OMEROS By Derek Walcott (Faber, $175) DURING my summer stay in Trinidad, the American astronauts landed on the moon. I was flattered when people congratulated me on behalf of my countrymen. The flattery turned to amusement when a fierce debate arose in the Port-of-Spain newspaper: the moon landing was a great triumph for mankind said one faction. No, it was a great fraud; the moon is covered with water and no landing could have taken place said the other. That week I was also introduced to an emerging West Indian writer. He wore a Brooklyn Dodgers cap, stroked a ukulele, and treated me to rum punches. I said I was amazed by the provincial illiterates and their ideas about the universe. ''Oh, man,'' he crooned, dropping the Oxford tones and slipping into a sonorous Calypso patois, ''oh, man, they somethin' else again. But you don't see yet. That so?'' He was teaching me something I hadn't seen among his countrymen. I've been learning from Derek Walcott, the 1992 Nobel laureate for literature, ever since. And Omeros, an immensely ambitious and beautiful epic poem, has a great deal to teach us. The poem is a great work in its intellectual and historical sweep, in its range of styles and resources, in its characters, in its devotion to the insistent theme: slavery is the story of human history. It is man's greatest evil but, somehow, man will not let slavery stand. An epic poem is a cultural compendium, an encyclopedia of the poet's life and times. Hence Marshall McLuhan called Homer the ''Ann Landers'' (the American newspaper advice columnist) of ancient Greece. And in the Christian era, Spenser and Milton felt the need to provide everything Homer and Virgil offered and then to surpass them through the grace of Christian vision. Mr Walcott goes the next step; he doesn't merely reflect Greek or Roman or English cultural power. He fuses the Greek myth to the Hebrew and Christian myths, underpins them with African myth and then transports them to the West Indies where a new Achilles and Hector and Helen become the Aeneases of the New World. The poem is, in the best sense, serious, but Mr Walcott's prevailing tone is good-natured. It is so because Mr Walcott's characters are drawn with an unpretentious clarity and acceptance. We don't doubt for a moment that Hector, the West Indian fisherman, can suffer heroic passion. We're not surprised when Achille ties a cinder block to his Achilles heel to help him dive deeper in search of underwater treasures. Indeed, the poem constantly juxtaposes a heroic and a mock heroic world. When the fishermen, Hector and Achille among them, set off for sea, their flotilla is a far cry from Agamemnon's fleet sailing from Aulis to Troy: Placed them parallel in the grave of gunwales like man and wife. They scooped the leaf-bilge from the planks, loosened knots from the bodies of flour-sack sails, while Hector, at the shallows' edge, gave a quick thanks . . . The heroic gestures and the daily dangers of death at sea are there, but the homely task of clearing the leaves and the (resourceful) poverty that demands flour-sacks turns the scene from heroic to mock-epic. But this is comedy within a far larger loom. Mr Walcott also provides an Englishman, a retired officer who studies history. He also employs Helen, the black maid. Plunkett's vision connects the African, Greek, Hebrew, Christian and New World myths. Thus,when Helen tries on one of his wife's bracelets, Plunkett observes this: The bracelet coiled like a snake. He heard it hissing: Her housebound slavery could be your salvation. You can pervert God's grace and adapt his blessing to your advantage and dare His indignation at a second Eden with its golden apple, henceforth her shadow will glide on every mirror in this house, and however that fear may appall, go to the glass and see original error in the lust you deny, all History's appeal lies in this Judith from a different people, whose long arm is a sword, who has turned your head back to her past, her tribe; you live in the terror of age before beauty, the way that an elder longed for Helen on the parapets, or that bed. Like an elder trembling for Susanna, naked. This is an astonishing passage, effortlessly drawn, yet I find myself choosing it almost at random from among 8,000 lines of such wonderful writing. Mr Walcott writes feelingly; he knows history and politics and art and human character. He can write in dense, witty lines or prosy conversations. He knows where he stands and with whom he stands.