OBSERVING PARALLAX
Most distances on the earth can be measured in meters or kilometers. The distances of objects in space are quite far which makes it very difficult to predict or imagine. Astronomers have found that the distance to even the nearest star is even too great to measure in kilometers. The distances are so great that the numbers are too large to work with easily. For example, the star Proxima Centauri is the closest star, other than the sun, to the earth. ( Proxima Centauri is 40 km from the earth.) With such a huge number, astronomers had to invent special units to measure distances in space. Astronomers often measure the distance to an object in space in 'light-years'.
A 'light-year' is equal to the distance light travels in one year. Light travels through space at a speed of about 300,000 km/sec. A 'light-year' is equal to almost 10 trillion kilometers. Light from the sun reaches the earth in a little more than 8 minutes. Light from the North Star, Polaris, takes about 300 years to reach the earth. The Sun is about 150 million km from the earth. This distance is referred to as an astronomical unit or 1 AU which is equal to 150 million kilometers.
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Astronomers can use parallax to find the distances to the closer stars. Parallax is the change in the position of a distant object when seen from two different places. A nearby star seems to move against a background of distant stars. By measuring how much the star appears to move, astronomers can calculate how far away the star is. Nearby stars have a large parallax whereas distant stars have a smaller parallax.
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MATERIALS:
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drinking straw
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transparent tape
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3 sheets of computer paper ( continuous feed )
PROCEDURE
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1. Remove the edges with holes from the computer paper. Tape this strip of paper with holes to a wall in the classroom.
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2. Tape the drinking straw upright on the metric ruler at the 15cm mark.
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3. While standing about 2 meters from the wall with stripped paper, hold the ruler horizontal to the floor.
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4. Tell student to close the left eye and line up the straw with the left end of the strip of computer paper.
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5. Tell student to open the left eye and then close the right eye. Student should count the number of holes the straw appears to move along the strip.
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6. Students will then move the strip to the 30cm mark on the ruler. Repeat steps 3-5.
QUESTIONS
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1. What does the strip of paper with holes represents in our galaxy?
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2. At the 15cm mark, how many holes did the straw appear to move?
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3. At the 30cm mark, how many holes did the straw appear to move?
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4. Why does the straw appear to change position?
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5. Which has a greater parallax, a nearby star or a more distant star?