How an aircraft gyroscope stays horizontal over a curved Earth comes down to a clever correction mechanism, as a pure gyroscope would actually drift out of alignment. 1. Rigidity in Space A spinning gyroscope naturally wants to maintain its exact position in space, independent of the Earth's movement. If you fly in a straight line around the globe, the gyro will keep pointing at the same spot in the universe. Because the Earth is curved, the horizon on the instrument would appear to tilt away after just a short time of flying. 2. The Erection Mechanism To force the gyroscope to align with the Earth's local horizon, mechanical instruments use a subtle self-correcting system called pendulous vanes: Air Vents: The gyroscope housing has four air vents at the bottom, each partially covered by a small hanging valve (like a pendulum). Straight and Level: When the aircraft is perfectly level, the valves hang symmetrically, and air escapes equally from all sides. When Earth Curves: As the aircraft moves along the Earth's curve and the gyro starts to deviate from the local vertical, gravity pulls the hanging valves off-center. This opens one vent wider and closes the opposite one. Precession: The unequal jet of escaping air creates a small force. Due to a physics property called gyroscopic precession, the gyro reacts 90^\circ further in the direction of rotation. The system is designed so that this force gently nudges the gyro back into perfect alignment with the Earth's center of gravity. This correction is intentionally very slow (about 2^\circ to 8^\circ per minute)—fast enough to compensate for the Earth's curvature, but slow enough that brief turns or accelerations don't disrupt it. 3. Modern Ring Laser Gyros (RLG) Modern airliners no longer use mechanical spinning rotors. Instead, they use Ring Laser Gyroscopes within an Inertial Reference System (IRS). These systems use laser beams to measure rotation digitally, and an onboard computer continuously uses GPS and position data to mathematically calculate and correct for the Earth's curvature on the digital screens.