Cycles Make the World Go Round
Sundaran, T. R., The World and I
In both natural phenomena and human endeavors, cyclical variations are amazingly wide-ranging in scope and profound in influence.
We tend to think of our daily lives as proceeding more or less linearly in time and space. But cyclical changes are intricately interwoven in nature and in human activities. Familiar repetitive phenomena include the day-night cycle based on the Earth's rotation, tidal cycles caused by the Moon's gravitational attraction, and seasonal cycles related to the Earth's circumnavigation of the Sun. Equally familiar are our hours of waking and sleeping, working and relaxing, eating, and so forth. Scientific research continues to reveal the ubiquitous nature and tremendous importance of cyclical variations.
What are cycles?
To avoid semantic confusion, it would be appropriate to begin with some fundamental definitions. In general, any event, natural or artificial, that occurs repeatedly and regularly over time constitutes a cycle. The regular, cyclical motions of the hands of a clock are an obvious example.
Cyclical variations are also part of other phenomena in which recurrence is central. They can be observed in the movements of animals--such as walking, swimming, flying, and slithering--and in body functions, such as breathing, blood circulation, and food ingestion and excretion.
Cyclical aspects are included in wavelike phenomena, which involve periodic variations in both time and space. Wavelike variations observed over a period of time at a fixed point in space constitute cycles, while variations in space at a fixed time compose a pattern. Thus, if one stands on the beach at the water's edge, the manner in which the ocean waves approach and recede is cyclical, whereas a snapshot of the waves at any instant reveals their pattern.
Just as water waves have an inherent cyclical nature, likewise all other types of waves--including sound, light, radio, and other electromagnetic waves--have cyclical aspects. Similarly, vibrations and oscillations of all kinds are inherently cyclical. Musical instruments, for example, produce sound from the vibrations of strings (violin, cello, harp), air passages (flute, trumpet), stiff metal (xylophone, cymbals), and so forth.
It is important to note that the term cycle can refer to a single completion of a recurring event as well as to a sequence of such events. It is in the former sense that we use the term recycling, when we refer to the reutilization of material from a product that is normally used only once.
Features of cycles
Typical cyclical variations can be represented in the form of different types of waves. For instance, the motion of a simple pendulum can be represented by what is commonly known as a "sine" wave. Sinusoidal motion is sometimes referred to as simple harmonic motion, because most musical instruments produce vibrations composed of such waves.
A "sawtooth" type variation occurs in the temperature of a heated room: When a controlling thermostat alternately switches the heater on and off, the room temperature increases and decreases somewhat linearly with time. The on-off cycle of the thermostat switch itself can be represented by a "step" wave, as can the behavior of a traffic light that alternates between green and red.
Any wave, and therefore any cycle, is characterized by certain key parameters. For instance, the amplitude of a wave refers to one-half of the maximum variation from peak to trough. In the case of sound waves or light waves, the amplitude gives a measure of the intensity of sound or light.
If one considers a wave traveling through a fixed point in space, the time that elapses between two consecutive peaks or troughs (the amount of time per cycle) is called the period. On the other hand, if one looks at a wavelike pattern at a given instant in time, the distance separating two peaks or troughs is known as the wavelength. …