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Chapter 22: Exploring Space
 Star Sightings For thousands of years, humans have used the stars to learn about the planet we live on. From star sightings, you can map the change of seasons, navigate the oceans, and even determine the size of Earth. Polaris, or the North Star, has occupied an important place in human history. The location of Polaris is not affected by Earth's rotation. At any given observation point, it always appears at the same angle above the horizon. At Earth's North Pole, Polaris appears directly overhead. At the equator, it is just above the northern horizon. Polaris provides a standard from which other locations can be measured. Such star sightings can be made using the astrolabe, an instrument used to measure the height of a star above the horizon.
 How can you determine the size of Earth?
 Think about what you have learned about sightings of Polaris. How does this tell you that Earth is round? Knowing that Earth is round, form a hypothesis about whether you can estimate the circumference of Earth based on star sightings.
 Goals Record your sightings of Polaris. Share the data with other students to calculate the circumference of Earth. Safety Precautions Do not use the astrolabe during the daytime to observe the sun. Data Sources
 Plan Obtain an astrolabe or construct one using the instructions posted on the Glencoe Science Web Site. Design a data table in your Science Journal similar to the one below. Decide as a group how you will make your observations. Does it take more than one person to make each observation? When will it be easiest to see Polaris? Do Make sure your teacher approves your plan before you proceed. Carry out your observations. Record your observations in your data table. Average your readings and post them in the table below.
1. Research the names of cities that are at approximately the same longitude as your hometown. Gather astrolabe readings at the Glencoe Science Web Site from students in one of those cities.
3. Determine the distance between your star sighting location and the other city.
4. To calculate the circumference of Earth, use the following relationship.
 Circumference = (360°)(distance between locations) difference between readings

 How does the circumference of Earth that you calculated compare with the accepted value of 40 079 km? What are some possible sources of error in this method of determining the size of Earth? What improvements would you suggest.

Results

 Your Location Date Time Astrolabe Reading Date Time Astrolabe Reading Average Astrolabe Reading: * City: * State: Select Your State Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware District of Columbia Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming Not Applicable * School:
* required