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The problem with PC clocks is not entirely due to errors in the 
displayed time but the internal software clock, which is typically a 
poor timekeepers since timing uncertainty is limited by the stability 
of interrupt requests.  Changes in interrupt request rates cause the 
clock to gain or lose time.  Also, if you leave your computer turned 
on for awhile, the software clock might be off by possibly a minute 
or more.  Poorly behaved software may also use the timer-counter for 
other purposes and change the interrupt rate making the clock keep poor time.

The display can also cause problems.  You can get software that will 
display the time to the nearest second not just to the minute.  So 
this is not a problem.  However, the software clock also has limited 
resolution in that it can only display values that are even multiples 
of time interval between interrupts.  So, a time as 00:00:01.00 might 
be displayed as 00:00:00.98 and 00:00:01.04.  Of course, this may be 
accurate enough for your purposes so the problem, again, is not with 
the display but with the internal software clock.

Thus, even if you synchronize right before observing, the software 
running on your PC during your observing session might screw up the 
time unless you constantly synchronize the clock.

Note that this problem seems to be the reverse of many GPS units 
which may have good internal times but inaccurate displayed times.

John Oliver mentioned that cell phones can produce accurate times.  I 
know little about this but some research suggested that some but not 
all cell phones might give good times.  However, there is another 
problem.  Many cell phones (like mine) only display the time to the 
nearest minute, which makes them less useful as accurate time pieces.

Howard

At 10:34 AM 1/24/2007, Tandy W. Carter Jr. wrote:
>Hi Howard,
>
>    I know the clock in my laptop doesn't keep good time. So, what I 
> do is I synchronize the clock right before I go out to do my 
> astronomy at home, or I synchronize it right before I load it up 
> into the car to go to the star party. I also run only the programs 
> absolutely necessary for my planned astronomy for the evening. Of 
> course, I wonder if it is the displayed time that is bad and not 
> the internal time. I would think that a timing chip running at the 
> speed of modern computers would keep the time spot on. However, 
> you're probably right that the displayed time is not accurate at 
> all. Since the time on my laptop is only displayed to the minute. I 
> don't know how SoftwareBisque handles the time it gets for the FITS 
> header CCDSoft uses.

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