[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Sep 27 01:38:27 EDT 2006

   Brooke Clarke wrote:

Hi Bruce:

I'm interested in automatically measuring the earth's period by looking 
close to straight up with a fixed telescope.
There are automatic star trackers that can "see" stars in the day time 
so the only time you would not see the star is when there's cloud cover.
By using a reticule it should be possible to detect individual stars and 
discriminate other bright objects.  I've started a web page on Stellar 
Time Keeping at: [1]http://www.pacificsites.com/~brooke/StellarTime.shtml
although many of the patents linked on that page are aimed at satellite
navigation there are a couple designed for surveying on the earth.  But
if the position of the instrument is known then you can get the time of
star meridian crossing.

Have Fun,

Brooke Clarke

w/Java [2]http://www.PRC68.com
w/o Java [3]http://www.pacificsites.com/~brooke/PRC68COM.shtml

Dr Bruce Griffiths wrote:

In article <44D8C0BB.1030608 at pacific.net [5]<https://www.febo.com/cgi-bin/ma
ilman/listinfo/time-nuts>>, Brooke Clarke
<brooke at pacific.net [6]<https://www.febo.com/cgi-bin/mailman/listinfo/time-n
uts>> writes

/Hi Geoff:

/>/Thanks for the reference to "Geodesy".  Do you know if the first edition
/>/has the Danjon information?
/>/I ask because the second editions are rather pricey and the 3rd and 4th
/>/are not available at all.
/>/Could someone on this list make a copy of the Danjon section?
/I'm quoting from memory about the workings of the Danjon Astrolabe - the
book was borrowed from my local library, and I don't remember which
edition it was.

Nor can I quote chapter and verse, I fear - although I will attempt to
re-borrow the book, and copy the relevant section. Later this week, I

AFAIR, the discussion was of the workings of various instruments for
determination of time from the stars, together with ways to mitigate the
systematic errors in each instrument. "Personal equation" figured large
in all this, which is why the PZT was preferred.
Geoff Powell

The explanation of the operation of the Danjon Astrolabe is somewhat
There was a motor driven optical mechanism that was used to keep the 2
stellar images superimposed for some time so that a sequence of
observations could be taken on a single star.
This significantly reduced the personal equation on the measurements
obtained with this impersonal version of the astrolabe.

I remember a book on Astrometry (sorry cant remember the title) that had
detailed cross sections of the astrolabe together with a very detailed
description of how it worked.
A modified version of the Dajon Astrolabe is currently in use at
Santiago Chile for Solar astrometry.


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   You may find the following  article of some interest with respect to
   daytime star observations:
   They used an IR filter to reduce the daytime sky brightness.


   1. http://www.pacificsites.com/~brooke/StellarTime.shtml
   2. http://www.PRC68.com/
   3. http://www.pacificsites.com/~brooke/PRC68COM.shtml
   4. http://www.precisionclock.com/
   5. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
   6. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
   7. mailto:time-nuts at febo.com
   8. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
   9. mailto:time-nuts at febo.com
  10. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
  11. http://bima.astro.umd.edu/memo/memo88.pdf

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