Washington College
Physics 100

Home Experiment #3: The diameter of the Moon.

When the full moon is visible in the sky (around February 10-14), close one eye, hold a pencil (the experiment works better with a pencil than with a coin) vertically and stretch your arm up to the point where the width of the pencil covers the entire width of the moon. At this point, the width of the pencil and the apparent width of the moon will have the same size. Knowing that the average earth-moon distance is 384,000 km, calculate the diameter of the moon in km (1 km = 1000 meters). Take at least two measurements at different times. Report time and day of the measurements. Use the formula explained in class,

Diameter of moon =(width of pencil)*(distance earth to moon/distance eye to pencil)

Find how many times larger than the moon is the Earth (diameter of Earth =12,800 km) and estimate the error of the average as percent of the average, 100x[(max. value - min.value)/(2*average)]. The factor 2 in the previous formula assumes two measurements of the diameter. Discuss sources of error in the execution of the experiment and how they affect the final result.