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Documentation on an old procedure wanted

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#1 mitsos68

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Posted 13 October 2017 - 01:37 AM

Dear All,

National Athens Observatory decided to offer a demonstration of the detemination of the local time using the well preserved Gautier-Henri 8 inch meridian circle (https://www.cloudyni...ns-observatory/). Unfortunately the last observer retired around 1965 and moved outside Greece. Can any one can help me on the required procedure with relevant documentation?

 

Kind Regards

 

Dimitris


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#2 beggarly

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Posted 13 October 2017 - 04:07 AM

Don't you need to know the direction of the local meridian before you can install a transit (meridian) circle?

 

a demonstration of the determination of the local time during the day? or at night?

 

 

http://www.thegreenw...s.php?article=6 paragraph "Aligning the ATC to the Meridian" ... "observations of the circumpolar stars".

 

https://www.shadowsp...n/sundials.html


Edited by beggarly, 13 October 2017 - 05:50 AM.


#3 mitsos68

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Posted 13 October 2017 - 05:51 AM

Don't you need to know the direction of the local meridian before you can install a transit (meridian) circle?

 

a demonstration of the determination of the local time during the day? or at night?

 

 

http://www.thegreenw...s.php?article=6 paragraph "Aligning the ATC to the Meridian" ... "observations of the circumpolar stars".

 

https://www.shadowsp...n/sundials.html

Hi the mridian telescope is installed already.



#4 Linn

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Posted 13 October 2017 - 11:50 AM

Here is a link to a chapter from the textbook Practical Work in Elementary Astronomy‚Äč :  https://link.springe...?no-access=true

 

To access one needs a log-in through a university or library with a research journal account or it can be purchased outright. 

 

Good luck!



#5 teast

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Posted 13 October 2017 - 04:01 PM

Dimitris:

 

I asked a fellow at the Cincinnati Observatory Center if he knew of any resources and he suggested the following:

 

Chauvenet, William, "A Manual of Spherical and Practical Astronomy", 2 Volumes.  It was published in the late 1800s, but Dover has reprinted it.

 

I hope this will be useful.

 

-Tom



#6 MCovington

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Posted 13 October 2017 - 04:25 PM

Basically, what you do is watch a star (of known right ascension) cross the wire.  At that moment, the r.a. of that star is the sidereal time.

 

The Astronomical Almanac has tables of the *current* r.a. of bright stars (this year).  Better yet, use Stellarium to get r.a. precessed to the actual time of the observation.

 

Most of the rest of the story is about how to make sure the meridian circle is properly installed, and how to adjust it.  If you assume that it is properly aligned and adjusted, as I understand it actually using it is very simple.



#7 KLWalsh

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Posted 13 October 2017 - 04:44 PM

I'm wondering if I'm missing something... (??)

Local time is determined from local noon. When the sun crosses the meridian, as viewed through the transit circle, the local time is, by definition, local noon.
Technically noon occurs at the instant the sun is highest in the sky, which means the center of the sun. If the transit circle eyepiece includes a reticle, or graticule, it would help to determine the precise instant the center of the sun crosses the meridian.
Needless to say, a proper solar filter needs to be fitted over the objective lens.

#8 KLWalsh

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Posted 13 October 2017 - 04:49 PM

...and if the observatory has a chronometer that gives precise Greenwich Time, the longitude of the observatory (transit circle) can easily be determined.

#9 MCovington

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Posted 13 October 2017 - 05:44 PM

I'm wondering if I'm missing something... (??)

Local time is determined from local noon. When the sun crosses the meridian, as viewed through the transit circle, the local time is, by definition, local noon.
Technically noon occurs at the instant the sun is highest in the sky, which means the center of the sun. If the transit circle eyepiece includes a reticle, or graticule, it would help to determine the precise instant the center of the sun crosses the meridian.
Needless to say, a proper solar filter needs to be fitted over the objective lens.

That would give you local noon, which is not the same as local mean noon because the earth's orbit is not a perfect circle; thus it will not differ from GMT noon by the same amount every day.  (The sideways axis of the analemma.)



#10 roscoe

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Posted 14 October 2017 - 01:44 PM

If you need accurate time, find someone with a portable shortwave radio, several countries transmit extremely accurate time signal pulses, the one in the US is within a microsecond of correct as transmitted, and real accuracy at the receiving site considers the distance/speed of light/radio waves in their time calculations ...... and the time blips on a computer are also within a very small portion of a second in accuracy, probably within 10 microseconds.


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