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that I had not anticipated before I became a pilot; it is one that arguably matters more, not less, the more you travel. On a long-haul pilot’s own map of the world some cities glow with frequent and recent experience, others less so, and some are entirely dark. As a relatively junior pilot, I have a map sparser than those of most of my colleagues. It still happens once or twice a year that I fly to an airport I have never flown to before, because the route is new, or the airport itself is new, or the route has switched to the 747 from another aircraft. For days in advance of such a flight I will look at the charts for the airport and for others nearby, or at the flight documents prepared for a previous day’s flight. It is common, when we meet our colleagues for a flight, for the captain to ask: Have you been there recently? Or: Have you been there before? We are sharing our maps.

    Aside from my personal borders between the places I have and have not been to, the most fundamental division of the world may not be an obvious one, such as whether you are over land or water, in cloud or in the clear, whether it is dark or light. Perhaps the simplest bifurcation of the heavens is between the regions of the world that are covered by radar and those that are not. On the ground at certain airports, markings on our charts exactingly delineate those aprons or taxiways that cannot be directly seen by the controllers in the tower. The whole world is divided in a similar way, by the presence or absence of radar coverage. A surprisingly large portion of the world has no civilian radar. There is none over the seas once you leave coastlines far behind. There is none over Greenland, large parts of Africa, or significant portions of Canada and Australia. Where I fly within a certain distance of a radar site or installation— radar head is the term sometimes used for the rotating part—the air-traffic controllers can “watch” my plane in a direct sense. Where there is no radar, they cannot, and we must report our positions via various increasingly sophisticated electronic means or by reading out to them on the radio our time and altitude for various locations, a position report that they must then read carefully back, to check that they have heard us correctly.

    This sense of being watched, or not, divides the world. To be outside radar range is not like being in a place without cell phone coverage, because we are still in communication with the controllers. It’s not like entering a tunnel in a car and losing your GPS location, because pilots know where they are. Nor is the difference comparable to situations in which you are made uncomfortable by being observed, because pilots prefer controllers to be watching them; there is relief when controllers tell us we are radar identified, and the sense that we are crossing into a less isolated portion of the journey, or nearing its end.
    Mountains above a certain height constitute another division of the world, a separate realm of sky. The altitude above which we are required to wear oxygen masks if the cabin pressure fails is 10,000 feet, and so this rough contour shaped by peaks and an added safety margin forms perhaps the map of the world that a pilot might draw most easily from memory, as if sea level had risen by about 2 miles. The world that remains exists largely in two great, distinct bands. An enormously long swathe of Eurasia, from Spain across to the Alps and the Balkans, from Turkey roughly eastward to China and Japan, crossing the highlands of such countries as Iran, Afghanistan, India, and Mongolia, forms the heart of the map. Another long line of minimum altitudes marked on our charts in red runs in an all but unbroken line along the western side of the Americas, from Alaska down through the Andes; from the Arctic to the Southern Ocean.
    On this map of world-height, the United States east of the Mississippi does not exist. Huge portions of Africa, Brazil, Russia, and