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A natural periodic phenomenon that repeats every day is the rising and falling of the sea levels or what we normally refer to as 'tides'. This is a clear example of a natural periodic activity as it happens at regular intervals daily due to the gravitational pull of the moon and the sun.

A trigonometric equation can describe the tide levels over time. Since both the high tide and low tide occur twice a day, the period of the tide levels is 12 hours. Normalising this value, in terms of radians, would mean a period of \( 2\pi \). If we denote the average sea level as 0, the maximum high tide might be at +a and the minimum low tide at -a.So based on these considerations, a possible model to define sea level, h(t), as a function of time, t, would be a sine function, like this: \( h(t) = a \times \sin(b(t - c)) + d \) This function results in a wave-like graph that oscillates between the extreme tide levels, driven by the 't' variable (which is the time), and repeats every \( 2\pi \). Since the period of this model is 12 hours (or \( 2\pi \) radians), we have 'b' equals to \( \pi/6 \). 'a' is defined as the amplitude (the maximum sea level deviation from average), 'd' as the average sea level, and 'c' as the time of the first high tide.

One question that could be solved by this trigonometric equation might be: 'At what time do the first high tide and the first low tide occur?' To answer this question, we would set h(t) equal to maximum and minimum sea level and solve for t, using the model defined on Step 2.