Determining the Size of the Moon

Physics 1052 (General Astronomy)

Instructions for writing a Lab report
The format of the laboratory repo rts may vary slightly depending on the project. A sketch alone
will not be sufficient for ANY laboratory; you should describe what you did in sufficient detail
that the grader should not need to know in advance which lab you are reporting on. Depending
on how well you write (communicate), and the nature of the lab, you may have anywhere from
three paragraphs to a page or two of writing. BE  CONCISE. Interpretation of your results is also
important. ALL LABS MUST HAVE A CONCLUSI ON (compare what you started out to do
with what you did and state the result).  Although sketches are not sufficient, they are usually
necessary. Accuracy is important  in the sketches; use a weak flashlight with a red covering for
the lens while you are sketching outside in the  night so you can see without ruining your night
vision. Do NOT return the lab hand-out with your report (unless directed otherwise for a specific
lab). Items listed below must be in each and every lab report:
?  (1 pt) Title of the Experiment
?  (1 pt) Your name.
?  (1 pt) Viewing location and names of  other students present, if any.
?  (3 pts.)  A statement of purpose. One or two sentences stating the objective(s) of the lab
(in your own words) will be sufficient.
?  (4-6 pts.)  An explanation of procedure . This section of your report is most important. It
should contain a concise and we ll-written description of what you observed/measured
and how you did it. NEAT diagrams are acceptable as long as they are accompanied by
appropriate explanation.
?  (6-4 pts.)  Data and Calculations. The length and importance  of this section will vary
depending upon the project. Always include the time and date of your observations
(remember the date changes at midnight). Us e tables and graphs where appropriate.
Include units and label the axes of graphs. Use SI units and scientific notation. Do not get
carried away with excessive precisions – thr ee significant figures are sufficient. Show all
your calculations. If the final answer is wrong, but the grader finds that everything was
okay until the last step, you will not lose much credit.
?  (4 pts.) Conclusions and analysis . State the results clearly  and discuss the relation
between your results and the expected or predicted results. Include possible sources of
errors.
Note: Keep up with due dates. Late labs won’t be accepted. You need to turn in hard copy by the
beginning of class on the due date.  Emailed lab wo rk will not be accepted. If you want to turn in
a lab earlier than due date you can  do so. Bring the completed lab to my office. If I am not in my
office you can turn in to the room 3801 (next to my office).  Room 3801 is UCD physics office
where you will find office secretary.

Department of Physics, University of Colorado at Denver
Physics 1052 (General Astronomy)

Card
Rule r
Card
Experiment #2 —Determining the Size of the Moon
The purpose of this experiment is to have you perform 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 kilometers 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 and step back until the dime just covers the moon
or other variations on this theme. You are encouraged 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  ?
in arc seconds, which means that the diameter  of the Moon we are looking for will be about
1800 360 ?? ? to get within our goal of 0.1
o
.
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 the 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.
?
d
D