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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