Two Greek mathematicians pioneered research to navigate and map the Earth. Eratosthenes of Cyrene, chief librarian at the Ancient Library of Alexandria, calculated the circumference of the Earth in the third century B.C. In the second century B.C., Hipparchus of Nicea first used mathematics to calculate latitude and longitude, invented the first astrolabe to observe stars and measure latitude, and observed lunar eclipses to propose longitudes for specific cities.
Not much is known about the work of Eratosthenes and Hipparchus, but what little survives is evidence of how people long ago thought to organize maps into standardized grids. Because Eratosthenes had discovered the circumference of the Earth, Hipparchus could work from his calculations to establish a line of zero latitude (the equator) and lines of longitude. Through his own observations, Hipparchus learned how to measure latitude by observing the movement of stars over time, and he proposed a system of timed measurements to calculate longitude.
Later, Ptolemy was the first to use a constant meridian for drawing maps. Early in the 18th century, English clockmaker John Harrison developed the chronometer, which compensated for ships’ pitch and roll, humidity and other factors. Coupled with accurate star charts developed by Royal Astronomers John Flamsteed and Edmund Halley in addition to a long series of observations from the Royal Observatory at Greenwich, England, this led cartographers to adopt the Greenwich Meridian as a worldwide standard in 1884.