A week ago, Randall Munroe finished "Time", XKCD's long, running, slow-updating, 3,000+ frame comic telling the story of two people who discover an impending superflood that would destroy their society. Randall's explained in detail what was going on there, from the geology of the thing (it's set millennia in the future, amid a civilization denied the ability to jumpstart itself by the paucity of remaining fossil fuels, and the flood is modelled on a real event that sealed off the Mediterranean Sea five million years ago) to the fictional language the upland culture speaks (designed by a linguist, and still mysterious).
More clues lay not only within the world, but above it. Astronomically-savvy readers finally got a chance to nail down the time period of the comic during a striking sequence later in the series where the night sky slowly rotated over the characters, revealing a distinctly different configuration of stars than our present sky.
"The Earth's axis wobbles over the millennia, and some individual stars move visibly, so I used a few different pieces of astronomy software–with a lot of hand correction and tweaking–to render the future night sky," said Munroe. He also consulted with astronomer and science blogger Phil Plait and learned that while most visible stars would still be around 11,000 years in the future, the red supergiant star Antares could go supernova and vanish from the starfield.
"When the Sun sets in the night sequence, one of the first things you see is the gap where Antares should be, which was the first clue that this is taking place in the far future," said Munroe. "Later in the night–which lasted for several days of real time–more astronomical details let readers pin down the date more precisely."
Creator of xkcd Reveals Secret Backstory of His Epic 3,099-Panel Comic [Laura Hudson/Wired]