NEW YORK - Nobody - except perhaps the omniscient Mr. S. Claus - knows for sure who wrote the famous children's poem beginning: "'Twas the night before Christmas ..."

That may be news to the millions of people, of numerous generations, who always have supposed that the author was Dr. Clement Clarke Moore. THe bibliographies, encyclopedias, histories and copies of the poem itself all give that name. So do the markers which show where the spacious and many chimneyed Moore house stood on West Twenty-third street in the old Chelsea district. Every Christmas eve, too, a lot of children gather in the court yard of the huge apartment development which now occupies the site of the Moore estate to sing carols and recite the poem in tribute to the memory of that notable scholar and somewhat less distinguished poet.

The family of Dr. William S. Thomas, however, will tell you that all this Clement Moore business is a mistake. It is their contention, backed by a family legend more than a hundred years old, that Dr. Thomas' great-grand-father, one Henry Livingston, Jr., wrote the poem himself.

They believe he composed it in his home near Poughkeepsie, N. Y., a few days before the Christmas of 1822, read it at the breakfast table the next morning, and gave a copy to a house guest who later showed it to Dr. Moore.

Scholar and a Poet
The facts on both sides of the case are meager but interesting. Dr. Moore himself was of unimpeachable character and couldn't be accused by anyone of intentionally appropriating some one else's verse. His father was the Episcopal bishop of New York who assisted at the inauguration of George Washington and administered the last rites to Alexander Hamilton. Clement Moore was a great scholar, but didn't go in much for whimsey. For example, his principal work was "A Compendious Lexicon of the Hebrew Language." Another bit that he dashed off was "Observations Upon Certain Passages in Mr. Jefferson's Note on the State of Virginia Which Appear to Have a Tendency to Subvert Religion and Establish a False Philosophy."

Dr. Moore did write verses. He published a volume of them in 1844, and "A Visit from St. Nicholas" - the original title fo the disputed poem - was included. Incidentally, it was by all odds the best of the lot, and there was only one other poem in the book which was done in the same metre.

Henry Livingston had been dead ten years when the volume was published, and his family evidentaly made no strenuous protest against

First Published Anonymously

Livingston seems to have just as unimpeachable a reputation as Dr. Moore. He was a statesman, a soldier and an author. And he also wrote a book of poems. "Twas the Night Before Christmas" was not among them, but the family legend contains an explanation for its absence. The minority faction in the literary feud also claims that many of the Christmas poem's phrasings and tricks of expression are used in the book. And it is a fact that nearly half of the verses are in the same meter.

The most baffling part of the whole mystery is that the poem first was published anonymously in a newspaperk, the Troy Sentinel, in 1823. Mr. Livingston was a reader of that paper. The editor put in a little note saying it was a fine piece of work but he didn't know who had submitted it. Neither Moore nor Livingston claimed authorship at that time, and neither left any writings mentioning it. The original manuscript, should it ever turn up, would be almost priceless. It also would settle an ancient argument.