Marinus’ geographical treatise (c. 120 AD) is lost and known only from Ptolemy’s remarks. He introduced improvements to the construction of maps and developed a system of nautical charts. His chief legacy is that he first assigned to each place a proper latitude and longitude. His zero meridian ran through the westernmost land known to him, the Isles of the Blessed around the location of the Canary or Cape Verde Islands. He used the parallel of Rhodes for measurements of latitude. Ptolemy mentions several revisions of Marinus’ geographical work, which is often dated to ad 114 although this is uncertain. Marinus estimated a length of 180,000 stadia for the equator, roughly corresponding to a circumference of the Earth of 33,300 km, about 17% less than the actual value.
He also carefully studied the works of his predecessors and the diaries of travelers. His maps were the first in the Roman Empire to show China. He also invented equirectangular projection, which is still used in map creation today. A few of Marinus’ opinions are reported by Ptolemy. Marinus was of the opinion that the World Ocean was separated into an eastern and a western part by the continents of Europe, Asia and Africa. He thought that the inhabited world stretched in latitude from Thule (Norway) to Agisymba (around the Tropic of Capricorn) and in longitude from the Isles of the Blessed (around the Canaries) to Shera (China). Marinus also coined the term Antarctic, referring to the opposite of the Arctic Circle.