Land Planisphere Showing Longitude

This 1696 polar projection world map by Jacques Cassini (1677–1756) is the replica and only surviving representation of the large, 7.80-meter diameter planisphere by his father, Jean-Dominique Cassini (1625–1712). The first director of the Paris Observatory, the elder Cassini had designed the planisphere on the floor of one of the observatory's towers, using astronomical observations performed by correspondents of the Academy of Sciences. The map shows 43 places, from Quebec to Santiago, from Goa to Beijing, each marked with a star, with latitudes accurately measured using a method that relied upon observation of the moons of Jupiter. The longitudinal measurements on the map are less accurate, as the determination of longitude remained problematic until the installation on ships, in the second half of the 18th century, of marine chronometers. Their significance was that they could precisely measure the time at a known meridian when out of sight of land, which then could be used to determine longitude based on the rotation of the Earth. The map also displays uncertainties regarding the northern borders of Asia and America, which persisted until the discovery of the Bering Strait in

Date

1696

Mapmaker / publisher

Cassini, Jean-Dominique, 1625-1712

Archive

Wikimedia Commons

Type

chart

Original size

1,024 × 1,024 pixels

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