Banville, the English novelist who fictionalized the life of one classical astronomer in the modestly impressive Doctor Copernicus (1976), now turns to another: Johannes Kepler (1571-1630)--seen here as a long-sufferer, a victim of religious persecution, a husband with complaining wives (especially first wife Barbara, fat and harping), the son of an accused witch. . . and a court mathematician who's most often called on to make up detestable horoscopes. An outcast from Graz in Austria, the penniless Kepler is taken in by Tycho Brahe in the Dane's Bohemian castle, and is "swaddled in the folds of a personality larger far and madder than his own." Tycho's retinue is disorderly and disreputable, the old man himself rude and spiteful and vain--but Kepler holds onto his position: "For he knew now that Mars was the key to the secret of the workings of the world. He felt himself suspended in tensed bright air, a celestial swimmer." And, after Tycho's death (and those of Kepler's wife and children), Kepler still labors on, a sort of Job figure--as he acquires a greater system (harmony of the heavens) if no proportional increase in respect. (Galileo, for instance, either shuns or toys with him.) Banville is a rich writer, his language pleasingly and solidly ornate; but the book's first section--Kepler in Tycho's castle--is the only one that really involves. In subsequent sections, as was the case with much of Dr. Copernicus, the novel descends to mere exposition--excessively dependent on letters and flashbacks. So, while unusually well-written, this historical novel loses its thrust about halfway through, becoming centerless, retrospective, and sometimes flatly crammed with facts.