Parts of Pern's dating system are based upon the orbit of the Red Star, which has a 250 year orbit. The Red Star is close enough to drop Thread on Pern for only fifty years. Thus, Intervals are defined as the remaining 200 year period, beginning the first year after the end of Threadfall and ending when Threadfall returns. However, according to observations by AIVAS, the true length of an interval varies by several years due to the erratic nature of the Red Star's orbit. Intervals are numbered in reference to the Pass they follow. Thus, the First Interval followed the First Pass.
In addition, there have been two intervals which have been much longer, around 450 years, when the Red Star never came close enough to drop thread on Pern. These intervals are known as Long Intervals. It is eventually revealed by AIVAS that the two Long Intervals were caused by forced shifts in the Red Star's orbit by antimatter engines detonated at specific points in time. Because of AIVAS's plan to eliminate Thread, the Ninth Pass will be the last pass. It remains unknown whether Intervals will be used to refer to future periods of time.
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