Perturbation
any disturbance or irregularity in the movement of a planet in its orbit. Every heavenly body, by the law of gravitation, possesses an attractive power over every other one. When, therefore, the orbits of any two approach, each causes a perturbation in the movement of the other. Magnetic perturbation, irregular declination of the magnetic needle. This may be produced by earthquakes, by volcanic eruption, by the aurora borealis, etc.