Ancient Greeks seem to have their GPS (Global Positioning System), according to the scientist of the Astrophysics Laboratory, Athens University. It is the well known Machine of Antikythira Island, Aegean Sea, which could allocate the geographical longitude of each area and be useful in the creation of reliable maps since the 2nd century BC.
Since there was no Greenwich and no equator at this time, Greek astronomers had determined a meridian that passed through Rhodes and Alexandria and was used to the estimation of the distance and the positions of several destinations.
The longitude was determined by using the moon's position in the sky. Specifically, when a ship was setting sail from Rhodes island to count the longitude in Syrakouses or Gibraltar, the sailor set in the system the moon's position, as it showed in Rhodes sky. When, after a few days, the ship arrived at Syrakouses or Gibraltar, the sailor counted the difference between the position of the moon in Rhodes with that they could see in the sky. This difference could tell the geographical longitude of the area they were situated.