Schoenherr's cover for this is one of my all-time favorites by him: capturing a sense of scale, immense age, and sense of wonder...

Catch a Falling Star - a novel by John Brunner. Revised/expanded from The 100th Millenium (Ace Double:1959), which itself was an expansion of "Earth Is But a Star" from Science Fantasy June 1958.

A hundred thousand years from now, it was discovered that a star was approaching the world on a collision course. Its discoverer, Creohan, figured there might be time to save the world if he could arouse everyone to the danger. But the Earth had become a strange and kaleidoscopic place in that distant era. Too many empires had risen and fallen, too many cultures had spread their shatterred fragments acrocc a planet whose very maps had long been forgotten. People were too busy with their own private dreams to pay attention to one more new alarm. Could nothing breach that monstrous wall of human indiffrence, before the air seared their throats and the hair burned crackling on their heads?