[time-nuts] Calculating sidereal time

[Björn]

Hi Brooke,

SV position in circular orbit repeats at 12h sidereal, but ground relative az/el repeats at 24h sidereal. This is because of the earth 24h rotation rate. After 12h your earth position will be 180deg away from (inertil) start position.

—

   Björn 

Sent from my iPhone

> On 21 Jan 2019, at 03:35, Brooke Clarke <brooke at pacific.net> wrote:
> 
> Hi Didier:
> 
> I'm still very much interested.  The main reason is the GPS satellites come close to following the same ground track.
> So I'd expect the elevation and azimuth to a given SVN to be the same on a 12 sidereal hour basis.
> 
> -- 
> Have Fun,
> 
> Brooke Clarke
> https://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> axioms:
> 1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
> 2. Everybody, with no exceptions, holds false beliefs.
> 
> -------- Original Message --------
>> Actually not a specific requirement. I made (and am in the process of
>> making a new batch of) an assembled kit which is used to monitor the data
>> stream from a Trimble Thunderbolt GPSDO and display time and vital
>> information on a small LCD display.
>> 
>> A long time ago, Brooke Clarke, on this list asked if I could make it
>> display sidereal time. I had no idea what it was so I researched it and
>> found out I needed to be able to process double precision calculations,
>> which the original 8 bit microcontroller was totally incapable of.
>> 
>> So I set this aside but I recently came across an inexpensive ARM chip and
>> decided to make a new version of my monitor with it. The toolchain is
>> excellent and supports double precision so I decided to fullfil Brooke's
>> request, who by now has probably lost interest in it but for the fun of it
>> and to learn something.
>> 
>> So while I understand sidereal time is used by astronomers, I am not one so
>> I have no personal practical use for it, which has the advantage of setting
>> the bar pretty low to decide when I am done :) That said, I am open to
>> suggestions to actually make it more useful if possible.
>> 
>> It is so easy to load an app in your phone if you actually need sidereal
>> time that I do not believe my kit will be used for that purpose but it was
>> an interesting exercise for me and I am glad it is working.
>> 
>> I learned quite a bit and discovered, through your postings and the
>> associated links how complex that subject actually is.
>> 
>> I love this list.
>> 
>> Didier KO4BB
>> 
>> 
>>> On Sat, Jan 19, 2019, 8:01 PM Steve Allen <sla at ucolick.org wrote:
>>> 
>>> On Sat 2019-01-19T12:15:28-0800 Steve Allen hath writ:
>>>> The most expedient place to find them are roughly pages B7 to B12 in a
>>>> current Astronomical Almanac.  See for example
>>>> 
>>> https://babel.hathitrust.org/cgi/pt?id=uc1.31822038913307;view=1up;seq=116
>>> 
>>> Emphasizing one point, it has always been important to use expressions
>>> for GMST and such things which are designed for use in the same
>>> reference frame as the star catalog.  Current star catalogs do not use
>>> an equinox as any kind of reference point, therefore any expression
>>> for GMST does not simply correspond to the longitude-like coordinate
>>> of a current star catalog.  GMST now coresponds to a new fictitious
>>> point in the sky which is defined with much more complexity than in
>>> the old days.
>>> 
>>> What exactly is the intended use case for this project?
>>> 
>>> --
>>> Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
>>> UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat
>>> +36.99855
>>> 1156 High Street               Voice: +1 831 459 3046         Lng
>>> -122.06015
>>> Santa Cruz, CA 95064           https://www.ucolick.org/~sla/  Hgt +250 m
>>> 
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> 
> 
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