When is a legend legend? Why is a myth a myth? How old and disused must a fact be for it to be relegated to the category "Fairy-tale"? And why do certain facts remain incontrovertible while others lose their validity to assume a shabby, unstable character?

Rukbat, in the Sagittarian sector, was a golden G-type star. It had five planets, and one stray it had attracted and held in recent millennia. Its third planet was enveloped by air man could breathe, boasted water he could drink, and possessed a gravity that permitted man to walk confidently erect. Men discovered it and promptly colonized it. They did that to every habitable planet, and then -- whether callously or through collapse of empire, the colonists never discovered and eventually forgot to ask -- left the colonies to fend for themselves.

When men first settled on Rukbat's third world and named it Pern, they had taken little notice of the stranger-planet, swinging around its adopted primary in a wildly erratic elliptical orbit. Within a few generations they had forgotten its existence. The desperate path the wanderer pursued brought it close to its stepsister every two hundred (Terran) years at perihelion.

When the aspects were harmonious and the conjunction with its sister planet close enough, as it often was, the indigenous life of the wanderer sought to bridge the space gap to the more temperate and hospitable planet.

It was during the frantic struggle to combat this menace dropping through Pern's skies like silver threads that Pern's tenuous contact with the mother planet was broken. Recollections of Earth receded further from Pernese history with each successive generation until memory of their origins degenerated past legend or myth, into oblivion.

To forestall the incursions of the dreadful Threads, the Pernese, with the ingenuity of their forgotten Terran forebears, developed a highly specialized variety of a life-form indigenous to their adopted planet. Such humans as had a high empathy rating and some innate telepathic ability were trained to use and preserve this unusual animal whose ability to teleport was of great value in the fierce struggle to keep Pern bare of Threads.

The winged, tailed, and fiery-breathed dragons (named for the Earth legend they resembled), their dragonmen, a breed apart, and the menace they battled, created a whole new group of legends and myths.

Once relieved of imminent danger, Pern settled into a more comfortable way of life. The descendants of heroes fell into disfavor, as the legends fell into disrepute.