A vast body of water can often be referred to as sea or an ocean – and the terms are used interchangeably – but there is a crucial difference between the two terms, to do with their size and geographical features.
What’s the difference between a sea and an ocean?
Oceans are the largest bodies of water on earth, covering about 70 per cent of the planet. Seas are smaller than oceans and are usually located where the ocean meets the land. They are found at the margins of the ocean, where oceans become partially enclosed by land.
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Ocean currents affect climate, geography and environmental conditions. The Atlantic Gulf Stream, for example, is powered by the North Atlantic Ocean current and brings warm air from the Gulf of Mexico to Europe.
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While we think of the five separate oceans – Pacific, Atlantic, Indian, Southern and Arctic – they are really one giant body of water, whereas seas are divided by land.
A sea is a part of an ocean where the ocean meets the land. The North Sea, for example, is part of both the Atlantic and Arctic oceans, but it’s classed as a sea because it’s located between two land masses – the British Isles and the European continent.
Seas are also much shallower than oceans, and there are about 50 seas in the world, as opposed to the relatively fewer – and much larger – five oceans.
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Are there any exceptions to this rule?
Unfortunately, yes! The Sargasso Sea doesn’t have a land border but is referred to as a sea. This is because it is surrounded and bound by four ocean currents, which form a gyre (vortex) around it.
And then you have inland seas. An inland sea is surrounded by dry land or linked to an ocean via a river, strait, or an 'arm of the sea.' Inland seas include the Black Sea, the Marmara Sea and the Baltic Sea.
Top image: A distant cruise ship in the Arctic Ocean (credit: Getty Images)
