page_title: History of longitude
section_title: Methods of determining longitude
hierarchical_section_title: History of longitude / Methods of determining longitude
caption_attribution_description: English: View of a transit circle (transit instrument) from: Wollaston, Francis&#32;(1793).&#32;"A description of a transit circle, for determining the place of celestial objects as they pass the meridian". Philosophical Transactions&#32;83: 133-153.
context_page_description: The history of longitude is a record of the effort, by astronomers, cartographers and navigators over the centuries, to discover a means of determining longitude.
  The measurement of longitude is important to both cartography and navigation, in particular to provide safe ocean navigation. Knowledge of both latitude and longitude was required. Finding an accurate and reliable method of determining longitude took centuries of study, and involved some of the greatest scientific minds in human history. Today, the problem of longitude has been solved to centimeter accuracy through satellite navigation.
context_section_description: The development of the telescope and accurate clocks increased the range of methods that could be used to determine longitude. With one exception (magnetic declination) they all depend on a common principle, which was to determine an absolute time from an event or measurement and to compare the corresponding local time at two different locations. (Absolute here refers to a time that is the same for an observer anywhere on earth.) Each hour of difference of local time corresponds to a 15 degrees change of longitude (360 degrees divided by 24 hours). 
  Local noon is defined as the time at which the sun is at the highest point in the sky. This is hard to determine directly, as the apparent motion of the sun is nearly horizontal at noon. The usual approach was to take the mid-point between two times at which the sun was at the same altitude. With an unobstructed horizon, the mid-point between sunrise and sunset could be used. At night local time could be obtained from the apparent rotation of the stars around the celestial pole, either measuring the altitude of a suitable star with a sextant, or the transit of a star across the meridian using a transit instrument.
  To determine the measure of absolute time, lunar eclipses continued to be used. Other proposed methods included:
caption: A Transit Instrument from 1793
exif: {}