Northern means belonging to the colder half of the world above the middle. Northern Europe, the northern lights, a northern accent.
About ninety per cent of the world's people live in the northern hemisphere, because that is where most of the land is. Capitalised it names regions rather than directions — Northern Ireland is a place, not a compass reading. Note the -ern ending on all four directions.
1adjectivenorthernBelonging to the north.
“It snows a lot in the northern part of the country.”
northern is a direction.
Old English northerne comes from north plus the -erne ending used for all four directions.
Proto-Germanic
*nurþrōnijaz
Proto-West Germanic
*norþrōnī
Old English
norþerne
Middle English
northerne
English, today
northern
From Middle English northerne, from Old English norþerne, from Proto-West Germanic norþrōnī, from Proto-Germanic nurþrōnijaz. Cognate with Old High German nordrōni and Old Norse norrœnn. Morphologically north + -ern.
Etymology from Wiktionary, CC BY-SA