The Size of the Moon - Lab Report Example

A meter stick was mounted at slightly greater than eye level, level to the ground. A plumb bob, actually a string with a nut tied to the end, hung at a head level exactly 0.5 meters from the meter stick. This served as an observation post with easily converted and consistent measures. As the moon fully rose past the horizon, two index cards were “butted” to either edge of the moon and the indicated length was recorded. This procedure was repeated later. The chart results:

time

d (cm)

D (cm)

conversion

θ

7:10

50

0.5

205265

2053

10:10

50

0.5

205265

2053

Where:            d=50 cm as set by the experiment.

            D= the measure between the cards

            Θ=the angle size of the moon in arc seconds calculated as =(D/d) x 205265

The diameter of the moon is calculated as: D= (205265 x 2053)/ 380000km    d=3800 km

The angular size of the moon by calculation is: 2053 arc seconds

The estimated diameter of the moon is: 3800 km

The angular size of the moon is consistent throughout a one-night period; however, the distance to the moon varies over time, so over years, the angular size changes.

The method used produces a good approximation of the diameter of the moon. The possible reasons for inaccuracy include: measuring to the millimeter with cards on a meterstick, the angle of the eye to the card if not dead center, the halo of the moon may make it appear wider and environmental lighting conditions can change the perceived width. The calculations use an average distance to the moon rather than the exact distance, which is why the calculations are consistent but inaccurate. The illusion of the moon changing size occurs because the horizon gives visual cues that make people think the scale of the moon is greater. Against a large tree, the moon looks wider. In the open, it doesn’t appear to change much.