The long winter season of the New York City Ballet, which ends Feb. 20 at the New York State Theater, has been a turning point in the company's life. With director George Balanchine absent for health reasons, the company has had to carry on without the personal inspiration and authority he has always provided. At the very least, the dancers have performed valiantly; and some performances of Balanchine classics have reached landmark quality.
The one major gap in the season has been a new Balanchine ballet, but company members Jacques d'Amboise and Peter Martins filled in the new-ballet slots with aplomb. The most successful of the three premieres was the last, Mr. Martins's ''Rossini Quartets,'' set to two sonatas for string quartet written when the composer was 12.
Martins apportions the music neatly: the fast parts go to an ensemble of six and the adagios go to Suzanne Farrell and Adam Luders. Since most of Martins's previous choreography has been allegro in temperament, the big news here is the stately duets he's made for Miss Farrell and Luders.
Farrell is like a complexly petaled flower at fullest bloom. Her effulgence is tempered by wisdom, by a certain separateness. Martins conveys that separateness by coding her duets with an unusual motif. Aided by Luders, she rolls onto an off-pointe without bending her knees; at other times she moves with enormous amplitude even though her limbs are pressed together. These devices work brilliantly, for they give ''Rossini Quartets'' the distinctive identity all works of art want.
The basic problem with ''Rossini Quartets'' is that the fast sections are too fussy. Martins is forever breaking up phrases between two dancers and chopping the music into more parts than it has. He's too involved with harmony, as it were, and not enough with melody.
The French composer Delibes is all melody, which perhaps is why the other Martins premiere, ''Delibes Divertissement,'' looks under-choreographed. The music, from the ballet ''Sylvia,'' cries out for long, singing lines, but Martins seems uncomfortable with simple grandeur.
Jacques d'Amboise has no hesitation in taking on grandeur in his new ''Celebration'' to various selections of Mendelssohn. It's a cast-of-thousands ballet with a full complement of children to boot. What ''Celebration'' lacks in depth, it makes up for in brio, and the kids are adorable.
D'Amboise uses children as a gesture of celebration for the School of American Ballet, the training ground of the New York City Ballet. The City Ballet is currently celebrating the 50th anniversary of its school and the arrival of Balanchine to this country from Europe. More substantial than d'Amboise's observance of the occasion are two events.
The first is a photographic exhibit of the School of American Ballet's dances and teachers at Lincoln Center's Library for the Performing Arts, which runs into the spring. The second is the March publication of ''Choreography by George Balanchine,'' a complete, annotated catalog of his 425 ballets. This monumental endeavor is published by the Eakins Press Foundation in New York.