Mary Ann Whitaker, age 13, of St. Augustine, Fla., for her question:

CAN YOU EXPLAIN DEAD RECKONING?

Dead reckoning is the basic method of navigation in which the position of a ship or aircraft is determined by calculation from a previous position, the speed of the craft and the time traveled.

The earliest known method of navigation, dead reckoning is now used only when bad weather makes celestial observation impossible.

To obtain even approximately accurate results by dead reckoning, allowances must be made  for the effects of ocean currents and winds, for the inaccuracy of the course steered, for compass variation and deviation and for inaccuracy in the measurement of speed.

Under the best conditions, a position determined by dead reckoning is subject to some error, and in storms or turbulent weather, the error is greater.

Dead reckoning positions may be calculated entirely by arithmetical methods, but they are usually determined by plotting the various distances involved on a chart and finding the position graphically by vectorialaddition.