What effects do lunar cycles have on living organisms?
https://oceanservice.noaa.gov/education/tutorial_tides/ - Tides and Water Levels Tutorial and Subject Review
http://www.edusite.com/sci/tides.htm – Links to internet sites with information about tides
Two or three 45-minute periods, plus time outside of class for research and preparation
Groups of four or more students
Tides are the periodic rising and falling of ocean waters caused by the gravitational forces of the sun and moon. The vertical motion of tides is accompanied by a horizontal movement of ocean waters called tidal currents. Oceanographers say that tides rise and fall, while tidal currents ebb (during a falling tide) and flood (during a rising tide).
For a simple explanation of tides, it is sufficient to consider only the effects of the moon (the magnitude of the moon's effect is about twice that of the sun, since the moon is closer to the Earth). At any point in a day, one side of the Earth will be closer to the moon than the opposite side. Ocean waters on the closer side of the Earth will experience a greater gravitational pull from the moon than waters on the opposite side of the earth (tide-generating forces vary inversely as the cube of the distance from the tide-generating object). This causes a "bulge" in the waters closest to the moon, and creates a high tide. At the same time, inertial forces on the opposite side of the Earth causes a similar "bulge" creating another high tide. Low tides occur at longitudes that are 90° from the longitudes of locations that are experiencing high tides. The sun also exerts a gravitational force on ocean waters. Depending upon the positions of the sun and moon relative to the Earth, the sun's gravity may enhance or diminish the tidal effect caused by the moon. When the sun and moon are aligned (at the time of full moon or new moon), their gravitational forces act in the same direction and produce more pronounced high and low tides that are called spring tides. When the sun and moon are at right angles relative to the Earth, the gravitational force of the sun partially cancels out the gravitational force of the moon. The result is less pronounced high and low tides that are called neap tides. The magnitude of tides is also affected by the actual distances between the sun, moon, and Earth: Gravitational attraction is increased when the Earth is closest to the sun (perihelion) or moon (perigee), For more information on the causes and types of tides, visit https://oceanservice.noaa.gov/education/tutorial_tides/.
For centuries, people who live and work near seacoasts have understood the importance of being able to predict tides and tidal currents. Low tides may prevent ships from entering harbors, while high tides may make it impossible for ships to pass beneath bridges. Tidal currents may significantly increase the speed of a vessel, or may carry it into dangerous waters. Recreational boaters who anchor near shore may find themselves stranded by a falling tide. Fishermen throughout the world have learned that catches are likely to be much larger during certain portions of the tidal cycle than others.
Many people who live thousands of miles from any ocean also pay close attention to the same lunar cycles that produce tides. Traditional agricultural practices often prescribe planting and harvesting during times when the moon is at a specific location relative to the Earth. Some traditional forestry practices link lunar cycles with specific properties of wood including strength, resistance to decay, and resonance for musical instruments. Lunar cycles have also been linked with many aspects of human biology, behavior, and folklore including crime, suicide, mental illness, birthrates, fertility, werewolves, and vampirism. In this lesson, students will research and analyze various phenomena in living organisms that have been attributed to lunar cycles.
1. If they have not already done so, have students complete the Tides and Water Levels Subject Review at: https://oceanservice.noaa.gov/education/tutorial_tides/. If you choose to have students work in groups, you may want to assign different tutorial sections to each student. Have each student or student group answer questions in the Tides Subject Review. To save class time, you may want to assign this portion of the lesson as homework. Lead a discussion of students' answers to the questions, highlighting ways in which knowledge of tides can be useful and important.
2. Have student groups research and prepare written reports on at least three phenomena in living organisms that are allegedly affected by lunar cycles. Assign one of the following topics to each student group:
Tell students that in addition to their assigned topic, they should identify and report on two additional phenomena in living organisms that are allegedly affected by lunar cycles. Reports should include hypotheses or discussions of the mechanism(s) through which lunar cycles might affect the phenomena being discussed. Direct all groups to review the discussion of lunar myths at: http://skepdic.com/fullmoon.html.
3. Have each student group present an oral presentation of their research results and analyses. When all groups have made their presentations, lead a discussion of the pooled results. Reports should reflect a mix of well-documented effects of lunar cycles, as well as "effects" that have no demonstrated factual basis. Discuss possible reasons for widespread belief in "effects" (such as emergency room admissions) for which there is little or no supporting evidence. The discussion at http://skepdic.com/fullmoon.html explores the influences of media, folklore, tradition, misconceptions, cognitive biases, and communal reinforcement. Encourage students to discuss how the same factors may affect other popular perceptions about natural phenomena and scientific investigations.
Be sure students realize that lunar cycles almost certainly are important in the timing of some phenomena (such as reproduction in many marine animals). Discuss possible mechanisms through which lunar cycles might affect these phenomena. In many cases these mechanisms are not well-understood, even when the cause-and-effect relationships are well-documented.
Neuro-endocrine systems are thought to be involved in many cases, but the precise ways in which these systems detect variations in lunar cycles are not known. Given the large gaps in our knowledge about these mechanisms, it is entirely possible that many biological phenomena are sensitive to lunar cycles. While skepticism for claims about lunar effects is certainly warranted, it is also important to keep an open mind. In an article about lunar influences on women's reproductive cycles, Winnifred Cutler and co-authors concluded, "Historical indication that fertility rites were scheduled with consideration for the phase of the moon may have been reflecting accurate perceptions which we have yet to discover."
www.vims.edu/bridge – Click on "Ocean Science Topics" in the navigation menu to the left, then "Physics," and then the "Tides" button at the top of the page.
Have students write a short essay on why knowledge about tides and water levels is (or might be) important in their own lives.
The rise and fall of ocean tides involve huge amounts of energy. Have students or student groups prepare a report on how this energy could be captured in a form that could be used by humans. Reports should include a plausible mechanism for converting tidal motion into useful energy, a strategy for transporting captured energy to potential users, and a rough estimate of the energy that might be available from tidal sources. A keyword search on "tide energy" will produce lots of starting points.
https://oceanservice.noaa.gov/education/kits/tides/supp_tides_roadmap.html – NOAA's National Ocean Service Web site's Roadmap to Resources about tides and water levels, with links to many other sources of tide data and background information
http://www.navmetoccom.navy.mil – Naval Meteorology and Oceanography Command Web site with information on waves and tides and other oceanography topics.
http://mdc.mo.gov/conmag/1998/11/50.htm - Article about "Lunar Lore" on the Missouri Conservation Commission Web site
http://skepdic.com/fullmoon.html – Article about evidence and beliefs concerning full moon and lunar effects.
http://www.athenainstitute.com/lunarmpl.html - B., W. M. Schleidt, E. Freidmann, G. Preti, and R. Stine. 1987. Lunar influences on the reproductive cycle in women. Human Biology. 59(6).
http://www.edusite.com/sci/tides.htm – Links to educational resources on ocean science topics
http://www.internet4classrooms.com/tide.htm – Links to internet resources dealing with tides
http://school.discovery.com/curriculumcenter/oceans/ – Discovery Channel School resources on ocean science topics.