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duffer1960 02-10-2003 08:35 AM

Longitude: How do yo tell what the exact ships time is.
Ok, it''s pretty obvious that the difference between GMT and ships time can tell you your longitude, but how do you know your exact ships time? Do you reset the clock every sunrise? Observe the sun until it peaks and say that is noon? What?

jcmannone 02-10-2003 09:44 AM

Longitude: How do yo tell what the exact ships time is.
Hello Duffer,

You must synchronize your ship''s clock with UTC. The Naval Observatory has the precise time as well as many other sources. I suspect your computer is linked to one of these sites so it can keep precise time. I know my Mac does.

You DO NOT reset your clock to peak sun. This is local solar time (also called sundial time). High noon will vary according to your position.

The rest here is an aside:

Sundial time was the problem in Colonial days when people could more quickly get from one part of the state to another. It was the railroads that instituted time zones in 1863 in this country to avoid scheduling confusion and to prevent train wrecks. England had already started railroad time in 1840. Greenwich Mean Time was started in 1847, but I don''t think the US switched over to it ''til after 1883 (I suspect this since Lewis and Clark used Washington, DC as their longitude reference in charting the West using the stars and a "crude" chronometer)


duffer1960 02-13-2003 09:14 AM

Longitude: How do yo tell what the exact ships time is.
Still doesn''t make sense. If you don''t know the exact time on your ship, at your present location, how can you calculate longitude?

Denr 02-13-2003 09:52 AM

Longitude: How do yo tell what the exact ships time is.
If the sun is directly over-head where you are at 18:00 hours GMT (for example) and your boat is at 42 degrees N latitude, you''re on Lake Michigan which is 90 degrees west. For every hour past GMT that the sun is over-head where you are, you''re 15 degrees west of GMT. It doesn''t matter what time it is at your location, it only matters when the sun is over-head and what time it is at GMT. One hour equals 15 degrees. Now get the hell out of my way, I''m on starboard tack!

jklewissf 02-13-2003 01:22 PM

Longitude: How do yo tell what the exact ships time is.
local apparent noon is the point at which the sun is as high in the sky as it is going to get for the day. You find local apparent noon by taking several sites of the sun starting a little before you expect it to occur. Once the sextant redings start getting smaller you know that local apparent noon has occured. you can plot the sextant readings and see the peak...the gmt of the peak tells lets you know how far west of grenwich you are.

An excellent book on this subject is Longitude. It''s a tiny book that can be read very quickly. It tells the story of John Harrison who is the guy who invented the first chronometer accurate enough to allow sailors to determine longitude at sea.

There is another way to determine longitude. Its called the method of lunar distances. The spherical trigonometry is too complicated for the level of education of sea captains in Harrison''s day so everyone figured an accurate clock was the only practical solution to the problem.

duffer1960 02-18-2003 05:48 AM

Longitude: How do yo tell what the exact ships time is.
I knew it had to do with observing the peak of the sun and calling that noon. What got me wondering was because I watched "Longitude" and they never explained how ships time was determined . So, every day, a ships clock is/was reset at noon?

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