Prytaneum (πρυτανεῖον) was the common house of an ancient Greek city or state in which a sacred fire was kept constantly burning in honor of Vesta. It was an appropriate building, where, in the name of the city or state, the magistrates, known as the Prytanes, brought suitable offerings to the venerated goddess. The fire-service observed in honor of Vesta was distinguished by the name of Prytanistis. The temple which was called prytaneum was of a round form, in order, as some have supposed, to represent the figure of the earth, and, according to others, to represent the centre of the universe. Plutarch thus speaks on the subject: It is also said that Numa built the temple of Vesta, where the perpetual fire was to be kept, in an orbicular form, not intending to represent the figure of the earth, as if that was meant by Vesta, but the frame of the universe, in the centre of which the Pythagoreans place the element of fire, and give it the name of Vesta and Unity. The earth they suppose not to be without motion, nor situated in the centre of the world, but to make its revolution round the sphere of fire, being neither one of the most valuable nor principal parts of the great machine. Plato, too, in his old age, is reported to have been of the same opinion, assigning the earth a different situation from the centre, and leaving that, as the place of honor, to a nobler element." If the sacred fire in the prytaneum was accidentally extinguished, or even if it continued burning, the vestal virgins invariably renewed it every year on the calends of March by collecting the solar rays in a concave vessel of brass. From the fire which was kept burning in the prytaneum of the parent state, the sacred fire was supplied to each of its colonies or dependent states. Thucydides states that, before the time of Theseus, a prytaneum was to be found in every city or state of Attica. The prytaneum of Athens was originally built on the Acropolis, but afterwards it stood near the agora, or forum.