What typically measures a period of time?

The correct choice identifies a method of measuring time by comparing it with another established period. This approach allows for a relative understanding of duration, as we often interpret a specific timeframe based on another that is already understood or defined. For instance, to understand the length of a decade, one may compare it to a year or a century.

Time can be subjective and relative, so utilizing a reference helps create a standardized measure. This is fundamental in various fields such as history, science, and even daily life where we track significant eras or intervals based on the events and their implications over those periods.

In contrast, measuring time by life expectancy or the events that occur within a timeframe focuses more on specific phenomena rather than establishing a measure of temporal duration itself. Similarly, geographical changes can reflect events over time but do not measure the passage of time in a direct context.