Definition from Wiktionary, the free dictionary
I would say that "northernmost" is a superlative form of "northern", along with "most northern". At least, in all languages I know "northernmost" would be translated by means of a superlative.
- I'm not sure if that applies to all languages. In Dutch you would usually say "noordelijker" (a comparative) when comparing two places: de noordelijker van de twee steden "the more northern of the two cities". But when comparing more than two places the superlative is used: de noordelijkste van de vijf steden "the most northern of the five cities". It is interesting that there is no synthetic comparative like *northernmore in English, but etymologically it makes sense as the -most isn't actually the word most but a suffix deriving from Germanic *-umist-, itself a compound of two comparative suffixes put together. —CodeCat 19:45, 9 January 2013 (UTC)