Sextant is an ode to the craft of nautical navigation, and the instrument which enabled the navigators of the Age of Sail to find their way across the seas. Barrie frames his history around his own crossing of the Atlantic in a small sailing yacht, which gives a human touch to his history of famous voyagers: Bligh, Cook, Vancouver, Fitzroy, and Shackleton foremost among them.

Picking up from Sobel's classic Longitude, Barrie demonstrates that even after the invention of the chronometer, celestial navigation was preferred as more reliable than mechanical gimcrackery. With a sextant for measuring the angles of the sun, moon, and key stars, along with a table of ephemera in a nautical almanac, a skilled navigator could get a fix of a few hundred meters.

But navigation is more than the best route between ports. These are stories about cartography, strenuous missions to get trigonometric fixes on the mazes of channels that make up the Pacific Northwest, the islands of the South Pacific, and the horrific shoreline of Tierra del Fuego. These explorers were an austere crowd, at least compared to the genocidal conquistadors of the Age of Exploration, or the mercantile interests that would follow the initial mapping. Barrie presents a rather uncritical view of what was a vital part of the British imperial project, but he has a talent for turning the logs of these taciturn men into thrilling adventurers, invoking the magic of sailing by your senses and little bit of spherical trigonometry in an age when precise coordinate to anywhere are in your pocket.