Only alien help can save humanity from being exterminated, but thanks to relativity, the request for help must start through a young Roman soldier, Gaius Claudius Scaevola, who must be abducted by a passing alien ship. To be taken to the right place the abduction must violate the major law against interfering in the development of emerging societies. Accordingly, he has to do three things: prove the Earth goes around the Sun, design a workable steam engine, and become a significant military strategist. Gaius is ordered by Tiberius to earn an agnomen based on what he will learn from Timothy on Rhodes, and it is on Rhodes that Gaius receives his prophecy and instructions from Pallas Athene. While Gaius does not believe in Gods, parts of the prophecy gradually come to pass.
All does not go well. Timothy proves to him the Earth cannot go around the Sun, Tiberius dies and Caligulae abolishes his military appointment, then in Alexandria, when Gaius sees the toy steam engine, he knows he cannot build it bigger. Then, in the anti-Jewish riots, a further part of the prophecy comes to pass, and when he does what he should, Caligulae appoints him as Tribunis Laticlavius in the Fulminata, where he becomes involved in the emerging Christianity, a battle against Parthians, and finally, the crisis at the Temple in Jerusalem. Besides having three near-impossible tasks, he must also survive the erratic Imperium of Caligulae. Besides being as historically accurate as I can make it, where relevant, the story also involves how to do science. You, the reader, know the answer, but could you prove the Earth goes around the Sun with what was known then?