Determination of Longitude In order to determine longitude , it is necessary to have an accurate watch . In this case , " accurate " means that the watch is either set to UTC ( Greenwich Mean Time ), or has a known offset from that time ( e . g ., a particular zone time ). The basic idea is that , because the Earth is rotating on its axis , time is longitude , and longitude is time . In addition , the time of Local Apparent Noon ( LAN ) is the same along every meridian of longitude ( e . g ., LAN at 65 o West will occur at exactly the same time , regardless of your latitude ).
Unfortunately , it is extremely difficult , if not impossible , to observe the time of LAN , even approximately , without suitable instruments ( e . g ., a sextant ). However , it is relatively easy to observe the time of sunrise and sunset and , as long as you are not moving fast ( which presumably would be the case in a life raft ), then LAN is the arithmetic average of the time of sunrise and sunset .
The determination of your longitude is based on comparing your observed time of LAN ( based on sunrise and sunset ) against the predicted time of LAN . Due to tilt of the Earth ' s axis , and the nature of the Earth ' s rotation , LAN does not occur at exactly 1200 every day at Greenwich . In fact , the
Because the Earth is rotating on its axis , time is longitude , and longitude is time .
time of LAN will vary by as much as 16 minutes , depending upon the specific day . This difference is known as the equation of time , and daily values are listed in the Nautical Almanac . If you were in a lifeboat without the Nautical Almanac , would you be out of luck ( at least navigationally speaking )? Not at all .
In Emergency Navigation , a makeshift prescription for calculating the equation of time is provided , based on memorizing four key dates ( Halloween and Valentine ' s Day being two of them ). The Barefoot Navigator ( by Jack Lagan ) also contains a poem ( credited to Tony Crowley ) to remember the dates : “ 14 minutes late around St . Valentine ’ s Day , 4 minutes early in the middle of May , 6 minutes late near the end of July , 16 minutes early when Halloween nigh , The differences last about two weeks , Around these pairs of troughs and peaks .”
Once you have " observed " the time of LAN ( using the arithmetic average of sunrise and sunset ), and calculated the predicted time of LAN ( using either a nautical almanac or the mnemonics above ), the difference between them enables you to calculate the offset of your longitude from the prime meridian ( one minute of time = 15 minutes of longitude ).
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