Long Intervals refer to extended periods of time when the Red Star doesn't pass close enough to Pern to drop Thread on the planet. Unlike a typical Interval, which lasts two hundred years, Long Intervals are four hundred years long. There have been two such instances in Pern's recorded history, the Fourth Interval and the Eighth Interval, which are referred to as the First and Second Long Intervals respectively.
Because Long Intervals were unpredictable before hand, the length of time without Thread sometimes results in the Pernese thinking that Thread will never return. During the the Second Long Interval, this thinking resulted in an increase in wood products due to increased cultivation of trees. In addition, Holds and Crafts would gain increased stature, as the lack of Thread meant that the Weyrs could not exercise their usual domination. This also resulted in the Weyrs losing stature in the eyes of many. The Ninth Pass occurs directly after a long Interval, and early problems revolve around the fact that Pern at that time did not have enough dragonriders to combat the threat.
After its discovery, AIVAS was initially unaware of the reasons for the two long intervals, though it had several theories. Eventually, however, it is revealed that Long Intervals occurred due to changes in the Red Star's orbit. These changes came about from riders in the Ninth Pass traveling back in time and detonating antimatter engines from the colony ships at precise moments that were calculated by AIVAS. A third detonation, scheduled for the present time, was designed as the final step to alter the Red Star's orbit enough that it would never pass close enough to drop Thread.