These may be from a fungus called fairy ring. As the fungus moves through the soil, it creates a dark ring (or arc) where it breaks down organic matter in the soil. You can mask the darker ring by fertilizing the lawn. Sometimes the ring is also associated with a dead patch. As the fungus moves through the soil, the dead patch spreads on the leading edge, but fills back in again with new grass. There's no effective control for the fungus.