Part 2: Solstices, Equinoxes, and Seasons

Part 2: Solstices, Equinoxes, and Seasons

How and why does the daily path of the sun across the sky change for different locations on Earth?

Introduction

In Section One, you saw how the daily path of the sun is different throughout the year but you only looked at Cumberland, ME and you determine why this change happens. In this Section, you will explore why both time of year and one’s location on Earth affect the apparent motions of the sun and why this is important.

To begin this Section, you will make claims about how the daily path of the sun changes if viewed from different locations on Earth.

Activity 1: Modeling Earth’s Tilt and Its Revolution Around the Sun

In this activity, you will explore models of the Sun-Earth system and consider what they show about the apparent motions of the sun as viewed at different times of year and from various locations on Earth.

Part 1: Modeling Earth’s Movements in Space

Part 2: Modeling Sunlight on Earth Using a Globe and Flashlight

Follow the class demonstration.

Part 3: Modeling Earth’s Tilt and Revolution

Follow the class demonstration.

1. Set up a “sun” on a stand in the middle of an open table. Position the globe on one side of the table so that its North Pole is tilted toward the sun. Note toward which edge of the table the North Pole points. You will need to keep it pointed toward this edge throughout the model.

2. Move the globe counterclockwise around the sun, keeping the axis tilted toward the same table edge and keeping the globe about the same distance from the sun.

4. Draw a diagram of Earth’s orbit around the sun and label it with information from the model. (Sheet provided)

a. Label where the North Pole is tilted most directly toward the sun, where the South Pole is tilted most directly toward the sun, and where the axis is “sideways” to the sun.

b. Label where along the orbit:

5. Read: Earth in Space.

The apparent movements of the sun across the sky are caused by the movements and positioning of Earth in space, as well as by the rotation of Earth on its axis.

• Earth rotates on its axis. One rotation, which takes 24 hours, defines one day.

• Earth revolves around the sun. One revolution, which takes 365.25 days, defines one year.

The term “orbit” is also used to describe Earth’s motion around the sun. To orbit is to revolve or go around, and the path of Earth as it revolves around the sun—indeed, the path traveled by any astronomical body around another—is called an orbit. The term “orbit” is thus both a noun—the path—and a verb—to travel the path.

The ecliptic is the plane of Earth’s orbit around the sun extended to meet the celestial sphere.

• Earth’s axis is tilted relative to the ecliptic. The angle of this tilt, which is about 23.5°, stays the same relative to the celestial sphere throughout Earth’s orbit.

The angle stays the same, but Earth’s position in its orbit around the sun changes. Thus, Earth is oriented differently relative to the sun at different times of year. This changing angle of Earth’s axis relative to the sun is the cause of seasons on Earth.

For part of the year, the North Pole is tilted generally toward the sun, while the South Pole is tilted generally away from the sun. Then, during another part of the year, this reverses, with the South Pole tilted generally toward the sun and the North Pole tilted generally away.

When a pole is tilted toward the sun, the sun never sets at that pole. In other words, the sun is visible in the sky all the time. For this reason, the sun in the far north in the summer is called the “midnight sun.” The midnight sun isn’t limited to just the poles. Areas near the poles also experience this perpetual day in the summer months.

Earth’s rotation causes the sun to appear to move across the sky over the course of a day. At the poles, the sun appears to move in a circle, parallel to the horizon. For all other locations on Earth, the sun’s apparent path is tilted relative to the horizon. For most locations, this tilt is enough to make the sun rise above and drop below the horizon, causing day and night. However, for areas near the poles, the tilt is sometimes not enough to do this. Instead, the sun dips close to the horizon, but doesn’t drop below it.

6. As you learned in Section One, the apparent daily path of the sun changes over the course of the year. Explain what causes this, giving examples from the models.

7. Is the daily path of the sun the same for every location on Earth? Explain this, giving examples from the models.

8. Critique the models from this activity. How are they appropriate? What faults do they have?

Checking In

9. Describe how Earth moves over a period of 30 days. Use the terms rotate, revolve, and orbit.

10. How does the apparent path of the sun across the sky differ near the poles from other points on Earth?

Activity 2: Observing the Sun from the Poles and Equator

In this activity, you will investigate the path of the sun as viewed from different locations on Earth. You will begin by gathering data from ASTRO UNL. Then you will analyze your data, connecting it to the models from Activity 1 and the movements of Earth through space.

Engage:

• Choose your birthday date
• Unclick “show the Sun’s declination circle” and “show the ecliptic.”
• Observer’s latitude = 90° N (North Pole) -- Click START ANIMATION.
• Change the latitude to 90° S. (South Pole) -- Click START ANIMATION.
• Change the latitude to 0°. (Equator) -- Click START ANIMATION.

1. Was the sun visible from the North and South Poles on your birthday? Date: Visible: N S

Part 1: The Poles and the Equator

2. Explain and/or draw a picture of the sun’s path as viewed from a pole.

3. How does this relate to the models from Activity 1?

4. What is the highest altitude the sun reached in the sky at transit (noon) as viewed from the equator? The lowest?

Part 2: Analyzing the Sun Data from the Poles and Equator

4. Add dates to your diagram from Part 3 of Activity 1.

Pause and Think

Activity 3: Solstices and Equinoxes

Follow the class demonstration.

Read: Solstices and Equinoxes

The seasons we experience here on Earth occur due to the tilt of Earth’s axis of rotation relative to the ecliptic, which is the plane of Earth’s orbit around the sun. There are four special points on Earth’s path around the sun. These correspond to four days of the year—the equinoxes and solstices.

Twice a year, Earth reaches a point in its orbit (points B and D) when neither the North Pole nor the South Pole is tilted toward the sun. On these two days—one in the fall and one in the spring—sunlight covers Earth from pole to pole; the sun is directly overhead at the equator; and the length of day and the length of night are equal—12 hours apiece— everywhere on Earth. These days are known as the equinoxes. There is a fall equinox, in September in the northern hemisphere, and the spring equinox, in March in the northern hemisphere.

On two other days in the year, Earth reaches a point in its orbit (points A and C) when one of its poles points at its maximum tilt toward the sun. On the two days this happens, the most sunlight possible is reaching either the northern or southern hemisphere; the sun is directly overhead at the most northern or southern latitude possible; and the difference between the length of day and the length of night is at its greatest. These days are known as the solstices. In the northern hemisphere, the winter solstice is in December and the summer solstice is in June. The most northern latitude where the sun can be directly overhead, 23.5° north, is known as the Tropic of Cancer. The most southern latitude where the sun can be directly overhead, 23.5° south, is known as the Tropic of Capricorn. The sun can only be directly overhead between the Tropic lines.

1. As viewed from the equator, how does the sun appear to move across the sky on the equinoxes? On the solstices?

2. What is special about the two imaginary lines on Earth—the Tropic of Cancer and the Tropic of Capricorn?

3. Add solstice and equinox labels to your diagram from Part 3 of Activity 1.

4. Where on Earth can the sun be directly overhead?

5. When is the sun directly overhead at various locations?

6. When is the sun highest in the sky at transit where you live? Lowest in the sky at transit where you live?

7. Explain the characteristics of the solstices and equinoxes in terms of Earth’s position in its orbit, Earth’s tilt relative to the sun, the amount of sunlight reaching the poles, where on Earth the sun appears to be at the zenith at transit, and anything else you can identify.

Activity 4: Reasons for the Seasons

Discussion:

1. Can changes in Earth’s distance from the sun be the cause of the seasons? Explain.

Follow the class demonstration.

As Earth moves around the sun, the angle between its axis of rotation and the ecliptic changes, and this causes Earth’s seasons. During the portion of the year when the northern hemisphere is tilted toward the sun (around position A), the northern hemisphere experiences summer, while the southern hemisphere—which is tilted away from the sun—experiences winter. The reverse is also true. During the portion of the year when the northern hemisphere is tilted away from the sun (around position C), the northern hemisphere experiences winter, while the southern hemisphere—which is tilted toward the sun—experiences summer.

Being tilted toward or away from the sun matters because it affects the amount of sunlight or electromagnetic radiation reaching a region. Not only does sunlight not reach all locations at all times of year, but even the areas receiving sunlight experience different amounts.

Part of why different areas on Earth receive different amounts of sunlight is simply the result of day length. Days are longer in the hemisphere tilted toward the sun, and longer days means more sunlight reaching that hemisphere.

In areas on Earth where the sun is high in the sky, the sunlight comes straight down and the rays are concentrated on a small region, so that region receives more sunlight. In areas where the sun is lower in the sky, the sunlight comes in at a lower angle and the rays are spread over a larger region, so that region receives less sunlight.

In turn, the amount of sunlight reaching an area affects the general temperature of that area. When light from the sun reaches Earth, some of that electromagnetic radiation is transformed into heat energy. The more sunlight reaching a region, the more energy is being received and transformed into heat, making the temperatures generally warmer.

Consider the following example. It is June 21, the solstice, and the sun is directly overhead at the Tropic of Cancer (23.5° north).

• At the location on the Tropic of Cancer where the sun is currently at transit, sunlight is coming straight down (from an altitude of 90°). This means that a large amount of sunlight is concentrated on that latitude, much of that sunlight is being transformed into heat, and the temperatures are generally high.

• At transit time for a more northern location, such as at 66.5° N, the sunlight is not coming straight down. It is coming in at a lower angle. However, while the angle of the sunlight is lower than at the Tropic of Cancer, it is still the highest angle that is ever experienced at 66.5° N! The resulting concentration of sunlight causes generally higher temperatures. It is the beginning of summer in the northern hemisphere.

• At transit time for a southern hemisphere location, such as at 23.5° S (the Tropic of Capricorn), the sunlight is coming in at the lowest angle that is ever experienced at that location. The amount of sunlight reaching the area is therefore also at its lowest, and the temperatures are generally lower than at other times of year. It is the beginning of winter in the southern hemisphere.

Some people wonder why the summer solstice is the beginning of summer and not the middle of summer. This seasonal lag is caused primarily by Earth’s oceans. Because of the nature of water, the oceans heat up and cool down very slowly. It takes a lot of energy to heat water, and it takes the loss of a lot of energy to cool water. Therefore, on the solstices, the oceans are still in transition from cool to warm or warm to cool, and the peak cool or warm temperature for each season occurs about a month and a half after the solstice occurs.

Many people mistakenly believe that summer occurs when Earth is at the closest point in its orbit around the sun, and winter occurs when Earth is at its farthest point from the sun. This doesn’t make sense, because when it is summer in the northern hemisphere, it is winter in the southern hemisphere, yet the distance between Earth and the sun is the same for both hemispheres. In fact, Earth is at its closest to the sun in January, which is winter in the northern hemisphere.

Checking In

1. Why do different areas on Earth receive different amounts of sunlight at different times of year?

2. What is the relationship between the height of the sun in the sky and the seasons?

3. Summarize what you know about the sun-Earth system during different seasons.

Make a chart and include the following: (sheet provided)

The altitude of the sun at transit

Include:

How and why does the daily path of the sun across the sky change for different locations on Earth?

To conclude this Section, you will revisit your claims about how the daily path of the sun changes if viewed from different locations on Earth.