Section 2 Locating Astronomical Objects in the Night Sky

Section 2 Locating Astronomical Objects in the

Night Sky What Do You See?

What Do You See?

Learning Outcomes Think About It In this section, you will When you try to find your way to a new address, what tools or • Construct a model of the night sky methods do you use? Do you find some ways of locating new to understand how star maps work. places more useful to you than others? How do you think it • Describe the coordinate system used would be different to find your way to a new place in space? to locate objects in the sky. • How do astronomers locate objects in the sky? • Identify patterns in the organization and distribution of matter in the Record your ideas about this question in your Geo log. Include a universe. sketch that demonstrates your ideas. Be prepared to discuss your responses with your small group and the class.

Investigate In this Investigate, you will be comparing and contrasting ways of finding places both on the surface of Earth and in space.

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Part A: Describing the Locations of On a globe, they form a series of circles Places on Earth that decrease in size toward the poles. 1. With your group, discuss ways for Latitude lines show locations north locating places on Earth. and south of the equator. Lines of longitude are imaginary lines that run a) List all the ways your from the North Pole to the South Pole. group identified. They show locations east and west b) Rank your list from the of the prime meridian. You will now most accurate to the least sketch latitude lines using a compass, accurate descriptions. protractor, and ruler. 2. Use a globe, an atlas, the Internet, or 8. Use a compass to draw a circle 15 cm other sources to find the most precise in diameter. location of your school as possible. 9. Draw a horizontal line through the a) Record the location of your school. center of the circle (a diameter). 3. Exchange your description with one 10. Identify the longitude and latitude from another group. coordinates for your school. a) How does the accuracy of the other a) Record the longitude and latitude group’s description compare to yours? coordinates for your school. 4. Identify the locations of two places on 11. Use a protractor to draw an angle that Earth that have physical and historical is equal to the latitude coordinate for importance. your school. Measure the angle along the a) Record each location on a separate horizontal line and from the center of the sheet of paper. Research and describe circle. Draw the angle from the center of in a few sentences the importance of the circle to the edge of the circle. the physical nature and history of each place. 5. Exchange your sheets of paper with those from another group so that your group has two new places. 6. Locate each new place on a map. a) Did you have any difficulties finding each place? b) What information helped you or could have helped you in finding the places on the map? 7. You most likely used longitude and 12. Following the same procedure as in Step latitude coordinates to precisely locate 11, measure angles from 0° to 90° from the your school and the places of physical horizontal, and mark these on the circle. and historical importance on Earth’s surface. Lines of latitude are imaginary 13. Obtain a globe and a world map. lines on Earth’s surface that run east Examine the lines of latitude on the to west and parallel to the equator. globe and map.

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a) How do these lines compare to the 3. Now imagine that you put the glass ball lines that you drew on your circle? into a much larger opaque ball without any markings on it. Your model now b) Describe how lines of longitude are consists of two concentric spheres, measured in comparison to lines with the opaque sphere at a distance of latitude. from the glass sphere. In your Geo Part B: The Celestial Sphere log, sketch what a cross section of this 1. Imagine Earth expanding and becoming “sphere within a sphere” would look 10 times its normal diameter. like. Again, compare your sketch to those of others in your group. a) Could you use the longitude and latitude coordinate system from a) What pattern do you think would be Part A for this larger Earth? Explain projected onto the inside of the larger your answer. sphere by the light shining out from the glass ball? Sketch this pattern. 2. To create a coordinate system for locating objects in the sky, you first need b) How would this pattern be similar to to make an imaginary model of Earth. the one on the glass ball? Imagine Earth’s core as a glass ball c) How would this pattern be different with two poles and an equator, just like from the one on the glass ball? Earth’s. Think about this ball as having lines of longitude and latitude just d) How could you use coordinates like those on a globe. These longitude to find objects located on the and latitude lines are opaque, which larger sphere? means light cannot pass through them. Think of the glass between the lines as transparent, which means light can pass through it. Learning Through Technology a) In your Geo log, sketch what this To use online resources to glass ball might look like. Share it practice finding objects in with others in your group. space, go to the EarthComm b) If you put a light bulb in the center Web site at http://www of the glass ball that radiated light .agiweb.org/education/earthcomm2/. outward in all directions, and turned There you will be able to explore the ways out the lights in the room where the in which astronomers locate stars, planets, ball was, what would you see on the and other celestial objects. wall and ceiling? Discuss this with your group.

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Digging Deeper

THE CELESTIAL SPHERE In the Investigate, you explored different ways of locating places on Earth. You then investigated how you could find objects in space. Locating objects in space has been a challenge for people for a very long time. For thousands of years, people from all parts of the planet recognized groups of stars in the night sky. The Greeks, Egyptians, Babylonians, and others gave names to the groups of stars. They told stories about them that allowed for their positions to be learned without needing instruments. Some scientists think that caves in France may contain drawings of the night sky from 17,300 years ago.

Figure 1 An ancient star chart showing animals and objects that represent the constellations.

Geo Words There are 88 groupings of stars in the sky. Astronomers call a grouping constellation: a of stars a constellation. Each constellation has a Latin name. The name is grouping of stars in given a three-letter abbreviation. For example, Cassiopeia is abbreviated the night sky that as Cas. Draco is abbreviated as Dra. Constellations are important reference forms a recognizable pattern. points for locating planets and other astronomical objects in the night sky. celestial sphere: an Many ancient astronomers thought that the stars were attached to a imaginary sphere giant hollow sphere that surrounded Earth. Astronomers now know of infinite radius that projects around that such a sphere does not exist. Stars are located throughout space at Earth, and whose different distances from Earth. However, the idea of a giant sphere, called interior is inscribed the celestial sphere, is useful. It provides a model on which to place and with a grid system locate stars, planets, and other objects in space. The word celestial means for taking celestial coordinates. pertaining to the sky. The difference between astronomical objects and celestial objects is that celestial objects do not include Earth.

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Figure 2 View of the celestial sphere looking toward the north celestial pole.

To describe the location of an object in the sky, astronomers use a special Geo Words reference system. It is known as the celestial coordinate system. This celestial coordinate system is similar to the one used for locating objects on Earth’s surface. It system: a coordinate system for mapping uses measurements from known reference points or lines. These lines are positions on the similar to lines of longitude and latitude. The main reference line in the celestial sphere. celestial sphere is the celestial equator. It is an imaginary circle created by longitude: an east- extending Earth’s equator into space. west measurement of position on Earth. latitude: a north- south measurement of position on Earth. celestial equator: an imaginary circle created by extending Earth’s equator into space; the main reference line in the celestial sphere.

Figure 3 Locations on Earth’s surface can be precisely located using longitude and latitude.

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Geo Words declination (dec): a The celestial coordinate system uses declination (dec) to measure how far coordinate used on north or south an object is from the celestial equator. Declination is similar the celestial sphere, just as latitude is used to latitude. It is measured in degrees. The celestial equator has a declination on Earth. of zero degrees. Objects north of the celestial equator have a positive right ascension (R.A.): value. Those south of the celestial equator have a negative value. a coordinate used on On Earth, longitude is measured east or west of the prime meridian. The the celestial sphere, just as longitude is prime meridian is located at 0°. It runs through Greenwich, England. Like all used on Earth. other lines of longitude, it runs north to south. It converges with all other vernal equinox: the lines of longitude at the poles. If you look down onto a globe at the North position of the Sun as Pole, you can see the 360° through which Earth rotates every 24 h (hours). it crosses the celestial It rotates through an angle of 15° every hour. According to these different equator into the measurements for longitude, the city of Miami, Florida lies 80°, or 5 hours Northern Hemisphere each spring. and 20 minutes, west of the prime meridian. Los Angeles, California lies 118°, or 7 hours and 52 minutes, west of the prime meridian. In the celestial coordinate system, the lines that correspond to longitude on Earth are called right ascension (R.A.). Right ascension is measured in hours, minutes, and seconds, much like lines of longitude can be measured. Right ascension is slightly more complicated to understand than longitude. Due to Earth’s counterclockwise rotation, objects appear to move across the sky at night. Earth’s eastward rotation causes stars to appear to move west. Thinking about the rising Moon might help you understand how this works. The Moon appears to rise above the horizon from a given point on Earth’s surface. As Earth rotates, that point moves away from the Moon, which appears to rise upward and to the right. Similarly, Earth’s rotation causes the positions of the stars in the sky to appear to change gradually throughout the night.

Figure 4 The trail of the rising Moon over Seattle.

In order to properly measure right ascension, a fixed point is needed. The sky lacks a fixed point. Even the stars are moving as they slowly orbit the center of the galaxy. As a result, astronomers have chosen the point on the celestial equator where the Sun crosses into the Northern Hemisphere in spring. This is a point in time known as the vernal equinox. Notice that this point does not coincide with the point where the prime meridian intersects the equator.

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The stars seem to travel across the sky at night and the Sun seems to travel across the sky during the day. While it may appear that these objects are moving, they are not. The reason for this motion is because Earth rotates 360° about its axis every 24 hours. The points about which the sky seems Geo Words to rotate are called celestial poles. One is located above the North Pole celestial pole: the and the other is located above the South Pole. Over time, stars located point about which the near the celestial poles make small circles in the sky. Currently, the star sky seems to rotate. called Polaris lies very close to the northern celestial pole. It therefore appears to be almost stationary in the night sky. As a result, this star has become known as the North Star or Pole Star. It has been of great use in navigation. The nearest star to the Sun is Proxima Centauri, located at right ascension 14 hours 38 minutes and declination –60° 46’.

Checking Up 1. What is the main Figure 5 Polaris is the Pole Star seen at the center of the concentric rings in this difference between photograph. The rings are the apparent paths of stars produced by Earth’s rotation a point on Earth’s about its axis. grid system and one on the Not all of the stars in the sky can be seen from a single location. A person celestial system? viewing stars in the Northern Hemisphere cannot see all the stars visible 2. What is special in the Southern Hemisphere. Some are located below the horizon. The about Polaris, the stars that can be observed from any given place also vary throughout the Pole Star? year. As Earth moves around its orbit, the North and South Poles point 3. Why do the toward different parts of the universe. As a result, different constellations objects you see in and other celestial objects come into view at different times. the sky at night change throughout the year? Think About It Again At the beginning of this section, you were asked the following: • How do astronomers locate objects in the sky? Record your ideas about this question now. Be sure that you describe the celestial coordinate system in your answer.

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Reflecting on the Section and the Challenge You used longitude and latitude coordinates to locate your school and two places of physical and historical importance on Earth’s surface. Drawing your own latitude lines using a compass, protractor, and ruler helped you to understand how lines of latitude are drawn on globes and world maps. You then transferred your understanding of coordinate systems on Earth to developing a coordinate system for locating objects in the sky. You applied this new system to a model of the night sky. This will help you to describe Earth and its place in the universe for the Chapter Challenge.

Understanding and Applying

1. Describe the objects in the night sky above your community. 2. How do the objects you can see in the sky at night change during the year? 3. Why is it important for astronomers to make observations of the night sky from many locations? 4. Explain why you can see above and below the celestial equator. 5. Why is it that new constellations gradually enter our field of view from the east and the old ones disappear to the west? 6. Preparing for the Chapter Challenge Continue developing your radio or podcast script for the Chapter Challenge. Explain the use of the celestial coordinate system in plotting positions in the sky. Use examples of astronomical objects to illustrate your explanation. Help your audience to understand how precisely locating objects in the sky helps to identify Earth’s place in the universe.

Inquiring Further 1. Earth’s axial tilt Over a period of 13,000 years, Earth’s axis wobbles like a spinning top. How does this affect the relative position of Polaris? Find out more about Earth’s wobble, the position of Polaris, and stars that have marked the south celestial pole. 2. Using star charts Get to know your community’s night sky and its movements. The constellations above the horizon change with time and with the seasons. Use monthly star charts to examine the evening sky. Hold the chart overhead and turn it to match the direction you are facing. Which way do the constellations appear to move during an evening? 3. The stories of the stars Different cultures give the various constellations different names and stories. Choose three constellations and recount the different cultural stories that have developed about them.

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