Answer:
Generally speaking, the range of time for each period gets shorter as we get closer to the present day because the more recent a period is, the more detailed and specific its events can be. This is due to the fact that more records are available for more recent periods than for those in more distant past, allowing us to track more exact dates and times for events. Additionally, since our understanding of history also increases as time progresses, we are able to assign more events to more recent periods.