Magnetic reversal, also known as geomagnetic reversal, is a change in a planets magnetic field such that the positions of magnetic north and magnetic south are interchanged. This is not to be confused with geographic north and south. During a magnetic reversal, the magnetic field may sometimes only undergo an excursion, rather than a reversal, where it suffers a large decrease in its overall strength. We have no complete record of the history of any reversal, so any claims we can make are mostly on the basis of mathematical models of the field behavior and partly on limited evidence from rocks that retain an imprint of the ancient magnetic field present when they were formed. Mathematical simulations suggest that a full reversal may take about one to several thousand years to complete, which is fast by geological standards but slow on a human time scale. The solid iron inner core of the Earth has been shown in these simulations to be important in controlling the reversal process. The U.S. Geological Surveys Geomagnetism Program monitors the Earths magnetic field and provides continuous records of magnetic field variations covering long timescales.