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Parts of Pern's dating system are based upon the orbit of the Red Star, which has a 250 year orbit. Passes are the 50 year period in which the Red Star comes close enough to Pern to drop Thread, while Intervals refer to the remaining 200 years. According to AIVAS, however, this method of dating is inaccurate, as the Red Star's orbit is erratic. Thus, the true length of a Pass can either be longer or shorter than 50 years. Passes are numbered sequentially. The First Pass was the first instance of Threadfall encountered by the settlers, while the most recent pass is the Ninth Pass. In two instances, however, the Red Star did not come close enough to drop thread on Pern, resulting in two time periods that became known as Long Intervals.
Passes on Pern are marked by increased activity by dragonriders, who are responsible for eliminating Thread before it can reach the ground. Because of the importance of this duty, dragonriders have developed several ways of judging when a Pass is due. Each Weyr has a stone monument known as an Eye Rock. Eye Rocks are specifically placed so that they will frame the Red Star at dawn at the beginning of a Pass. In addition, several natural phenomenons are known to occur near a Pass. Finally, queen dragons are known to rise to mate more often in the years leading to a Pass, and during the early years of the Pass. Because of AIVAS's plan to eliminate Thread, the Ninth Pass will be the last Pass.