When an air traffic controller refers to the 12-hour clock, he is using the aircraft's

When an air traffic controller refers to the 12-hour clock for aircraft positioning, they are using the concept of ground track. Ground track is the path over the ground that the aircraft follows as it moves through the air, often represented relative to a reference point, similar to how we use the hour markers on a clock.

This reference is particularly important for air traffic control to efficiently manage air traffic and provide appropriate clearances, as it gives a clear and understandable way to convey the orientation and movement of the aircraft in relation to the earth's surface.

Using ground track helps controllers coordinate the movement of multiple aircraft, ensuring safe separations and effective traffic management, especially in busy airspace. In contrast, true course and magnetic heading are more technical references to an aircraft's direction in relation to geographic north and magnetic north, respectively, and do not directly correlate with the simpler and universally understood 12-hour clock format used in communications between pilots and air traffic controllers.