In the excerpt from Dorothy Dunnett's "Checkmate," the characters have arrived at the open sea, guided by some navigational charts and the visible sky above. This moment signifies a pivotal point in their journey, emphasizing a sense of adventure and exploration as they venture into the unknown waters.

The quote, "We have reached the open sea, with some charts; and the firmament," highlights the mix of guidance and uncertainty faced by travelers. It underscores the reliance on limited knowledge and tools to navigate vast, uncharted territories, symbolizing both physical voyage and life's broader journey.