Leitenstern

Leitenstern [Guiding Star] is, in fact, not a star, as the name would suggest, but a planet of sufficient size and with satellite of sufficient brightness, owed to its ice-covered surface, and orbital regularity, that allow its usage for coordinate calculations.
Leitenstern is close enough to the planet so that the observation of the clock-like motion of its satellite, fittingly called Uhrzeiger - clock hand - can be done merely with a very good spyglass or a small reflective telescope, easy to use and not burdensome. The motions of both the planet and the satellite are very regular and well-understood by the Imperial Astronomical Society. Freiherr Eberhard von der Wald, the man credited with the calculation of Uhrzeiger's orbital period, which is only marginally smaller than two days, was the one to propose the method in the year 1053 after the Founding.
Having the accurate (for the Emperor Willibald's observatory in the capital) time tables for starts and ends of the satellite's eclipses, one, von der Wald argued, could first estimate the longitudinal difference with the help of a map, thus obtaining an approximation of the time of the eclipse. Then one can observe the said eclipse, carefully noting the time. Once it is done, the difference in time with the one noted in the tables could be turned into degrees of longitudinal shift, if one recalls that twenty-four hours correspond to a shift of a complete circle.
Von der Wald's celestial clock theory was lauded by the Imperial and, truthfully, the world's scientific community, but its practical implementation was significantly delayed. Only more than seventy years later, in the year 1127 from the Founding, Count of the Overseas Leopold-Dietrich von Steinbrücken performed an experiment that proved von der Wald's method readily usable by obtaining a longitudinal reckoning of his seat at Ostkap that was soundly verified by a chronometric comparison.
After von Steinbrücken's experiment the method was quickly adopted by the Imperial navy. Although its usability for home defence was marginal due to the narrowness of the sea that separates the overseas possesions from the western coast, the usefulness of such an algorithm in longer voyages, especially exploratory ones, is hard to overestimate, as latitude has long been easy to estimate with the help of the Sun, and a combination of latitude and longitude uniquely defines one's geographical position.