Tomorrow is Johann Sebastian Bach's 292d birthday. For the first time in many years, the world can celebrate the great man's anniversary by hearing some music of his that has only recently come to light.

The discovery in 1975 of 12 previously unknown canons adds a whole new chapter to the already fascinating story of the Goldberg Variations by Bach. It's like this:

Two years ago Christoph Wolff, a musicologist from Columbia University, was in Europe doing research into the Goldberg Variations. Looking through a private collection of manuscripts owned by an organist in Strasbourg, France, Wolff found Bach's own printed copy of the Variations. That was in itself in invaluable discovery, since Bach had made important corrections to the famous work on the printed copy.

But when Wolff turned over the back page of the music he found the kind of thing of which musicians' dreams are made: A hand-written manuscript by Bach labeled "Verschiedene Canones ueber die ersteren acht Fundamental-Noten vorheriger Arie" - "Diverse canons upon the first eight notes of the preceding aria ground." Wolff was looking at an unknown major work by Bach.

His discovery, quickly authenticated by the Bach Institute at Goettingen as unquestionably genuine, set off an international round of bidding for the manuscript. The National Library in Paris won the prize, paying $150,000, which is probably the largest sum of money ever paid for a single sheet of music manuscript. The publishing firm of Baerenreiter-Verlag in Kassel published the canons in Wolff's edition.

The new music is a set of 14 canons which Bach wrote in the years 1742-1746. (The complete set numbers 14 of which 12 were completely unknown. Two of the 14 had been known previously in other connections, but had not been known to be part of a set.)

He had just finished writting the Goldberg Variations for his pupil, Johann Gottlieb Goldberg, to play for Count Hermann von Keyserlingk, the Russian ambassador to the court of Saxony. The count, bothered by insomnis, had once told Bach that he would like to have some pieces which Goldberg, who lived in his house, could play for him during his sleepless nights. He suggested that they should be of a "smooth and lively character" that would cheer him up.

Out of his unlikely stimulus Bach created one of the greatest sets of variations in the world. The count was so pleased by them that he sent Bach a golden goblet filled with 100 louis d'or which was probably the largest amoutn of money Bach ever received for any of his compositions.

Thereafter, Count Keyserlingk, never tiring of the new music, used to say to Goldberg, "Dear Goldberg, do play me one of my variations."

There is a further element in this whole story that provides a very special kind of satisfaction. For many years one of the most celebrated paintings of Bach has been that done by Elias Haussmann in 1746 when the composer was 61. In the portrait, Bach is holding in his right hand a single sheet of music paper clearly headed "Canon triplex a SV." This is No. 11 of the newly discovered canons. Its heading refers to a complex kind of round here printed on three staffs and in six "voices."

What we see from Wolff's discovery is that Bach, after completing one of his greatest masterpieces, was still full of ideas for further contrapuntal structures suggested by it, and specifically by the first eight notes in the bass line.

Eventually Bach wrote a set of 14 canons - pieces in counterpoint in which one melody is imitated by those that follow it, as for example in "Row, row, row you boat," - and each canon is based on those same eight notes. The fact that there are 14 of these canons is, by the way, no accidental matter. If you number each letter in the name B-A-C-H according to its place in the alphabet - two plus one plus three plus eight - you will see, as Bach did early in his life, that the total is 14. This is only one of dozens of cases of Bach's making use of numerical-musical parallels.

Following the publication of the new musci, Alan Curtis, a friend of Wolff's, and a Curtis student, Bruce Brown, gave the world premiere of the canons last May on the Berkeley campus of the University of California. They played the music on two harpsichords.

Last July 3 in Marlboro, Vt., Rudolf Serkin and a chamber ensemble gave the first performance of the canons in a version for various instruments. That performance was recorded and is available through the Marlboro Recording Society on MRS 12. It can be ordered through the society, 5114 Wissioming Rd. NW, Washington, D.C. 20016, for $7.50, postpaid.

On the Marlboro recording, Serkin frames the canons with the aria that opens and closes the Goldberg Variations. (So famous a historical event is recalled with his performance that it must be recounted here. When Serkin made his debut in Berlin in the early 1930s, he scored such a triumph that the audience demanded an encore. He played them the entire Goldberg Variations, which take nearly an hour to perform!)

After the opening aria, the first eight notes of whose bass line undergirds all that follows, the canons unfold. They are as fascinating in their simplest forms, as in the first four, as they are in the most complex and vividly exciting, as in the ninth, 11th and 13th. The latter two might well be marked "adagio espressivo" if Bach had used such indications.

While Bach would be the last man to suggest so immodest a theory, there is reason to believe, after examining this "new" music from his unparalleled imagination, that his genius had, literally, no limits. After the last of the 14 canons, instead of writing the usual fine," Bach wrote "et cetera," as if to suggest that the contrapuntal explorations he carried on in these canons, all based on that single fragment of the original Goldberg Variations, are actually journeys that could be continued forever.

They could if Bach were only around the keep them going. Happy Birthday, J.S.B.!