Cycles of Nature: The Difficulties in Seeing Cycles

When scientists plan and carry out research, they have time-honored methods they apply to every aspect of the study.  Chronobiology is a branch of science where some of the standard methods of measurement are having to be revised, to effectively identify cycles.  Using tides as an example, let's look at how to effectively identify a cycle.  (Keep in mind, often scientists may not know if there is a cycle present, or what the length (period) of that cycle is.)

Jessica likes to dig for clams, but she can only find them when the tide is out.  She has decided to map the tides to predict the timing of low tide.  She is using a stick in the sand to measure the height of the tide.

For the first couple of days she takes measurements at 8am and noon.  (Figure 1.1, days 1 & 2) The results are very puzzling — on the first day the tide seems to go up from morning to noon, and on the second day it seems to go down.  Jessica decides she will have to watch the tide much more carefully to figure out what is going on.  So the next day she packs a lunch and notes the heights on her stick at every hour, creating the graph in Fig 1.1, day 3.  Jessica now sees the unmistakable cycle of the tide. The water will lower gradually until it reaches the lowest point, and then it begins to rise.  But why did the first two days seem reversed? 

Figure 1.1

Figure 1.1
Tidal measurements on days 1,2 & 3.

Jessica continues her watch on the beach for a few days (Figure 1.2, Days 4, 5, and 6).  Although her data only show daytime values, she could extrapolate the curves across several days — the tides come in and out in a cycle of about 13 hours (with a peak during the day, and probably another at night), so high tide shifts just a little later each day.  Extending this pattern to her first two days, she can see how her earliest results fit this pattern — and why they had been so confusing!  She had measured at different points in the curve, giving misleading results!  (Fig 1.3)

Figure 1.2

Figure 1.2
Tidal measurements of days 4, 5 &6

Fibure 1.3

Figure 1.3
Extending the pattern from days 3-6 back onto days 1 & 2.

This is an important concept when measuring fluctuating, or cyclic variables!  Whether studying cycles in the tide or anything else, it is important to have enough data. If you don't, your results can be confusing.  A true cycle can look like random changes-or random changes may look like a cycle.