This is performed by a single member of the 3rd U.S. Infantry Regiment, who marches along a 63-foot walkway in exactly 21 paces, before turning to face the tomb for exactly 21 seconds, turning to face the opposite direction for another 21 seconds, and then retracing his steps to repeat the process. Each turn the guard makes is precise and is instantly followed by a loud click of the heels as he snaps them together. The guard is changed every half hour during daylight in the summer, and every hour during daylight in the winter and every two hours at night (when the cemetery is closed to the public), regardless of weather conditions.
(More help from Wikipedia):
The Memorial itself is 270 feet high and appears to be soaring; its array of arcs against the sky evokes a modern image of flight by jet and space vehicles. At the same time, it enshrines the past in permanent remembrance of the pioneers of flight who came before, and pays homage to those of the future.
Once the decision was made to have vertical elements, the number three became important, three being resonant with significant associations for the Air Force, including the three core values of today: Integrity first, Service before self, and Excellence in all we do. It is also the smallest number of elements needed to define and enclose a space. The spires are asymmetrical, dynamic, each a different height so that the view of the Memorial changes from every angle.