The South Dakota winter gives a man time to think. One subzero morning, as Dan O'Brien approaches his fiftieth year, the autumnal equinox of his life, he takes stock. Feeling a waning sense of purpose, he decides to devote himself entirely, for the first time in his life, to his greatest loves--falconry, his bird dogs, and the prairie he calls home.   That summer he obtains a remarkable falcon chick who immediately distinguishes herself by her ferocity. He names the bird Harley and trains her in the ways of falconry. Harley's powers of flight are awe-inspiring, her hunting success astounding, and like a lover, she captivates him. O'Brien hunts with her obsessively, reveling in her prowess and beauty. What he learns from her and from what happens one wind-driven day lead him to see fully things he had only just begun to glimpse. In this lyrical evocation of the grasslands, Equinox is a story of a life lived close to the natural cycles of the earth and of a midlife revelation of the importance of staying connected to all things held dear.