NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2011 Oct 30, 13:06 -0700
| I never got your question but perhaps this will help, I previously posted it on the TIGHAR website which is devoted to the Amelia Earhart search, Itasca was the Coast Guard vessel waiting for the plane at Howland island: ------------------------------------------------------------------------------------------ "Time" is a human construct so has no meaning on an uninhabited island since the land crabs do not wear watches. People choose whatever "time" is convenient for them. In the olden days every town kept its own time with towns further east seeing the sun rise before towns further west so their clocks were ahead and displayed a later time. With the arrival of railroads it became necessary to coordinate schedules across distances east and west so the concept of "zone time" was invented. Standard time zones are 15 ° wide because the sun travels 15° west every hour so this makes the "zone times" differ by exact hours. They extend 7.5 degrees (7° 30') each way from the standard meridians (which are spaced every 15° starting with the Greenwich Meridian) for that zone. This is the system used at sea but is often modified on land for convenience by not using the exact dividing lines between zones so that entire political units, such as states , can be on the same time and also some countries change their time to "daylight savings time" in the summer. So, for example, the standard meridian for the time in California is 120° west longitude and the zone extends from 112.5° (112° 30') to 127.5° (127° 30') west. To make it easy to convert from a zone time to GMT ("Z" time, or Zulu time) the zones are given numbers (called "Zone Descriptions, Z.D.) that you add to the zone time to find GMT. For California time it is + 8 so you add eight hours to California clocks to find Zulu time. (It changes for daylight savings time to + 7, but only in the U.S. It remains + 8 at sea.) There is an exception in that there are two zones, each only 7.5° wide (7° 30'), abutting the 180th meridian. To the east the Z.D. is + 12 while to the west it is - 12 to account for the clocks saying the same time in each of these zones but the date being different. An example will make this clear. If you are at 174° 30'east longitude (west of the 180th, Z.D. - 12) on July 3rd and your clock says 2100 you subtract 12 hours and the GMT is 0900 on July 3rd. If you are at 174° 30' west longitude (east of the 180th, Z.D. + 12) your clock also says 2100 but your calender says July 2nd. You add 12 hours and and find the same answer, 0900 the next day, July 3rd. The Navy regulation in force since 1920 required that their ships maintain standard zone times but provided exceptions when it was operationally convenient to use a different, non-standard, offset from Zulu time. The standard correction factors for converting one time to another used by navigators and, by regulation, the Navy is to use the sign that will convert local time to Zulu time, this is called the "Zone Description" (Z.D.) and must be recorded in the log book and next to the clocks. Itasca kept their clock set to 11:30 slow on Zulu so to convert Itasca time to Zulu their Z.D. was +11.30. Other users however see the problem in the reverse sense and use a correction factor that will convert Zulu time to local time so they would call to correction factor for Itasca time as -11:30 which added to Zulu time will produce the local time kept on Itasca. That is what I meant by the different "sign conventions." Either method will work as long as you keep it straight and don't get confused. gl --- On Sun, 10/30/11, John Brown <jdb0302@yahoo.co.uk> wrote:
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