Introduction

Chapter 2. Rotation and Revolution

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Author: Scott Miller, Pennsylvania State University

Editor: Grace L. Deming, University of Maryland

Polaris Trail

The goals of this module: At the end, you should be able to:

  1. Explain how the rotation and revolution of Earth affect what we see in the sky.
  2. Determine the time of day based on the position of the Sun.
  3. Understand the difference between a solar day and a sidereal day.
  4. Realize why stars appear to rise and set a little earlier each night.
  5. Explain why some stars never appear to rise nor set, and determine which stars behave this way.

In this module you will explore:

  1. How Earth's rotation affects the apparent motion of the Sun and stars.
  2. How Earth's revolution around the Sun causes the positions of the stars to change over time.
  3. How an observer's position on Earth determines whether a star will appear to rise and set or not

Why you are doing it: People have been observing the motions of the Sun and stars since before recorded time. Our basic units of time, such as a day and a year, are determined by their apparent motions. Navigators also use the positions of the Sun and stars at different times of the day and year to determine their location on Earth. By understanding how the apparent motions of these objects are caused by the rotation and revolution of Earth, an observer can understand why the appearance of the sky changes daily and annually.

Probably the most noticeable effect of the Sun on the Earth is that it causes the periods of day and night. As Earth rotates, the half of it which is illuminated by the Sun changes. When viewed from above Earth's North Pole, Earth appears to rotate counter-clockwise. To make it clear that the Sun illuminates the day side of the Earth, this animation has shrunk the size of the Sun and moved it closer to Earth.

Click on play and watch the Earth rotate.

People on the side of Earth facing the Sun experience day, while people on the other side experience night. As Earth continues to rotate and we cross from the illuminated side to the unilluminated side, the Sun appears to set below our horizon. As we cross from the unilluminated side to the illuminated side, the Sun appears to rise above our horizon.

Day and Night Animation

Question 1.

Assign the letters, shown in the animation, to the corresponding time of day:

A:

B:

C:

D:

3
Incorrect. Try again.
Correct. Noon is defined as the time when the Sun is highest in the sky, and midnight is the middle of the night. Sunset occurs as we go from day to night, and sunrise occurs as we go from night to day.
Incorrect. Noon is defined as the time when the Sun is highest in the sky, and midnight is the middle of the night. Sunset occurs as we go from day to night, and sunrise occurs as we go from night to day.

As Earth rotates about its axis, the Sun appears to rise and set each day. This is not due to an actual motion of the Sun, but rather due to Earth's rotation.

Rotation of the Earth Animation

Question Sequence

Question 2.

Watch the red dot on the surface of Earth. It takes 24 hours to complete one full rotation about Earth.

A.
B.

3
Incorrect. xxxxxxxxxx Try again.
Correct. The length of time between successive noons is, on average, 24 hours. This is known as a solar day. A solar day is the amount of time it takes Earth to complete one full rotation relative to the Sun.
Incorrect. The length of time between successive noons is, on average, 24 hours. This is known as a solar day. A solar day is the amount of time it takes Earth to complete one full rotation relative to the Sun.

Summary

Because Earth rotates west-to-east, objects in the sky appear to move in the opposite direction (east-to-west). When observing the Sun over the course of a day, you will notice that it rises in the eastern sky, moves across the sky, and sets in the western sky. You may have heard the sentence: "The Sun rises in the east and sets in the west." This summarizes the general motion of the Sun across the sky on a given day.

I'm sure you've noticed that the Sun is not the only object observable in the sky. At night, when the Sun sets below the horizon (because Earth blocks our view of it), it is possible to see lots and lots of stars. Visual groupings of stars are known as constellations. Astronomers use constellations as a way of defining regions of the sky, such that when we discover new objects in space, we can say in what general area they are located.

Constellations Animation

The red dot is located on the state of California.

Question Sequence

Question 4.

Approximately what time is it in California according to the animation?

A.
B.
C.
D.
E.

3
Incorrect. Notice that the dot is not too far past sunset. Try again.
Correct. It's 8 PM, since the dot is located roughly 1/6th of the way past sunset.
Incorrect. It's 8 PM, since the dot is located roughly 1/6th of the way past sunset.

Summary

As you can see, the stars at night move across the sky from east to west, just like the Sun does during the daytime. This is because both apparent motions are due to the same actual motion, the Earth's rotation. We can use a similar general statement for the stars as we did for the Sun in the previous section: "Stars rise in the east and set in the west."

It turns out, though, that while Earth is rotating once a day, it is also revolving around the Sun once a year. The animation will demonstrate the motion of Earth about the Sun during one rotation about its axis (this motion will be slightly exaggerated for effect). Click the "Play" button to begin this animation. Keep in mind that in reality the stars in the constellations are a LOT farther away than shown.

Revolution of Earth Animation

Question Sequence

Question 8.

After one 360° rotation of Earth, is Andromeda still directly overhead for a person at the red dot? After one 360° rotation of Earth, is the time once again noon for a person at the green dot?

A.
B.
C.
D.
E.

3
Correct. Andromeda is overhead again for a person standing on the red dot, but for a person standing on the green dot, it is not quite noon yet.
Incorrect. Andromeda is overhead again for a person standing on the red dot, but for a person standing on the green dot, it is not quite noon yet.
Try Again. Watch the animation closely. After one 360° rotation is Andromeda directly overhead for a person on the red dot? Is the Sun highest in the sky for a person on the green dot?

Summary

Because Earth is revolving around the Sun at the same time it is rotating on its axis, the constellations will rise and set at different times throughout the year. In general, you can say that "Stars rise a little earlier each night."

Over the course of time, as Earth revolves around the Sun, the stars will appear to rise earlier and earlier each night, as you discovered previously. In the animation below, Earth revolves a portion of the way around the Sun, such that two months time elapses.

Press play and wait until the animation stops.

The Sky at Different Times of the Year Animation

Question Sequence

Question 10.

At this point in the animation, what constellation is overhead at midnight?

A.
B.
C.
D.
E.

3
Correct. A person at midnight would observe the constellation of Perseus overhead at this position.
Incorrect. A person at midnight would observe the constellation of Perseus overhead at this position.
Try Again. Midnight is the location opposite from the Sun. If you drew a straight line through the green and red dots on the Earth into the night sky, which constellation would it hit?
Polaris Trail

While many stars will appear to rise and set,not all of them will, when viewed from most places on Earth. In the northern hemisphere,as we look north, we notice that some stars will always be above the horizon. The star Polaris holds a unique position in the sky in that it sits almost directly above Earth's North Pole. It's not a particularly bright star(in fact, it can be a little hard to find if you don't know where to look), but it's very useful because of its location above the North Pole.Due to Polaris' location, as Earth rotates, all of the stars appear to move except Polaris.Polaris appears to remain fixed in the sky,and the other stars appear to revolve around it.

Question 14.

In which direction do the stars appear to revolve around Polaris?

A.
B.
C.
D.

3
Correct.
Incorrect.
Try Again. From which direction do stars rise? Towards which direction do they set?

It turns out that, using simple geometry; it's easy to demonstrate that the altitude above the horizon at which Polaris sits is equal to the observer's latitude on Earth. A person at 35° N will observe Polaris 35° above the northern horizon. Stars close to Polaris will appear to move counter-clockwise around Polaris, never rising nor setting. We call these stars circumpolar stars.

Question Sequence

Question 15.

In the animation above, navigate to direction 270° (North) and view from earth.

Circumpolar stars are always near Polaris.

A.
B.

Correct. Since the stars appear to revolve around Polaris, this region would be described by a circle on the celestial sphere centered on Polaris. All of the stars within this region would be circumpolar stars.
Incorrect. Since the stars appear to revolve around Polaris, this region would be described by a circle on the celestial sphere centered on Polaris. All of the stars within this region would be circumpolar stars.

Let's travel now to a different location and see how many circumpolar stars can be observed at other latitudes.

Select "view from earth", change direction to 270° and change to a lower latitude (but still within the northern hemisphere).

How do the number of circumpolar stars observed by this person compare to the number of circumpolar stars observed by the person at 35° N latitude?

Now change to a higher latitude than 35° N and observe the number of circumpolar stars observed. (You may have to adjust the "lookup angle" so that you'll be able to see Polaris).

Question Sequence

Question 17.

How does the number of observable circumpolar stars depend on the observer's latitude? As the latitude increases, the number of stars

A.
B.
C.

3
Correct.
Incorrect.
Try again.

Indepth Activity: Rotation and Revolution

Question 20.

A sidereal day is:

A.
B.
C.
D.
E.

Correct. A solar day is about 4 minutes longer.
Incorrect. A solar day is about 4 minutes longer.

Question 21.

Stars set on the western horizon:

A.
B.
C.
D.
E.

Correct. Because they rise a little earlier, they must set a little earlier.
Incorrect. Because they rise a little earlier, they must set a little earlier.

Question 22.

Polaris is important to astronomers because it:

A.
B.
C.
D.
E.

Correct. Polaris is important to astronomers because it marks the point in the sky above the Earth's North Pole.
Incorrect. Polaris is important to astronomers because it marks the point in the sky above the Earth's North Pole.

Question 23.

You observe the constellation Cygnus on the eastern horizon at 9 PM. One hour later, at 10 PM, you expect to see Cygnus

A.
B.
C.
D.
E.

Correct. Stars appear to move from east to west.
Incorrect. Stars appear to move from east to west.

Question 24.

Where on Earth would you be located if you were unable to observe any circumpolar constellations?

A.
B.
C.
D.

Correct. All stars are observed to rise and set at the equator, so none are circumpolar.
Incorrect. All stars are observed to rise and set at the equator, so none are circumpolar.

Question 25.

The constellation of Andromeda is directly overhead California at midnight. Where would a person in Washington, D.C. look to view Andromeda at the same time (9 PM eastern time)?

A.
B.
C.
D.
E.

Correct. California is to the west of Washington D.C., so the constellation overhead would be in our western sky.
Incorrect. California is to the west of Washington D.C., so the constellation overhead would be in our western sky.

Question 26.

As you travel from the North Pole to the Equator, the number of circumpolar constellations

A.
B.
C.
D.

Correct. At the North Pole, all stars are circumpolar and at the equator there are no circumpolar stars, so as you go toward the equator there must be fewer circumpolar stars.
Incorrect. At the North Pole, all stars are circumpolar and at the equator there are no circumpolar stars, so as you go toward the equator there must be fewer circumpolar stars.

Question 27.

If the center of the constellation Andromeda is directly overhead at midnight tonight, when will it be directly overhead at midnight next?

A.
B.
C.
D.
E.

Correct. Stars rise about 4 minutes earlier each night, so 365 days later they will be at the same position at the specified time.
Incorrect. Stars rise about 4 minutes earlier each night, so 365 days later they will be at the same position at the specified time.

Question 28.

A star is observed to be directly below Polaris at 6 PM Where do you expect to find the star later that night at 6 AM (twelve hours later)?

A.
B.
C.
D.
E.

Correct. It has appeared to move half of its 24-hour path.
Incorrect. It has appeared to move half of its 24-hour path.

Question 29.

The fact that stars appear to rise a little earlier each night is due to

A.
B.
C.
D.
E.

Correct. Rising and setting are due to rotation; the earlier rising time is due to revolution.
Incorrect. Rising and setting are due to rotation; the earlier rising time is due to revolution.

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