Prior to the invention of the marine chronometer, accurate timekeeping being required for the calculation of Longitude, navigators generally had to resort to sailing only due North, South, East or West or not sailing far out of sight of land. The vessel would sail North or South and measure her Latitude daily. When she reached the parallel of Latitude that her destination port was on, she would then sail East or West along that parallel until a landfall was made. This method of sailing is known as Parallel sailing but as there are few ports directly East or West of one another, and apart from short legs of a coastal passage which may require steaming due East or Westerly courses, there is little requirement for this method of navigating nowadays. It should however be noted that there is often a crossover of the mathematics used between different navigational disciplines. The method of calculation of the position of the Intercept Terminal Point (ITP) used in celestial navigation position fixing, is governed by exactly the same mathematical theory, but projected on the celestial sphere as opposed to the terrestrial sphere, the surface of Earth.