But it is when we come to Pytheas of Massilia that we reach the border-line which separates fact from fable. This eminent astronomer and geographer of Marseilles brought together a knowledge of northern countries which was based not on premonition, not on speculation, not on hearsay, but on actual experience. So original, so accurate, and so far-reaching was his work, that for the next fifteen hundred years he dominated all geographical knowledge. We can fix his time if we remember that he flourished probably about the year 330 B.C. He was the first person in history to introduce astronomical measurements for ascertaining the geographical situation of a place, and thus became the founder of the science of navigation--the science which has enabled seas to be crossed in safety and continents to be discovered; which has given to the ship of all species a freedom to employ her speed without sacrificing safety. Indirectly arising from these may be traced the development and civilising and peopling of the world which have so entirely modified history. By means of a great gnomon, Pytheas determined “with surprising accuracy” the latitude of Marseilles, and in relation to this laid down the latitude of more northerly places. He observed that the Pole of the heavens did not coincide, as the earlier astronomer Eudoxus had supposed, with any star. What Pytheas did find was that it made an almost regular rectangle with three stars lying near it. (At that time the Pole was some distance from the present Pole-star.) And since Pytheas steered by the stars, the Pole of the heavens was obviously of the highest importance to him. A gnomon, it may be explained, was the pillar of projection which cast the shadow on the various Greek forms of dial. In the case under discussion the gnomon was a vertical column raised on a plane.