Let the North Star Tell You Where You Are

Lesson Objectives

By the end of this activity, students will be able to:
• find the Big Dipper in the night sky
• locate Polaris (North Star) in the night sky
• understand how the angle to Polaris and latitude are related
• understand what an astrolabe is
• determine the latitude by using an astrolabe

Related National Standards

National Science Education Standards (National Research Council)
Science Standard D; Earth and Space Science
Earth in the solar system
• Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the moon, and eclipses.

Tools and Materials Needed

• cardboard (manila folder type)
• 7-inch piece of string
• paper clip
• scissors
• tape
• metric ruler
• local night sky map
• Making an Astrolabe activity sheet
• access the the Internet

Estimated Time to Complete Lesson

The activity should take one-and-a-half class periods. It will take one period to make the astrolabe, which students can then take home at night to determine their latitude. The next day students can compare their results and calculate an average latitude based on the class results.

Teaching Strategy

Background information
An astrolabe is an ancient scientific instrument that the Greeks used in the second century B.C. to determine the latitude at which they were located. The astrolabe, which allows the user to measure vertical angles, was very important to sailors in ancient times. Determining the angle between your position on Earth and Polaris determines your latitude. A person standing at the North Pole will find Polaris straight overhead (90°), which is her latitude. A person standing at the equator will find Polaris at the horizon (0°), also her latitude. Polaris is not visible below the Equator. A local night star map would be helpful for this activity. Polaris is not a very bright star and students may need to identify the Big Dipper, which will then point them to Polaris. (If students follow the two stars at the end of the cup up and out of the Big Dipper, the next bright star they run into will be Polaris.)

Procedure

1. Provide each student with a Making an Astrolabe activity sheet.

2. Have students cut out the astrolabe silhouette from the activity sheet (heavy black lines).

3. Have students tape this silhouette onto their manila folder sheet and cut out this figure.

4. Have students make a tiny hole at the index point (marked by an "X," which is the midpoint of the horizontal axis), push the end of a 7-inch piece of string through the hole and tape that end to the backside of the cardboard. Have students attach a paper clip to serve as a weight at the other end of the string that falls over the degree markings on the front of their astrolabe.

5. Have students fold the cardboard along the dashed line as indicated on the diagram so that the sides with the triangular notch form a right angle with the face of the astrolabe.

6. Have students take their astrolabes home to use at night. First they need to locate the Big Dipper, which will point them to Polaris. (If students follow the two stars at the end of the cup up and out of the Big Dipper, the next bright star they run into will be Polaris.) Refer students to a local star map. Then have students face Polaris and raise their astrolabe until they can sight Polaris through both notches. At this point they should press the string against the cardboard and record the angle at which the string crosses the scale. This angle is the altitude of Polaris, which is also their latitude value.

7. Next day in class have students pool their latitude data and determine the class average for the latitude value. How does this average compare to the actual latitude value found on a map? Ask students to identify reasons for error in the determinations they made.

8. To conclude the lesson, discuss other kinds of navigation. One form of navigation that Ernest Shackleton's crew used was a sextant. To help students better understand how a sextant works, have them do the online activity, Escape from Antarctica. To acquaint students with more modern navigation techniques, have them check out NOVA Online's GPS: The New Navigation.

Ursa Minor
http://www.astro.wisc.edu/~dolan/constellations/constellations/Ursa_Minor.html
This page shows a diagram and provides text for how to locate the star Polaris, which lies in the constellation Ursa Minor.
U.S. Census Bureau—U.S. Gazetteer
http://www.census.gov/cgi-bin/gazetteer
Find your city's actual latitude (and longitude) by typing your town name and zip code in at this site.
Assessment Recommendations

Students may be assessed through:
• their participation in the activity.
• the accuracy of the astrolabe they constructed.
• the accuracy of their latitude determination.

1. Calculate the percent error in their latitude determination. Students can use the following formula to do this:

```% error = difference between the students value and the true value (from map) x 100
true value (from map)```
2. Calculate the percent error of the class average. Do the same as described in step 1 but for the entire class. How does this average compare to their own individual value? Have students discuss the merits of an average value versus one person's result.

3. Search for other constellations. They can look for other constellations such as Orion, Perseus, and the Little Dipper in their night sky and diagram those they found.

4. Research other methods past sailors used to determine their location. Students can start with this NOVA Online site, Longitude, which tells the story of how one man figured out how knowing what time it was in your home port could tell you where you were at sea.

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