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# Mobile Stars: The Apparent Movement of Stars (page 2)

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Author: Janice VanCleave

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1. How does the distance of a star from the Earth affect its parallax distance? Show how the parallax distance of stars at different distances from the Earth can be compared. Repeat the experiment three times, with the star at 5 cm, 10 cm, and 25 cm from the edge of the paper nearest your eyes. Mark these locations along the line on the paper. Without moving your head during the experiment, have a helper put the star on the 5-cm mark. Sight the star, then have your helper record the measurements and move the star to the 10-cm mark. Sight the star as before, then repeat for the 25-cm mark. Science Fair Hint: Use diagrams of each measurement, like the one in Figure 6.2, to compare the parallax distances of stars at different distances from the Earth.
2. How does the length of the baseline affect the parallax distance? Repeat the experiment, first looking at the star with your right eye from the right corner of the paper next to table's edge. Then look at the star with your left eye from the left corner of the paper. Compare the parallax distance in this experiment with that of the original experiment.

1. Astronomers use the parallax angle of stars close to the Earth to determine the stars' distance from Earth. The parallax angle (p) of a star is called its stellar parallax and is one-half of the star's total angular shift when observed from opposite sides of the Earth's orbit around the Sun. To measure the parallax of a star, astronomers photograph the sky on one night, then photograph it again six months later when the Earth is on the opposite side of its orbit. This provides the longest baseline, with viewing spots separated by a distance equal to the diameter of the Earth's orbit around the Sun, about 187 million miles (300 million km). A comparison of the two photographs shows that the other stars seen with the sighted star are different in each photo. Prepare a drawing similar to the one in Figure 6.3, which shows the parallax angle (p) when photos are taken from opposite sides of the Earth's orbit.
2.
1. In the Northern Hemisphere, stars appear to move around Polaris (the North Star, located above the Earth's north axis). This apparent movement is due to the rotation of the Earth on its axis. Stars that never sink below the horizon of the observer but appear to revolve around a point in the sky above the Earth's axis are called circumpolar stars. At latitudes of 40° N or greater, the stars in the Big Dipper's bowl are circumpolar and appear to revolve around Polaris.
2. Prepare a star clock to observe the apparent movement of the Big Dipper's stars by drawing two circles on stiff paper, such as a file folder. Make the diameter of one circle 8 inches (20 cm) and the diameter of the second circle 6 inches (15 cm). Use a paper punch to make a hole in the center of each circle. Draw a star, Polaris, in the center of the small circle and the stars of the Big Dipper near the circumference, as shown in Figure 6.4. Add a dashed line from Polaris through the two stars in the Big Dipper's bowl and to the circumference. Mark an arrow at the end of this line. Lay the small circle in the center of the large circle. Insert a paper brad through the center of the two circles, then write "Facing North" and the directions "West" and "East" along the circumference of the large circle.

On a clear, dark night, go outdoors. Use a compass to find north or, if you do not have a compass, face in the direction that the sun rises each day, then tum your head and look in the direction of your left shoulder. This is approximately north. Hold the circles in front of you with the direction "Facing North" at the bottom. Find the Big Dipper in the northern sky and tum the small circle so that in relation to the northern horizon the position of the Big Dipper on the paper matches the position of the Big Dipper in the sky. Use an astronomer's flashlight (see Appendix 3) to view the circles. On the large circle, make a mark at the end of the arrow and record the time and date. Repeat this every hour for as many hours as possible during the night. Use the paper model to determine the direction of the apparent movement of stars due to the Earth's rotation. Note: If you live at a latitude south of 40° N, use a single circumpolar star in the Big Dipper or in the constellation Cepheus.

For information on circumpolar stars, see Janice VanCleave's Constellations for Every Kid (New York: Wiley, 1997), pp. 45–55.

3. The movement of the Earth around the Sun results in the rising of the stars about 4 minutes earlier each evening. Four minutes each day for 30 days is 120 minutes, or 2 hours; thus the stars rise about 2 hours earlier each month. AB a result, the position of stars at the same time of night changes slightly each night. This is a noticeable change from month to month. Repeat the previous experiment at the same time of night once a month for as many months as possible to compare the changing positions of the Big Dipper. Use the models to represent the results.