(adv.) in a northerly direction
If you’re heading or travelling septentrionally, then you’re heading north.
If you’re wondering where that word comes from (and are somewhat puzzled by the apparent inclusion of the ‘sept–’ you’d find in words like septuple and September) then the answer actually lies in the night sky.
Septentrion is an old name for the constellation better known to us today as Ursa Major, or the Great Bear—or, even more familiarly, the Plough. That name is Latin, and refers to the seven (Latin septem) stars of which the constellation is comprised, to roughly form the shape of a plough blade (the Latin word triones meaning ‘ploughing oxen’).
Because the ‘handle’ of this astronomical Plough points north and onwards to the North Star, its name because a byword for anything concerning the north or a northerly direction. In medieval documents, indeed, you’ll find the word Septentrion used as a byword for north itself, or for someone or something heading or coming from a northerly direction—a use that survived in formal and poetic contexts right through to the nineteenth century and beyond.
It is quite clear why the Italians have no word but recitare to express acting, for their stage is no more theatric than their street, and to exaggerate in the least would be ridiculous. We graver-tempered and -mannered Septentrions [i.e. people from northern Europe] must give the pegs a screw or two to bring our spirits up to nature's concert-pitch.
James Russell Lowell, Italy (1854), in Leaves from My Journal (1890)
Septentrionality, ultimately, is northerliness, or northernness. Something that is septentrionic is located in the north. To septentrionate is to point north. And to move septentrionally is to head in a northerly direction.