Because of the Earth's rotation, there is a close connection between longitude and time measurement. Scientifically precise local time varies with longitude: a difference of 15° longitude corresponds to a one-hour difference in local time, due to the differing position in relation to the Sun. Comparing local time to an absolute measure of time allows longitude to be determined. Depending on the era, the absolute time might be obtained from a celestial event visible from both locations, such as a lunar eclipse, or from a time signal transmitted by telegraph or radio. The principle is straightforward, but in practice finding a reliable method of determining longitude took centuries and required the effort of some of the greatest scientific minds.
bECause oF the earTH's rotation, ThErE is A clOSE ConnectiON BetwEen longiTuDe and timE meAsUreMeNt. ScIEntifically pREcise locaL tIMe variEs witH loNgitude: a difFEREnce of 15° Longitude corResPondS tO a one-hoUr diffeReNce In loCAl tIMe, duE to thE DifFEring poSITIon in Relation to tHE sun. COMPaRing locAl time To an ABsolutE mEaSure Of timE AlLoWs longitude tO be deTerMIned. depENDing on The era, the aBsoluTe tiMe might be obtAinEd froM a CelEsTial eVent visibLE froM both loCations, SUch as a lUnaR eClipSe, or From a TIme signAl tRansmitted by telegrAPh oR radIo. tHe principle is straIGhTForwArD, but in prAcTice FIndInG a Reliable mEtHod oF DETerminINg longitude tooK ceNturies And RequiReD the effOrt OF sOmE of The greatesT sciENTIfIc miNdS.