Name
Name ______________________________________________ Period ______
Lab – Parallax
Purpose: We want to use parallax to find the distance to an object in the same way that an astronomer uses parallax to find the distance to stars in the sky.
Procedure: Refer to the diagram below.
[pic]
1. Stand facing the whiteboard at a distance much greater than the length of your arm. Hold your thumb out at arm’s length, and with one eye closed, line up your thumb with a line drawn on the board. Switch eyes, and notice how the apparent position of your thumb has shifted with respect to the board. Have a group member make a mark on the board so the new apparent position of your thumb covers this new mark. You have just experience parallax for your thumb.
2. From where you are standing, use your thumb or fist as an angular measuring tool (remember our first lab?) to measure the angular separation of the two marks on the board. This is the parallax angle. Record this angle in degrees and convert to radians.
3. Have a lab partner measure the distance between your pupils while you stare straight ahead. This distance is the baseline.
4. Using the equation, distance = (baseline) x (parallax angle in radians), calculate the distance to your thumb.
5. As a comparison, directly measure the length of your arm when holding your thumb up.
6. Determine the percent error using this equation:
[pic]
Data:
|Baseline (cm) |Parallax angle (degrees) |Parallax angle (radians) |Computed arm length (cm) |Measured arm length (cm) |
| | | | | |
Calculation:
Perform your % error calculation in the space below. Circle your result.
Questions:
1. List 3 possible sources of error in finding the distance. Which one do you think contributes the most uncertainty?
2. For astronomers, what is the baseline they use for their observations?
3. You use parallax to determine the distance to 2 stars. Star A had a parallax of 1 arcsecond and star B had a parallax angle of 0.89 arcseconds. Which star is closer?
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eye
eye
thumb
whiteboard
White board
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