2 The Earth’s oceans and seas

An ocean is a vast body of salt water of considerable depth. On Earth there are reckoned to be five main oceans; all of them are connected and together they form the World Ocean. Two of the oceans, the Pacific and Atlantic, are bisected on maps by the Earth’s equator, so sometimes it is said that there are seven oceans. Assuming the Earth to be a perfect sphere with a constant radius from its centre to a smoothed-out sea level (known as a geoid), the surface areas of the oceans can be compared with each other and the total surface area of Earth (i.e. land and water combined) as in Table 3.

Thus the oceans make up around 70% of the Earth’s surface. The expression ‘sail the seven seas’ actually refers to the oceans. A sea in itself is also a body of water somewhat smaller in surface area and shallower than an ocean and bounded fully or partially by land masses – to a greater extent than oceans at any rate. Table 4 shows a partial list of well-known seas.