Astronomy Lab

Astronomy Lab Department of Physics, University of Colorado at Denver Physics 1052 (General Astronomy) Card Ruler Card Experiment #2 —Determining the Size of the Moon The purpose of this experiment is to have you perf orm a measurement of angular size in the sky, and to confirm or discredit a folk – tale about the apparent size of obj ects are high in the sky compared to when they are low on the horizon. A ngular size is the apparent size of an object measured in degrees, with no information on the obj ect’s actual size (in kilo meters or miles) or distance. First measure the angular diameter of the Moon to the nearest 0.1 o (tenth of a degree). The best method for making this measurement is to take a ruler (with divisions at least as small as 1 mm) and hold it at arm’s length so th at it is in front of the Moon. Us e your thumb – nails (or two index cards) to mark the edges of the Moon cross the ruler as shown in Fig above. At the same time, have a friend measure the distance from your eye to the ruler. Make sure you are measuring the size at the widest point of the Moon. There might be more accurate ways to do this measurement say, taping a dime to the inside of the window a nd step back until the dime just covers the moon or other variations on this theme. You are encour aged to be inventive. Now use the small angle approximation 206265 D d ? ?? ? method to estimate the Moon’s angular diameter. Th e distance between your thumb – nails is the short side of the skinny triangle, D ; the distance from your eye to the ruler is the long side, d and the angle of the triangle is the angular diameter of the Moon. Recall that above equation gives You will want to comment on the possible sources of error in the measurement will make on the final outcome of the experiment. For example, how will the distance from your eye to the ruler, or the measured size of the Moon affect your outcome? If you find a more accurate method for making the measurement, include that in your report, too. Also test the conjecture that th e size of the Moon in th e sky is larger when it is near the horizon. How do your results compare? What do you think of the folk – tale now? Finally, estimate the actual diameter of the Moon in km. Now the diameter is the short side of a triangle and the long side is the average distance from the Earth to the Moon, about 5 3.8 10 km, ? center to center. Give both th e angular size and actual diamet er you measured in your report.