1620s, "Northern Lights," literally "northern dawn," said to have been coined by French philosopher Petrus Gassendus (1592-1655) after a spectacular display seen in France Sept. 2, 1621; see aurora + boreal. In northern Scotland and among sailors, sometimes called the dancers, pretty dancers, or merry dancers. Related: Aurora australis (1741).
"evening before the first day of the new year," c. 1300; "þer þay dronken & dalten ... on nwe gerez euen." The Julian calendar began on January 1, but the Christian Church frowned on pagan celebrations of this event and chose the Feast of the Annunciation (March 25) as its New Year's Day. The civic year in England continued to begin January 1 until late 12c., and even though legal documents then shifted to March 25, popular calendars and almanacs continued to begin on January 1. The calendar reform of 1751 restored the Julian New Year in England. New Year's was the main midwinter festival in Scotland from 17c., when Protestant authorities banned Christmas, and continued so after England reverted to Christmas, hence the Scottish flavor ("Auld Lang Syne," etc.). New Year's gathering in public places began 1878 in London, after new bells were installed in St. Paul's.
parting salutation, 1860, of unknown origin, perhaps from a German idiom (compare German parting salutation adieu so lange, the full sense of which probably is something like "farewell, whilst (we're apart)"); or perhaps from Hebrew shalom (via Yiddish sholom). Some have noted a similarity to Scandinavian leave-taking phrases, such as Norwegian Adjø så lenge, Farvel så lenge, Mor'n så lenge, literally "bye so long, farewell so long, morning so long;" and Swedish Hej så länge "good-bye for now," with så länge "for now" attested since 1850 according to Swedish sources. Most etymology sources seem to lean toward the German origin. So long (adv.) "for such a long time" is from late Old English.
Earlier guesses that it was a sailors' corruption of a South Pacific form of Arabic salaam are not now regarded as convincing. "Dictionary of American Slang" also adds to the list of candidates Irish slán "safe," said to be used as a salutation in parting. The phrase seems to have turned up simultaneously in America, Britain, and perhaps Canada, originally among lower classes. First attested use is in title and text of the last poem in Whitman's "Leaves of Grass" in the 1860 edition.
An unknown sphere, more real than I dream'd, more direct, darts awakening rays about me — So long!
Remember my words — I may again return,
I love you — I depart from materials;
I am as one disembodied, triumphant, dead.
Whitman's friend and fan William Sloane Kennedy wrote in 1923:
The salutation of parting — 'So long!' — was, I believe, until recent years, unintelligible to the majority of persons in America, especially in the interior, and to members of the middle and professional classes. I had never heard of it until I read it in Leaves of Grass, but since then have quite often heard it used by the laboring class and other classes in New England cities. Walt wrote to me, defining 'so long' thus: "A salutation of departure, greatly used among sailors, sports, & prostitutes — the sense of it is 'Till we meet again,' — conveying an inference that somehow they will doubtless so meet, sooner or later." ... It is evidently about equivalent to our 'See you later.' The phrase is reported as used by farm laborers near Banff, Scotland. In Canada it is frequently heard; 'and its use is not entirely confined to the vulgar.' It is in common use among the working classes of Liverpool and among sailors at Newcastle-upon-Tyne, and in Dorsetshire. ... The London Globe suggests that the expression is derived from the Norwegian 'Saa laenge,' a common form of 'farewell,' au revoir. If so, the phrase was picked up from the Norwegians in America, where 'So long' first was heard. The expression is now (1923) often used by the literary and artistic classes.