The true meridian is the intersection between the plane (the meridional plane) of the Earth's rotation axis and the surface of the earth. Therefore, the true meridian of the ground (true north direction) is in the same vertical plane as the axis of rotation of the Earth. When the gyroscope's gyro rotates at a high speed and its axis of rotation is not in the vertical plane of the ground's true meridian, the gyro's axis of rotation will be precessed by the moment of rotation of the earth, and it will be close to the vertical plane where the true meridian and the Earth's rotation axis are located. So the gyroscope The axis of rotation can automatically indicate true north direction.
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