Could the $50 purchase of an ancient coin by a Rutgers astronomer have unlocked the mystery of the Christmas Star? For years, scientists have looked, with little success, to astronomical records for an explanation of the magical star that guided the Magi to Christ's manger. Intrigued by the image he found on the latest addition to his coin collection, Michael Molnar thought there might be more to learn by looking, instead, at the teachings of ancient astrologers.

Molnar argues in his book that the Star of Bethlehem was not a star at all, but rather a regal portent centering around the...