describe how the sun’s altitude angle in Carbondale changes over the course of the year? (3) The altitude angle in Carbondale increases until the Summer Solstice and then decreases until the Winter Solstice.

2. Using the information in columns 2-4, describe two major differences between the sun’s altitude angle in Carbondale and the angle experienced at the Equator? (4) Two major differences between the sun’s altitude angle in Carbondale and the angle at the Equator are that the suns rays only come from the South in Carbondale while at the Equator they come from the North after the Spring Equinox and from the South after the Fall Equinox. Another major difference is that the sun’s altitude angle is greater at the Equator than in Carbondale except for in June and July.

3. What is the difference between the solar declinations on the following dates (in degrees) (use the data from Table 1 Column 1), and what direction is the subsolar point moving between those dates? Fill in your answers in Table 2 (3) Table 2 Difference (degrees latitude)

Direction of movement of subsolar point (N or S)

a. Dec 1 and Dec 21

23.5 S - 22.1 S = 1.4 degrees

S (moved from 22.1S to 23.5S)

c. March 1 and March 21

7.7 degrees

S

e. June 1 and June 21

1.5 degrees

N

g. Sept 1 and Sept 21

7.3 Degrees

S

4. The word “Solstice” means “Solar stand-still,” explain why using some information in the tables and the visualization to support your explanation (3) Solstice means Solar stand-still because on the Winter and Summer solstice latitudes poleward of 66.5 degrees go through either 24 hours of daylight or 24 hours of Night depending on the solstice occuring.

5. Areas such as Northern Alaska and Norway that are north of the Arctic Circle experience 24 hours of daylight on the days or weeks around the summer solstice, and total darkness for days or weeks during the winter. Explain why this occurs. (4 pts.)?

Page 5 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society These areas experience 24 hours of daylight or total darkness for days or weeks because the latitude of the Arctic Circle is 66.5 degrees and all latitudes higher than 66.5 degrees encounter a solar angle of 90 degrees which results in either 24 hours of light or 24 hours of darkness depending on the hemisphere and whether it’s the summer or winter solstice.

Page 6 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society Part 2 Beam Spreading and Atmospheric Beam Depletion Figure 3 The sun's rays all arrive at the earth from the same direction. The curvature of the earth results in a varying solar altitude angle at different latitudes and at different times of the day and year. The solar angle determines the amount of beam spreading and atmospheric beam depletions (and therefore the reduced amount of energy received).

As you can see in Figure 3 above (even though it is not to scale), the sun is much larger than the earth. The sun’s energy all arrives from the same direction, but strikes the planet’s surface at a different angle depending on latitude and season. Whe

re does the variation in solar altitude angle come from? It comes from the fact that the planet is (nearly) spherical. The curvature of the surface of the earth means that it bends away from the incoming light. This affects how we on the surface see the sun angle. The sun angle varies through the day, from zero at sunrise, through the highest value at noon, back to zero at sunset. Solar angle also varies throughout the year. If we consider Carbondale, at a latitude of 37.7oN, the noon solar angle on the summer solstice is about 76o above the horizon (it’s not ever directly overhead here), and at the December solstice it is only around 29o above the southern horizon. Figure 4 will help you visualize the way changes in the solar angle result in beam spreading, reducing the energy received at the surface as the angle decreases with latitude.

Page 7 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society

Figure 4 Beam Spreading increases as latitude increases (and as solar angle gets smaller). You can see the area illuminated by the red dot gets larger as the dot moves farther north, and the earth curves away from the beam of light. The same amount of energy covers a larger area, so that area is receiving less energy per square meter. On the left, we see concentrated energy that occurs with a high sun (large solar angle). In the middle, we have moved further north, and the solar angle is smaller – resulting in decreased energy per square meter. Finally, at the very high latitudes, the sun does not get far above the horizon even in the summer, so energy spreads out even more thinly. Even with long summer days in the high latitudes, the sun’s energy is weak due to beam spreading and atmospheric beam depletion.

Atmospheric Beam Depletion The variations in solar angle result in another modification of how energy reaches the surface. Sunlight must pass through the atmosphere, and some of that light is reflected away by matter in the atmosphere. Clouds obviously prevent light from making it to the surface, but so do pollutants and natural aersols in the atmosphere, such as sea salt, smoke, and dust. When the solar angle is low, the sun’s light must pass through more of the atmosphere (Figure 5), giving it a higher likelihood of encountering something in the air, and allowing the atmosphere to “deplete the beam of energy” or reduce the energy received at the surface. Like beam spreading, the solar angle causes atmospheric beam depletion, but unlike beam spreading, atmospheric beam depletion is due to the interaction of energy with the atmosphere that increases with lower solar angles.

Page 8 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons

Southern Illinois University, Department of Geography and Environmental Resources

GEOG 104: Weather, Climate, and Society Using the information in this lab and the Topic 3 lectures, answer the following questions: 1. Assume it is March 21, the March Equinox. The solar declination is therefore at the Equator, 0o latitude, and so the solar altitude angle at the Equator at noon is 90o. Describe the relationship between the solar altitude angle and latitude that occurs on the Equinox (ie, how does it change as you move to higher latitudes – away from the equator? Is it the same for both the northern and the southern hemispheres?). (2) The solar altitude angle decreases as you move toward higher latitudes. The angle remains the same for the Northern and Southern Hemispheres.

2. Describe the relationship between solar altitude angle and atmospheric path length. (1) A high solar angle allows sunlight to pass through the atmosphere with a relatively short path. Lower sun angels requires the energy to go through more of the atmosphere.

3. On the June Solstice, where (at what latitude) is the solar altitude angle 90o ? (1) 40 degrees N 4. Figure 5 above, represents the Earth-Sun relationship on what day of the year? (1) Winter Solstice, December 21st Watch these two videos Timelapse of Norway in 24 hours of Daylight and Explainer Demo on Norwegian 24 Hours of Sunlight and answer the questions below. 5. Describe the change in the solar angle throughout the day in Spitzbergen, Norway on the Solstice (as seen in the first video). (3) At midnight the solar angle is close to 1 degrees as the day progresses, the solar angle increases until noon when it is at about 23 degrees, after noon the angle begins to decrease again until midnight.

6. With 24 hours of sunlight, why is it still cold in Spitzbergen, Norway in the summer? Explain in terms of solar angle, beam spreading and beam depletion. (5) The temperature in Spitzbergen remains cold even when it is exposed to 24 hours of sunlight because it’s latitude is 77 degrees. This means the solar angle encountered at Spitzbergen is lower, which increases the chances of beam depletion. Beam depletion reduces the energy that is received at the surface, which also means the temperature will be lower. The lower solar angle also increases the beam the spreading, the increased beam spreading means the energy from the beam is diapersed further than at latitudes closer to the equatorr.

Page 9 ! Geography 104 Lab Worksheet Earth-Sun Geometry and Seasons...