Afpectus Lunae: Does the Moon Rotate on Its Axis?
Last summer I often walked down the rough path to our pond and stepped out onto thedock. There I would lie down on my back and stare up at the night sky. The lowrumble of the distant highway was easy to shut out when a shooting star flewby. The bright face of the moon often sparked my curiosity. Looking at it mademe think about the telescope that sat in our closet, gathering dust.A few years ago mygrandfather gave my family a nice backyard telescope for Christmas.Unfortunately, it sat in the house for four years because it was "too much workto figure out." This fall I set it up to do
a school project with my scienceteacher. I wanted my project to be centered on the moon. Obvious possibletopics were the moon's phases, the rising and setting of the moon, and lunarand solar eclipse phenomena. But after more pondering, I realized that I didnot know if the moon, like Earth, rotates on its axis. I thought that I mightbe able to discover this simply by viewing the moon over several months andrecording its appearance. My prediction was that if the moon rotates, theappearance of its surface would change, and if it does not rotate, the surfaceof the moon would always look the same. I hypothesized that the moon rotatesbecause the same forces that affect Earth should affect the moon.
I planned to observe andrecord the appearance of the moon's surface as often as weather conditions andtime permitted during October, November, and December 2003. The kit I createdfor viewing the moon included a compass, a homemade astrolabe, twodifferent sizes of telescope lenses (9mm and 25mm), several pencils, and myfield journal, which consisted of data sheets. At the top of each ofthese sheets I left a space to sketch the moon. From these sketches I hoped toconfirm my hypothesis.
Most "viewing nights" beganwith my checking a moon phase calendar to see where in the sky the moon wouldbe and in what phase it would be. Then I took our telescope out onto our lane.We live in the country about six miles from a small town, so there is nosignificant light to obscure my view. The only obstructions are all the trees,so the cleared lane was an ideal viewing spot.After leveling thetelescope, I got out my tools. Using the lower-magnification lens (9mm), Ilocated the moon and focused the telescope. Then, in order to draw the detailsof the moon accurately, I replaced that lens with the higher-magnification lens(25mm) and refocused the telescope.Onmy data sheets, I recorded the date, time, weather, location ofviewing, compass direction, and the elevation of the moon above the horizon. Atfirst I used the altitude scale on my telescope to record the elevation of themoon. Later, my teacher showed me a science activity book that led me throughthe steps of making an astrolabe. While the astrolabe functioned nobetter than the telescope, it was more fun to create my own simple tool.The next spot on my datasheet was for recording my observations. I tried observing the moon with myunaided eye, but I needed more detail for my drawings. Using binoculars wascumbersome because I could not observe and draw at the same time. The telescopefreed my hands for drawing and gave me the necessary magnification; therefore,I used it exclusively for the remainder of my observations.Thelast step for filling out a data sheet was to sketch the moon. I made clear,simple drawings with a focus on distinct surface features. During this process I becamefamiliar with the main features of the moon and their appearance. Whenever Isaw a picture of the moon in a book or some other place, I was able torecognize some of the features and noticed that all the sources depicted thesame view of the moon.
The simplest feature for meto locate is called the Sea of Crises. The features on the moon were namedhundreds of years ago when they were thought to be seas. In my observations,this particular "sea" was always in the top left-hand portion of the moon. Being able to recognize certain features gave me a great sense ofsatisfaction. Often I ran excitedly to my mom, dad, or best friend, pointed atthe sky and said, "Ah! Look there. That's the Sea of Serenity!Isn't that cool!" They would look back at me blankly and shake their heads, butI think they understood.After viewing the moon for afew nights and noting its depiction in other sources, I began to think that myhypothesis was false. If the moon rotates on its axis, I should have seendifferent features of the moon on each viewing. So far, the nearside of themoon had not changed at all. ("Nearside" refers to the portion of the moonexposed to Earth. I will use this term henceforth.) Did the moon spin on its axis, or not? It sure didn't appear to, but it seemed oddthat the moon would hang still in space. I continued to think that since theEarth spins on its axis and the moon is affected by the same forces as theEarth, the moon should spin as well. Like any good naturalist, I went to the"literature." The first place I looked was in a children's book entitled The Moon Seems to Change . This gave me aquick and easy overview of the moon. I learned that the moon's "day" is almostone month long. If the moon could have a "day," it must rotate on its axis! Iwas thrilled to discover that the moon did rotate. Now that that question was answered, I realized that I would have tocontinue to investigate with a new question in mind: Why does the moon rotateon its axis, and yet observers on Earth only see one view? With more reading Ilearned that it takes about the same amount of time for the moon to rotate onits axis (27.3 days) as it does for the moon to make one revolution aroundEarth (29.5 days). I wondered if this was just a coincidence, or if this pieceof information was going to prove to be valuable. One thing that struck meabout this information was that the moon is, in this respect, very differentfrom Earth. Earth takes dramatically different amounts of time to rotate on itsaxis (24 hours) and to revolve around the sun (364.25 days).
Since I didn't learn theanswer to my new question from this reading, I decided to search the Internetfor information. I learned that the moon rotates but does it at such a slowspeed, relative to Earth's speed, that it always keeps the same part facingEarth. I had trouble picturing this, so I decided to prove it to myself bymaking a model. I took two oranges and conducted an experiment.First I put a dot on one side of one orange. That orange was Model Moon, andthe dot represented the nearside (visible surface) of the moon. I then set theother orange, Model Earth, on the table and made a circular path around it withModel Moon, always keeping the dot facing one direction. In other words, themoon was not rotating. Model Earthwas exposed to different views of Model Moon. But when I revolved Model Moonaround Model Earth and at the same time slowly rotated it, the dot always facedModel Earth. It made so much sense now! The moon does rotate on its axis. Onerotation takes nearly as much time as one revolution around Earth. If the moonwere to rotate quickly (several times each month) or not rotate at all, Earthwould be exposed to all sides of the moon (i.e. multiple different views).
With further research Ilearned that millions of years ago the moon actually did rotate much fasterrelative to its current speed. Over time it has slowed down because of theeffect of Earth's gravity. Astronomers call this a "tidally locked" statebecause it will now remain at this speed.Looking back at myhypothesis, I found that it was correct, but my predictions as to how I woulddiscover this were not. I've been told that astral observations often lead tocounterintuitive conclusions, and my experiment was no exception. Originally, Iwas unable to see what was ultimately made obvious by my research and my modelusing oranges.
Several days after drawingthese conclusions, I interviewed Dr. Norman Siems, a professor of astronomy atJuniata College in Huntingdon, PA. I wanted to confirm that all my conclusionswere correct and that there was nothing left unfinished. Professor Siems and Idiscussed all my findings up to this point, and he confirmed my conclusions.Then he brought up a very important point. How could I be sure that the moondidn't simply make one full rotation a day? Maybe that was why I saw the samefeatures with each viewing. For a moment I was baffled. Then it came to me that24-hour rotations would only be possible if I viewed the moon every night atthe same time and saw the same features. If it truly did take the moon 24 hoursto make one rotation on its axis, the moon would look different when I viewedit at 8 p.m. compared to when I viewed it at 11 p.m. And it didn't. The moon always looked the same, regardless of the time or phase.
After ruling thispossibility out, Dr. Siems and I considered whether there were any othersituations that would fit the data I had collected. We could not think of any otherpossibilities, so we resolved that I had narrowed everything down to my answer. The moon rotates, but in doing so alwayskeeps the same face toward Earth. Now sometimes I walk downto our pond and lie back on the dock. I look up at the sky. A shooting starflies by, and I turn my attention to afpectuslunae , the face of the moon. I have learned much in the past few monthsabout the moon, from its countless myths to its countless facts. But knowledgeabout the moon has not been all that I have gathered. I have also learned howto use a telescope and have gotten hands-on experience with the scientificmethod. I have been introduced to a new field, astrophysics. And best of all, Ihave had an unbeatable excuse to stay up late. "Oh, too bad, it'll be a whilebefore I can go to bed, Mama. I have to view the moon."
Branley, Franklyn M. The Moon Seems to Change . New York: HarperCollinsPublishers, 1987.
Couper, Heather, and Nigel Henbest. How the Universe Works . Readers Digest Association, 1994.
Kitt, Michael T. The Moon: An Observing Guide for Backyard Telescopes . Wisconsin: Kalmbach Books, 1992.
Wood, Robert W. Science for Kids: 39 Easy Astronomy Experiments . New York: Tab Books, 1991: 42-44.
Moon. KIDS DISCOVER . October 2001, Volume 11, Issue 10.
Siems, Norm. Interview by Olivia Grugan. 5 January 2004.
Our Moon . Project Astro. Retrieved from the World Wide Web in December 2003. http://www.nfo.edu/ astro/ moon.htm
The Moon . Mount Allison University. Retrieved from the World Wide Web in December 2003.http://www.appletree.mta.ca/ courses/ physics/ 1001/ Misc/ Moon/ MOON.htm
Why Do Observers on Earth See Only One Side of the Moon? Ask an Astronomer. Retrieved from the World Wide Web in December 2003. http://www.ucolick.org/ mountain/ AAA/ answers/ moon/ moll.html
Why Do We See Only One Side of the Moon? Starry Skies. Retrieved from theWorld Wide Web in December 2003. http://staryskies.com/ The_sky/ Events/lunar-2003/ eclipse9.html
More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2004 investigates why we see only one view of the Moon's face. Olivia's narrative essay (with photographs, illustrations, and references) discusses:
- her realization that she didn't know whether the Moon, like Earth, rotates on its axis
- her prediction that she could determine if the Moon was rotating by looking for changes in the appearance of its surface
- the two-month observation she planned and the kit she created for viewing the Moon
- the early results of her experiment and the research discovery that led her to change her question to: Why does the Moon rotate on its axis, and yet observers on Earth only see one view?
Less than 1 period
Supplement a study of astronomy with an activity drawn from this winning student essay.
- Tell or remind students that the Moon rotates on its axis. Then ask: So why do observers on Earth only see one view?
- Send students to this online article, or print copies of the essay for them to read.
- Divide the class into small groups, and have students re-create the model from the article.
OriginYoung Naturalist Awards