The DI works using a gyroscope to establish an inertial platform, which will remain fixed in space. The DI is arranged so that only the horizontal axis is used to drive the display, which consists of a circular compass card calibrated in degrees. The gyroscope is spun either electrically, or using air from a vacuum pump driven from the aircraft's engine. Because the earth rotates (15° per hour), and because of small accumulated errors caused by friction, the DI will drift over time, and must be reset from the compass periodically. Failure to do this is a common source of navigation errors among beginner pilots.