Part 1: Movement of the Sun in the Sky

Introduction

How does the position of the sun in the sky change throughout the day and throughout the year?

People have always looked into the sky and tried to make sense of the patterns they have seen. Just a few hundred years ago, scientists believed that all objects in the “heavens” circled around the Earth. Children often still believe this today because that is what the movement looks like!

To begin this Section, you will make claims about the motions of the sun in the sky.

Guiding Questions

1. Describe and/or draw how you think the sun moves across the sky throughout the day:

a. Where does it rise?

b. Where does it set?

c. Where and when does it reach the highest point in the sky?

2. Describe and/or draw how you think the path of the sun changes throughout the year:

a. Does the position of the sunrise change? If so, how?

b. Does the position of the sunset change? If so, how?

c. Does the position of the highest point the sun reaches in the sky change? If so, how?

Claims

Activity 1: Modeling Earth’s Rotation

In this activity, you will model Earth’s rotation and consider how this relates to the apparent motions of the sun over the course of a day.

a. Turn counterclockwise.

• When is your nose in daylight? When is it in night?

• When is your nose moving from day into night? When is it moving from night into day?

b. Stand with your nose pointed straight toward the sun.

• What time of day is it on your nose?

• What time of day is it on the back of your head?

• What time of day is it on your right ear—sunrise or sunset? (Hint: As you rotate counterclockwise, is your right ear moving to face more toward or away from the sun?)

c. Turn counterclockwise and stop when your right ear is toward the sun.

• What time is it now on your right ear?

• What time is it now on your nose?

d. Continue to explore. For example:

• How much do you have to turn to go forward in time two and a half days?

• How do you have to stand to make it about 3:00 PM on your nose?

3. Draw a diagram (like that in the figure to the right) that shows Earth’s direction of rotation. Then add:

(Not to scale) Hey what does this mean anyway???

• Shading on the nighttime portion of Earth

• Labels at sunrise, noon, sunset, and midnight

4. Record the evidence you have gathered about how the daily path of the sun appears in our sky. Use both words and drawings.

Pause and Reflect

5. Why does the sun appear to move across the sky over the course of a day?

6. Read “Earth’s Rotation.”

Earth’s Rotation

Our ancestors must have observed the motions of the sun, stars, and planets across the sky each day and night and believed that the entire universe was moving around us here on Earth. With the advent of telescopes, astronomers were able to observe that other planets and moons rotate, or spin. This led them to understand that Earth also rotates.

Each planet and moon spins around its axis, an imaginary straight line through the center around which that body rotates. The places where the axis meets the planet’s surface are called poles, or geographical poles. A planet’s axis also determines its equator. The equator is the imaginary circle around the planet at a distance halfway between the poles.

As different parts of the planet turn toward and away from the sun, day becomes night and night turns into day. Because Earth rotates toward the east, the sun rises on the east coast of the United States before it rises on the west coast. The rotation of Earth also is the reason why the sun appears to move across the sky each day, and why the stars and planets appear to rise and set each night.

The rotational rate of a planet determines the length of its day. For example, Earth rotates on its axis once every 24 hours. Some planets, such as Jupiter, have shorter periods of rotation; some, like Mercury, have longer rotation periods.

Checking In

1. As a planet rotates, which moves more—a point near a pole or a point on the equator? Explain.

2. If Earth’s north pole is considered “up,” does Earth rotate clockwise or counterclockwise? Explain.

3. You modeled the Earth’s rotation with your body. Is this a good model? How could you make it “better?” Be specific.

Activity 2: Observing the Sun from Outside Your School

The Celestial Sphere

The celestial sphere is the huge imaginary sphere on which all the objects in the sky, such as the sun and stars, were once considered to be attached. This sphere was thought to be centered on Earth. Although we now know that Earth is not the center of the universe, or even the center of our solar system, it is convenient to think of imaginary celestial spheres to describe the apparent motion of the objects in the sky. Just as Earth has important points or features used for reference, such as the North Pole, South Pole, and equator, the celestial sphere also has reference points.

• The North Celestial Pole (NCP) and the South Celestial Pole (SCP) are the extensions of Earth’s geographical poles into space. Polaris, also known as the North Star, is currently located at the North Celestial Pole.

• The Celestial Equator is Earth’s equator projected outward to the celestial sphere.

From any position on Earth, only half of the sphere is visible at any one time. This is the hemisphere of sky you see above you. If you are at the North Pole, it is the entire northern celestial hemisphere that is visible. Similarly, if you are at the South Pole, you would be able to see the entire southern celestial hemisphere. At points between the poles, one can see part of the northern celestial hemisphere and part of the southern celestial hemisphere.

The horizon, where Earth and sky appear to meet in the far distance, is a circle on the celestial sphere—one that changes depending on your position on Earth. The zenith is the point on the celestial sphere directly above the head of the observer.

Just as geographers use latitude and longitude to describe a location on Earth’s surface, astronomers use the coordinates, azimuth and altitude, to describe a location on the visible celestial hemisphere.

• Azimuth is a measure of how far around the circular base of the hemisphere or how far along the horizon, relative to true north, an object is. It is a horizontal measure from 0 degrees (true north) and around clockwise to 360 degrees. This means, if you are facing north, 0° is in front of you; 90° is at your right shoulder (east); 180° is directly behind you (south); and 270° is at your left shoulder (west).

• Altitude is a measure of how high on the hemisphere or how high above the horizon an object is. It is a vertical measure from 0° (the horizon) up to 90° (zenith).

For example, the position of the sun in the sky can be given as the altitude and azimuth of the sun at a specific time and as viewed from a given location. At both sunrise and sunset, the sun is directly on the horizon, so the sun’s altitude is zero degrees. Between sunrise and sunset, the altitude of the sun is first increasing and then decreasing. The time when the sun is at its highest altitude, as viewed from a given location, is sometimes called solar noon or transit.

Measuring the Altitude and Azimuth of the Sun

1. Use your hand positions to find the sun’s altitude.

a. Find the sun in the sky. DO NOT LOOK DIRECTLY AT THE SUN!

b. Facing the sun, shift your hand positions up in steps, as you did while calibrating, until your hand reaches the sun. The number of degrees you measure is the sun’s altitude.

2. Use a compass and your hand positions to find the sun’s azimuth.

a. Find the compass direction—north, east, south, or west—that is closest to the direction of the sun.

b. Using your hand positions, measure from that direction sideways to the sun. The number of degrees you measure, along with the number of degrees of your initial direction, is the azimuth of the sun.

3. Record the altitude and azimuth of the sun, as well as the date and time of your measurements.

Checking In

1. Describe and draw a diagram showing where on the celestial sphere the sun appears to be if it is at an azimuth of 120° and an altitude of 10°.

2. If the sun is two 10° hands to the left of west, what is the sun’s azimuth?

Activity 3: Analyzing the Sun’s Daily Path at Different Times of Year

In this activity, you will explore the daily path of the sun and if/how it varies over the course of a year.

Part 1: Observing the Sun’s Path Across the Sky at ASTRO UNL.

Change the date slider to your assigned date and 4:00 AM. (Notice the clock is a 24 hour clock.)
Change the latitude to 43.8 N (This is Cumberland, ME).
Unclick “show the Sun’s declination circle” and “show the ecliptic.”
Click START ANIMATION.
Complete the table on the recording sheet provided:

1. Did the sun “rise in the east and set in the west” on the days you observed? Explain.

2. Was the sun at the zenith (highest point) at transit (noon)on the days you observed? Explain.

Part 2: Graphing the Sun’s Altitude

1. Graph your data from Part 1, create a graph of altitude of the sun (y-axis) vs. time of day (x-axis), as measured by hours since sunrise (use the sheet provided).

2. Using your sunrise and sunset times, calculate the length of the day.

3. On each of your graphs, record your other data.

• Label sunrise and sunset, and include the time the sun rises and sets.

• Label the highest altitude reached by the sun on the graph (called solar noon) and include the transit time you recorded.

• Include both the length of day and sunrise azimuth on the graph.

4. Which day is longer, the one with the higher sun altitude or the one with the lower sun altitude?

Part 3: Preparing a Class Display of the Sun Graphs

1. Organize the class’s graphs from Part 2 by date—January to December—and post them as a display.

2. Examine and discuss the display, looking for patterns over the course of the year.

• What are the highest altitudes at transit? The lowest at transit?

• At what time(s) of year is the sun’s altitude at transit the highest? The lowest?

• What time(s) of year is the duration of day longest? Shortest?

• What is the range of sunrise positions? Sunset positions?

• What time(s) of year does the sun rise closest to directly east? Set directly west?

3. Over the course of a year, does the sun ever get directly overhead (have an altitude of 90°) where you live? Explain.

4. How does the sun’s altitude at transit relate to the length of the day?

5. The sun is said to “rise in the east and set in the west.” Do the class’s data fit with this saying? Explain.

6. How do the sun’s rising and setting positions relate to the length of the day?

To conclude this Section, you will revisit your claims about the motions of the sun in the sky.

How does the position of the sun in the sky change throughout the day and throughout the year?

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