A series of shimmering spheres has appeared all over the globe, hovering a few feet above the ground. A primitive, barely upright mother sees one. A 19th-century British army outpost sees another. A 21st-century U.N. peacekeeping team sees yet another. And without warning, the denizens of various eras are thrown together, forced to make sense of an Earth that's been fractured in time and mashed back together. A great idea -- one would expect no less from Clarke (''2001: A Space Odyssey'') -- but what sets Time's Eye apart from other time-travel yarns is its examination of every facet of a century-striped planet. By the end, when two of history's most ambitious conquerors meet, we are so thoroughly invested in the characters, we can't wait for the sequel.