Creation, by Gore Vidal. New York: Ballantine Books. 593 pp. $3.95

Set in the 5th century, this novel is narrated by Cyrus Spitama, grandson of Zoroaster, who dictates his memoirs to his nephew. As the ''king's eye,'' Cyrus visits Greece, India, and Cathay, and marries twice. The novel's slow progress across Persia and Cathay allows Vidal to introduce endless historic tidbits. These curious details lead to the question of whether or not such information constitutes a novel? The answer is: probably not. Another question arises from the aloofness of the ''king's eye.'' He remains apart as a spectator, not an actor. And the reader is left with no one he really knows or cares deeply about. A weakness of ''Creation'' is that, although the Persian version of the Peloponnesian War is told, it sounds like a distant twang - a dynasty passes, and the reader hardly notices.