[time-nuts] Time-zones and World time..

[Bruce Griffiths]

Hal Murray wrote:
>> One can, of course, observe meridian passage for a variety of stars at
>> night, and from that determine the time (given a calendar and the
>> appropriate almanac data), so you could directly observe midnight. 
>>     
>
> What does "directly observe" mean?
>
> Stars aren't conveniently located where I want them.  I think I'd have to do 
> something like time when a star that's close to midnight crosses a 
> north-south line in the sky that's above my location.  So in addition to the 
> almanac, I also need a (good?) compass.
>
>   
A compass isn't actually required for determining the meridian.
If you have located the Meridian and have identified the star, then its
not necessary to use one that has a meridan transit at local midnight.
In any event if you dont know exactly where you are identifying local
midnight may be problematic.
A PZT or equivalent may be easier to setup.

> Maybe I don't understand how navigation by sextant works.  They get latitude 
> from the elevation of the sun above the horizon.  Do they also get time 
> (local noon) by noting when the sun is highest?  I think that would be hard 
> to do since you are looking for the peak of a sine curve.  That has a broad 
> flat top.  There isn't any conspicuous peak.  Do the numbers work out?  Can I 
> get the peak close enough to be useful for navigation?
>
>
>   
An accurate watch or equivalent source of UTC is required.
Various techniques for determining the time from Astronomical
observations are somewhat impractical at sea.
>> Don't forget too, that solar noon varies quite a bit (minutes) from
>> "mean solar time" over the year.  If you're navigating your ship with
>> noon sun sights, this is pretty important. 
>>     
>
> What do I google for if I want that correction?
>
>   
Googling isnt a reliable technique for marine navigation, at least
during the journey.
> There are quite a few time scales.  Which one do navigators use?
>   

UTC is usually required in the calculations, as it is (currently)
closely tied to the Earth's orientation angle.

Bruce