Gravitation was an Age written by Atrus. While writing it he spent months of calculations and computations for the multiple orbits in diagrams and formulas. Apparently he wrote it for experimentation and entertainment purposes.
The Age is a planet in a complex system, inside gravity fields that interfaced with each other. The planet had six night suns of various inetnsities and colors that attracted each other; moons of various sizes with complex orbits, some rising quickly, slowing to a halt and then danced off at right angles or rested at the horizon. Meteors skimmed the atmosphere that brightened the suns.
However, as Atrus intended, the Age was naturally very unstable, and while he enjoyed the "show" from his vantage, he knew it would be destroyed when all the suns collided.
The book was probably burned by Sirrus and Achenar.