Since the early 1600s, astronomers have been studying sunspots. These relatively dark spots occur individually or in groups on the surface of the sun. These areas represent magnetic storms and appear darker because the temperature in that region is lower than the temperature on the surrounding surface of the sun. Most sunspots have a distinct dark central portion called the umbra and a less dark outer region called the penumbra.
The number of sunspots in any year seems to occur in a cycle. Data that contains cycles that repeat at regular intervals is called periodic data. A cycle in the graph of periodic data is a part of the graph from any point to the first point where the graph begins to repeat the same pattern.
To determine if data has a cycle, graph the data and try to find an identical pattern that appears repeatedly in the graph. The following graphs show one cycle in red.
The length of one cycle is called the period of the graph. See how the period is determined in Example 1.