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The Dragonlovers guide to pern
VII. THREADFALL CHARTS
The first Fall the colonists experienced was actually the fourth Fall of the Pass. The other three had fallen in the sea, in the polar ice caps where the mycelia could do no damage, or onto unoccupied terrain.
An aerial survey of the areas attacked by Thread revealed that the Thread appeared at predictable intervals in predictable locations. It would traverse three time zones for a period of four hours, covering twenty degrees longitude northeast to southwest. In the Second Long Interval, much of this vital documentation was lost and had to be tediously reconstructed by F'lar of Benden Weyr in the Ninth Pass.
Thread always fell within five degrees north or south of the strip predicted. As the Pass went on, the attacks came more and more frequently but did not vary from this pattern, which spiraled from north to south, matching the rotation of the planet.
The colonists used the term “Leading Edge” to denote the wave of Thread advancing during a Fall, and “Following, or Trailing Edge,” to refer to the line behind which no Thread fell. Once the Following Edge was in view, the fighters knew that the Fall was about to end. At sea level, Thread is visible on the horizon thirty minutes before it will be overhead.
Passes lasted fifty years as the Red Star drags its trail of Oort matter through Pern's atmosphere. Thread would begin to fall in the first warm weather after the time in midwinter when the Red Star appeared bracketed by the Eye Rock and the sun balanced on the Star Stones, rock formations that were placed on the lip of every Weyr to provide eternal pointers.
The Red Star passes through and around the Rukbat system on a revolution of 250 years. For 50 of those years, it descends, intersecting the plane of the ecliptic like a chain link. During the remaining 200, the Red Star makes the rest of its journey outside Rukbat's solar system until it reaches the point of aphelion, where it begins its pass once more.
Because the planet has an eccentric orbit, it does not always drag its trail of Oort matter close enough to Pern to cause a Pass. Twice in human memory on Pern, the Red Star failed to come into contact with the third planet.
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