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Nobody has mentioned NTP in this thread so far.  NTP (for Network Time 
Protocol) is a way of synchronising your computer's clock with that of 
others.  NTP is quite sophisticated, for example taking into account 
propagation delays over the Internet, and which of the servers that you are 
synchronising from is best.  It also figures out the "drift" of your 
computer's clock (this is much the same as the "rate" when talking about 
chronometers).  If the computer clock runs fast or slow, it is periodically 
adjusted, by small increments, so that the time is always correct (within 
some error limit).  It will continue to work in this way while disconnected 
from the Internet, albeit with decreasing accuracy.  You can use NTP in a 
less sophisticated way, by performing a one-off synchronisation, and this is 
what Windows computers do by default, synchronising once a week, without any 
small adjustments to the clock in between; this is not nearly so accurate.  
Mac OS X and Linux come with full NTP.  Your ISP may provide an NTP server, 
and there are many publicly-available servers, see for example 
www.pool.ntp.org.

Where do NTP servers get their time from?  Well, anybody can set up an NTP 
server, so it depends.  In most cases, the answer is "another NTP server".  
But so-called stratum 1 NTP servers get their time from another source, which 
could be anything from an atomic clock to one of the other sources already 
discussed in this thread, such as a GPS receiver or radio sources (WWV, MSF 
etc.).

How does this impact the celestial navigator?  If you are far from land, and 
do not have the capability or desire to maintain a satellite connection to 
the Internet, then NTP is not useful.  But for the armchair navigator, with a 
good Internet connection (Wi-Fi or not), then NTP will provide more than 
sufficiently accurate time.


Phil


On 2 Mar 2012, at 19:00, Zvi wrote:

> I have been using GMT provided over the Internet (via Wi-Fi) to set my time 
pieces by for celestial navigation, but when compared to a radio clock 
(AccTim Galaxias) the two time sources seem to differ, mostly by a second or 
two but sometimes by up to 5 seconds. Intuitively I would have thought that 
the radio clock would be the stable and accurate one and that the Internet 
GMT, delivered to a laptop or to a smartphone via Wi-Fi would suffer short 
delays and inconsistencies. Any thoughts or experience?

   

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