"Y Môr Tawel"
Translation:The Pacific Ocean
The difference is that 'Y Môr Tawel' has an adjective describing the ocean, ie literally 'the tranquil(pacific) ocean' whereas 'Môr Yr Arctig' (Arctic Ocean) is literally 'ocean of the Arctic' where the name of the ocean comes from its geographical location. The second pattern is much more common.
I though the same thing! Why is no one else wondering this? I have literally JUST learned both words and now they are switched around??
In a way, but 'tawel' can also be translated as 'calm', 'tranquil' and 'peaceful' according to the most comprehensive dictionary 'Geiriadur Prifysgol Cymru' - 'The dictionary of the University of Wales'