Investigating Parallax
Astronomers use parallax to measure the distances to nearby stars. In this lab, you will investigate the effect of distance on parallax. You will observe how an object appears to move against a distant background as you look at it first through one eye, and then through the other.
Problem How is parallax related to the distance to a star?
Materials
unlined paper
tape
marker
pencil
meter stick
index card
graph paper
Skills Measuring, Using Models
Procedure
Tape three sheets of unlined paper to the wall horizontally, at eye level. Use a marker to draw 11 vertical marks 5 centimeters apart on the paper, as shown in the photo on the following page. The marks represent a distant background against which you will view a closer star. Label the marks in multiples of five from left to right, starting with 0 and ending with 50.
Construct a copy of the data table shown above.
Facing the sheets of paper, stand directly in front of the 25-cm mark. Then, use a meterstick to measure a perpendicular distance of 7 m from the mark. Move to that spot and face toward the 25-cm mark. You must remain at this position until you have finished collecting all data.
Have a partner hold a pencil vertically at your eye level as shown in the photo. The pencil should be 1 m in front of you. Your partner should use a meter stick to measure this distance. The pencil represents a nearby star.
Hold an index card over your left eye and look at the pencil with your right eye. Your right eye represents Earth's position at one point in its orbit. Move your head so the pencil lines up with the mark labeled 0.
Now, without moving your head, hold the index card over your right eye and look at the pencil with your left eye. Your left eye represents Earth's position at the opposite end of its orbit six months later. Note the number of the mark that lines up with the pencil. If the pencil is between two marks, estimate its position to the nearest whole number. Your partner should record this number in the appropriate place in your data table.
to determine the parallax, subtract the right eye measurement (zero, in this case) from the left-eye measurement. Record the parallax in the appropriate place in your data table.
Predicting How do you and your partner think the parallax will change as your partner moves the pencil away from you? Your partner should record your prediction.