[time-nuts] Re Danjon Astrolabe

Bill Hawkins bill at iaxs.net
Tue Sep 26 14:18:15 EDT 2006

I wonder if it is possible to use radio astronomy. You'd
have to find an object that doesn't require a very large
antenna to acquire its signal.

Collins Radio once built a sun tracker for the Navy that used
microwave radiation from the sun. The antenna was driven by a
servo that maximized the signal.

I understand that optical tracking of a star crossing a hair
is more precise than the peak of a radio signal, but perhaps
modern signal processing would fix that.

Or maybe you don't have as many cloudy days as we do.

Bill Hawkins

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Brooke Clarke
Sent: Tuesday, September 26, 2006 12:48 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Re Danjon Astrolabe

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: 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 http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

Dr Bruce Griffiths wrote:

>>In article <44D8C0BB.1030608 at pacific.net 
>><https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>>, Brooke 
>>Clarke <brooke at pacific.net 
>><https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>> 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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