[time-nuts] Calculating sidereal time

Didier Juges shalimr9 at gmail.com
Sat Jan 19 12:51:14 UTC 2019


I am using an Android app and the USNO web site. The USNO web site is using
server side scripting so there is a few 100mS delay. Considering this small
delay, they match very well but the app is more convenient so I have been
using that as my reference, checking it periodically against USNO. Without
knowing any better, I would like to think the USNO is probably right.

I have seen a couple other calculators but they have some offset and they
do not publish their code.
The one Tom sent me the link for is accessible on line. The web site where
it is hosted produces results that are off by a significant amount
(minutes) compared to the USNO, but the GMST calculation is using a very
similar formula to what I am using. It could be that the server time is
off, not every web server has NTP running. I have made an attempt to
compare the results after converting the JavaScript to C but I must have a
bug in my conversion as it's just broken at the moment.

I have not found yet how to tell Lady Heather to display sidereal time but
have not looked very hard, that will be for today.

As of this morning, my implementation is still off by 57 seconds, so at
least it is consistent.

On Sat, Jan 19, 2019, 5:02 AM Mark Sims <holrum at hotmail.com wrote:

> When I was playing with the sidereal time code I found lots of buggy/bogus
> implementations and also lots of web calculators that were either totally
> wrong or off by some amount.    Same for sunrise/sunset code and
> equation-of-time code.   It's hard to know what code / sites you can
> trust.  Heather could be off...
>
> A lot of the web based ones seemed to have their system clocks not all
> that accurate.
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