voter's name, Themistocles had set his eyes on a radical new constituency" However, he took care to ensure that he did not alienate the nobility of Athens. He began to practice law, the first person in Athens to prepare for public life in this way. His ability as attorney and arbitrator, used in the service of the common people, gained him further popularity. Archonship Themistocles probably turned 30 in 494 BC, which qualified him to become an archon, the highest of the magistracies in Athens. On the back of his popularity, he evidently decided to run for this office and was elected Archon Eponymous, the highest government office in the following year (493 BC). Themistocles's archonship saw the beginnings of a major theme in his career; the advancement of Athenian sea-power. Under his guidance, the Athenians began the building of a new port at Piraeus, to replace the existing facilities at Phalerum. Although further away from Athens, Piraeus offered three natural harbours, and could be easily fortified. Since Athens was to become an essentially maritime power during the 5th century BC, Themistocles's policies were to have huge significance for the future of Athens, and indeed Greece. In advancing naval power, Themistocles was probably advocating a course of action he thought essential for the long-term prospects of Athens. However, as Plutarch implies, since naval power relied on the mass mobilisation of the common citizens (thetes) as rowers, such a policy put more power into the hands of average Athenians—and thus into Themistocles's own hands. Rivalry with Aristides After Marathon, probably in 489, Miltiades, the hero of the battle, was seriously wounded in an abortive attempt to capture Paros. Taking advantage of his incapacitation, the powerful Alcmaeonid family arranged for him to be prosecuted. The Athenian aristocracy, and indeed Greek aristocrats in general, were loath to see one person pre-eminent, and such maneuvers were commonplace. Miltiades was given a massive fine for the crime of 'deceiving the Athenian people', but died weeks later as a result of his wound. In the wake of this prosecution, the Athenian people chose to use a new institution of the democracy, which had been part of Cleisthenes's reforms, but remained so far unused. This was 'ostracism'—each Athenian citizen was required to write on a shard of pottery (ostrakon) the name of a politician that they wished to see exiled for a period of ten years. This