The United States is a country in the Northern Hemisphere, Western Hemisphere, and the Eastern Hemisphere. It consists of forty-eight contiguous states in North America, Alaska, a peninsula which forms the northwestern most part of North America, and Hawaii, an archipelago in the Pacific Ocean. There are several United States territories in the Pacific and Caribbean. The term "United States", when used in the geographical sense, means the continental United States, Alaska, Hawaii, Puerto Rico, Guam, and the Virgin Islands of the United States. The country shares land borders with Canada and Mexico and maritime (water) borders with Russia, Cuba, and the Bahamas in addition to Canada and Mexico.
Maritime history is the study of human activity at sea. It covers a broad thematic element of history that often uses a global approach, although national and regional histories remain predominant. As an academic subject, it often crosses the boundaries of standard disciplines, focusing on understanding humankind's various relationships to the oceans, seas, and major waterways of the globe. Nautical history records and interprets past events involving ships, shipping, navigation, and seafarers.
Maritime history is the broad overarching subject that includes fishing, whaling, international maritime law, naval history, the history of ships, ship design, shipbuilding, the history of navigation, the history of the various maritime-related sciences (oceanography, cartography, hydrography, etc.), sea exploration, maritime economics and trade, shipping, yachting, seaside resorts, the history of lighthouses and aids to navigation, maritime themes in literature, maritime themes in art, the social history of sailors and passengers and sea-related communities.
The Waldseemüller map, Universalis Cosmographia, is a printed wall map of the world by German cartographer Martin Waldseemüller, originally published in April 1507. It is known as the first map to use the name "America". The map is drafted on a modification of Ptolemy's second projection, expanded to accommodate the Americas and the high latitudes. A single copy of the map survives, presently housed at the Library of Congress in Washington, D.C. Waldseemüller also created globe gores, printed maps designed to be cut out and pasted onto spheres to form globes of the Earth.
The sea, the world ocean, or simply the ocean, is the connected body of salty water that covers over 70 percent of the Earth's surface. It moderates the Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been travelled and explored since ancient times, the scientific study of the sea—oceanography—dates broadly from the voyages of Captain James Cook who explored the Pacific Ocean between 1768 and 1779. In geography, "sea" is used in the names of smaller, partly landlocked sections of the ocean, for example the Irish Sea, while "ocean" is used in the names of the five largest sections, such as the Pacific Ocean.
The most abundant solid dissolved in sea water is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and many other elements, some in minute concentrations. Salinity varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean; however the relative proportions of dissolved salts vary little across the oceans. Carbon dioxide from the air is currently being absorbed by the sea in increasing amounts, lowering seawater pH in a process known as ocean acidification, which is likely to damage marine ecosystems in the near future.