As the pilot maneuvers into position for final approach, the aircraft banks to the right and you get your first clear view of the airport below. Lights leading up to the runway threshold flash in sequence like a rapid-fire movie marquee. Crossbars of bright red and white lights draw the eye toward the touchdown zone. The runway itself is outlined in multicolored lamps, with another row of lights down the center. As the airplane descends, you notice the rotating beacon atop the control tower, flashing alternately white and green. Once the aircraft is on the ground there are still more lights—a forest of dim blue ones mapping out the taxiways. What are all these lights and signals? And what about the stripes, bars, chevrons, numbers, and other markings painted on the pavement? What do they all mean?

Aviation breaks free of the earth—that’s what it’s all about—and yet it has left large marks on the landscape. Major airports are highly conspicuous; some of them are as big as cities, with populations to match. Away from the airport are other telltale signs of the airplane’s influence, such as the flashing beacon lights and the red-and-white stripes or checkerboard patterns on water towers and the tall masts of radio and television transmitters. Another important part of the aviation infrastructure is almost invisible: the network of air routes that most aircraft follow from city to city. Even though the air routes themselves are out of reach far overhead, you may come upon some of the navigational beacons that act as signposts along the way.

The airport

Airports are not among our best-beloved public places. Railroad terminals get preserved as historic landmarks even as the passenger railroads themselves wither away, but airports are at best tolerated as a necessary evil to be hurried through on the way from here to there. Passengers complain of the inhuman scale, the sterile architecture, the unwelcoming environment. It will be interesting to see whether the disdain for airports persists in generations to come. Who knows—maybe 50 years from now there will be a popular movement and fund-raising campaign to save O’Hare International from the wrecking ball.

Airport terminals have undergone a surprisingly complicated evolution since commercial air travel began in the 1930s. The prototypical passenger terminal was just a building that served as the buffer between ground transport and air transport. You drove up to the ground side of the terminal building, bought your ticket and checked your bags, then you walked through to the air side, going out the back door, across the tarmac, and up the stairs into a waiting airliner. At the end of the flight you passed through a similar building in the opposite direction.