Reduction

Changing the form of an expression without changing its value; to change hours, minutes and seconds into are is called reduction of time; reduction of the elements in the nautical almanac is accomplished by interpolation or by proportional logarithms; reduction to the meridian consist of applying a calculated quantity in arc to the observed altitude, taken either in the A. M. or P. M., when working an ex-meridian sight, in order to ascertain the meridian altitude of the body for the place of observation.