Chapter 2. Rotation and Revolution

2.1 Introduction

AstroTutorials

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To advance to the next page of the tutorial you need to submit every question; currently you have not finished all the questions on this page. Leaving a tutorial page without submitting all the questions results in you receiving no grade in the gradebook.

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Author: Scott Miller, Pennsylvania State University

Editor: Grace L. Deming, University of Maryland

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

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

In this module you will explore:

How Earth's rotation affects the apparent motion of the Sun and stars.
How Earth's revolution around the Sun causes the positions of the stars to change over time.
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.

2.2 Day and Night

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.

Question 2.1

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

A: M0lHbWRg6CHtK/KODKe7eN5ZGYg1ffW2odm/4JjRyzLgvzdQe2Lai6q+IOYfL2K7

B: s0DynG8uMqEW+f408Vg8TGNMRrERQ1SdS91lJROI1cnT0YRC2eVe+ezU6QEYMNtm

C: 0nPz+h1sgd3IdsqkjlwsAOIxTi8wDzDuZOwGFLfXJLOK4LA7RqNr1PN4gnzJ7Hqp

D: rnMu5EfkJzBIeRj7Wq0PxNV+kWiIR8Mxh/m0sMA85wGtL56suhlK3q51nGufZqg1

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.

2.3 Rotation of the Earth

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.

Question Sequence

Question 2.2

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Incorrect. 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.

Question 2.3

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Incorrect. Watch closely. Which coast enters daylight first? Which coast is between the Sun and Earth first? Which coast enters night time first? Try again.

Correct. Earth rotates west-to-east, such that the east coast enters daylight, experiences noon, and enters night time before the west coast does.

Incorrect. Earth rotates west-to-east, such that the east coast enters daylight, experiences noon, and enters night time before the west coast does.

Try again. Watch closely. Which coast enters daylight first? Which coast is between the Sun and Earth first? Which coast enters night time first?

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.

2.4 Constellations

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.

The red dot is located on the state of California.

Question Sequence

Question 2.4

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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.

Question 2.5

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Correct. According to this figure, if you looked straight up at 8 PM, you would see the constellation of Cygnus flying overhead.

Incorrect. According to this figure, if you looked straight up at 8 PM, you would see the constellation of Cygnus flying overhead.

Try again. In which direction is overhead? Since we are looking at Earth form somewhere outside of it, overhead is not "up". When you look "down" on Earth, you are looking at the ground, "up", therefore, is away from the ground. In this figure, which constellation is directly opposite California from the rest of Earth?

Question 2.6

This is the constellation which is overhead at 8 PM on this particular date, but remember, Earth is rotating. Because of Earth's rotation, the constellations will appear to move across the sky during the night as well. Different constellations will appear overhead at different times of night. Press the play button on the bottom-left corner of the animation.

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Correct. Looking at the animation, Andromeda will be overhead at midnight, when the observer is directly on the opposite side of Earth from the Sun.

Incorrect. Looking at the animation, Andromeda will be overhead at midnight, when the observer is directly on the opposite side of Earth from the Sun.

Try again. Notice that Andromeda is exactly opposite to the Sun when it's in the highest part of the sky.

Question 2.7

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Correct. According to the figure, to observe Cygnus from California at midnight on the date shown, you would look towards your western horizon (out over the Pacific Ocean) to see it.

Incorrect. According to the figure, to observe Cygnus from California at midnight on the date shown, you would look towards your western horizon (out over the Pacific Ocean) to see it.

Try again. Where would you be located in order for Cygnus to be overhead at midnight? In which direction is that relative to California?

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."

2.5 Revolution of Earth

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.

Question Sequence

Question 2.8

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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?

Question 2.9

Because the constellations are very far away, we can consider them as background stars. Our motion around the Sun does not affect our perception of where they appear to be located, and they appear to be fixed over time. One full Earth rotation relative to background stars is known as a sidereal day. As you can see, it differs slightly from a solar day. Earth will have to rotate a little bit more (almost 4 minutes longer) until a person standing on the red dot once again experiences noon.

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Correct. As you observed in the animation, on the second night, Andromeda was overhead again a few minutes early than midnight, as it was the night before.

Incorrect. As you observed in the animation, on the second night, Andromeda was overhead again a few minutes early than midnight, as it was the night before.

Try again. Watch the animation. Andromeda is overhead at midnight on the first night. When is it overhead on the second night?

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."

2.6 The Sky at Different Times of the Year

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.

Question Sequence

Question 2.10

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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?

Question 2.11

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Correct. You would look towards the west! Since the constellations rise earlier and earlier each night, Andromeda will be further westward in its position than it was two months ago.

Incorrect. You would look towards the west! Since the constellations rise earlier and earlier each night, Andromeda will be further westward in its position than it was two months ago.

Try again. Imagine yourself standing at the red dot. Would you look toward sunset (west) or sunrise (east)?

Question 2.12

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Correct. By midnight, Cygnus has already set.

Incorrect. By midnight, Cygnus has already set.

Question 2.13

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Correct. You would see Cygnus early in the night if you looked in the western skies.

Incorrect. You would see Cygnus early in the night if you looked in the western skies.

Try again. Remember that the constellations rise earlier and earlier each night.

2.7 Circumpolar Stars

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 2.14

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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 2.15

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

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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.

Question 2.16

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Correct. Look at the opposite end of the celestial sphere. There is a circle in the southern hemisphere which defines a region of stars which will never be observed from a latitude of 35° N. This region is as big as the circumpolar region in the northern hemisphere. What about the stars in between these two regions? These stars will appear to rise and set over the course of the night.

Incorrect. Look at the opposite end of the celestial sphere. There is a circle in the southern hemisphere which defines a region of stars which will never be observed from a latitude of 35° N. This region is as big as the circumpolar region in the northern hemisphere. What about the stars in between these two regions? These stars will appear to rise and set over the course of the night.

2.8 Circumpolar Stars at Other Latitudes

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 2.17

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Correct.

Incorrect.

Try again.

Question 2.18

Based on your findings above, how many stars in the sky do you expect will be circumpolar stars for someone standing at the North Pole? Change to latitude 90°N. Change the "look up" angle to 90°.

Someone standing at the North Pole would observe all of the stars in the sky as circumpolar stars, never rising nor setting. They would all simply appear to revolve around Polaris.

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Correct.

Incorrect.

Question 2.19

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Correct.

Incorrect.

2.9 Quick Check Quiz

Indepth Activity: Rotation and Revolution

Question 2.20

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Correct. A solar day is about 4 minutes longer.

Incorrect. A solar day is about 4 minutes longer.

Question 2.21

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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 2.22

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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 2.23

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Correct. Stars appear to move from east to west.

Incorrect. Stars appear to move from east to west.

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Question 2.24

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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.

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Question 2.25

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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.

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Question 2.26

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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.

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Question 2.27

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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.

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Question 2.28

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

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

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Question 2.29

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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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