Re: [time-nuts] Information on the Danjon Astrolab
In article [EMAIL PROTECTED], Geoff Powell [EMAIL PROTECTED] writes 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 hope. 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. Geodesy wasn't on the shelves when I checked yesterday, but I did find a copy of Plane and Geodetic Surveying for Engineers by the late David Clarke, Vol. 2, 6th Edition, Constable Press, London 1973, reprinted 1986, ISBN 0 09 459120 2, which has a description of a Prismatic Astrolabe, a device which is similar to the Danjon instrument. I've put a PDF (200 dpi, 2.6MB) of the relevant pages on my website at http://www.g8kbz.demon.co.uk/images/PrismaticAstrolabe.pdf including the discussion of errors in the observations. It's not linked from anywhere else, so no search engine will see it except via the list archive, and it won't be there long, either, since I only have 20MB of space. Get it while it's hot! Geodesy is on order, but from the date, it's the 1st Edition. -- Geoff Powell ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
In article [EMAIL PROTECTED], Brooke Clarke [EMAIL PROTECTED] writes Hi: After seeing a reference to Splitting the Second by Tony Jones on Tom's web page I read it and found that the Photographic Zenith Telescope was replaced by the Danjon Astrolabe. But Google provides almost no information about it. A patent search has turned up a number of optical systems, mostly for spacecraft, that use star separation for navigation, and if used at a know location on the earth could be used for timing. For these see my Stellar Time Keeping web page: http://www.pacificsites.com/~brooke/StellarTime.shtml Does anyone know how a Danjon Astrolabe works? IIRC, the Danjon astrolabe uses a mercury pool, and a semi-reflecting prism. The optical telescope is aligned horizontally, and the prism provides 2 light paths, one aimed upward at some angle (normally 45 or 60 degrees) and the other downwards. The mercury pool reflects the downward beam upwards. As a result, when you look through the telescope, you see double images. As the skies go through their apparent diurnal rotation, objects will pass through the nominal viewing angle, and the image doubling will go away. The observer presses a button at the magic moment to record the time. Advantages - can use more stars, not just those at the zenith. can observe more stars in a night, leading to improved accuracy in time determination. Disadvantage - requires manual operation, with resultant errors due to personal equation - the eye-hand delay. Accuracy of the Danjon astrolabe and the PZT were considered similar, but the automated capabilities of the PZT won out - I think. Source for this - the book Geodesy by Bomford. Sorry, no ISBN or publisher (I'm typing from memory) Have Fun, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts -- Geoff Powell ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
In article [EMAIL PROTECTED], Geoff Powell [EMAIL PROTECTED] writes Does anyone know how a Danjon Astrolabe works? snip Source for this - the book Geodesy by Bomford. Sorry, no ISBN or publisher (I'm typing from memory) Geodesy Bomford, Guy Clarendon Press, Oxford 2nd Edition (1965) (Out of Print) 3rd Edition (1971) ISBN 0198519192 (Out of Print) 4th edition, from 1980. ISBN 0-19-851946-X Or you can try this Biblioquest search http://www.biblioz.com/main.php?action=5u=24b729e881416a4d4feb2da5a9d34 ed3author=Bomfordtitle=Geodesy It is considered one of the primary references for geodetic surveying, albeit somewhat dated. -- Geoff Powell ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
In message [EMAIL PROTECTED], Geoff Powell writes: IIRC, the Danjon astrolabe uses a mercury pool, and a semi-reflecting prism. Hmm, but doesn't the local value of horizontal affect the precision also ? You would really need to know the direction of local gravity for this to be any good... -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
From: Poul-Henning Kamp [EMAIL PROTECTED] Subject: Re: [time-nuts] Information on the Danjon Astrolab Date: Tue, 08 Aug 2006 07:59:00 + Message-ID: [EMAIL PROTECTED] In message [EMAIL PROTECTED], Geoff Powell writes: IIRC, the Danjon astrolabe uses a mercury pool, and a semi-reflecting prism. Hmm, but doesn't the local value of horizontal affect the precision also ? You would really need to know the direction of local gravity for this to be any good... Down? :-) Wouln't a number of measurements and comparision with other methods and measurements help to reveal such deviations? Then again, question is if the deviation is so large that it has a real impact. Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
In message [EMAIL PROTECTED], Magnus Danielson wri tes: You would really need to know the direction of local gravity for this to be any good... Down? :-) There's a big difference between down enough that cows don't easily tip over and passes through the center of the earth. I know the Carlsberg Meridian had to take it into account when it was moved down to the Canary Islands. Wouln't a number of measurements and comparision with other methods and measurements help to reveal such deviations? Then again, question is if the deviation is so large that it has a real impact. Obviously, it all depends how precise you want it. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
On Tue, 08 Aug 2006 08:33:32 +, Poul-Henning Kamp [EMAIL PROTECTED] wrote: There's a big difference between down enough that cows don't easily tip over and passes through the center of the earth. LOL ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
On Tue, 08 Aug 2006 01:44:45 -0700, Rex [EMAIL PROTECTED] wrote: On Tue, 08 Aug 2006 08:33:32 +, Poul-Henning Kamp [EMAIL PROTECTED] wrote: There's a big difference between down enough that cows don't easily tip over and passes through the center of the earth. LOL Oh, you reminded me... A friend of mine once told me a long elaborate story about a mountainous section of the world where they had two breeds of cattle -- The clockwise and the counter-clockwise. It depended on which legs had evolved longer to let them stand upright and eat the grass on the steep mountain sides. Obviously, cross breeding between the two breeds was impossible in nature, and if man got involved with his special equipment to help them breed, you never knew what to expect. You could get a normal counter-clockwise or a clockwise or you could get a new breed of flatland cow or you could get a cow that could only stand upright on a hill so steep that they would just slide to the bottom. (A wall hugger?) Sorry. Hope my off topic posting doesn't start a trend here. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
From: Poul-Henning Kamp [EMAIL PROTECTED] Subject: Re: [time-nuts] Information on the Danjon Astrolab Date: Tue, 08 Aug 2006 08:33:32 + Message-ID: [EMAIL PROTECTED] In message [EMAIL PROTECTED], Magnus Danielson wri tes: You would really need to know the direction of local gravity for this to be any good... Down? :-) There's a big difference between down enough that cows don't easily tip over and passes through the center of the earth. Hence the smiley. I know the Carlsberg Meridian had to take it into account when it was moved down to the Canary Islands. Ofcourse they would have to, they moved down on the longitude on an ellipsoid and then ofcourse they have a different local gravity and the gradient of that will differ (and hence the angle of the gravity will differ slightly from that of the normal on the ellipsoid). Ever looked at a gravity map of the earth? Wouln't a number of measurements and comparision with other methods and measurements help to reveal such deviations? Then again, question is if the deviation is so large that it has a real impact. Obviously, it all depends how precise you want it. Indeed. Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
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? The PZT used a large pool of Mercury to define Up. The source of many errors was considered in the design and as far as I know were all eliminated. The PZT worked by exposing a glass plate at 4 known times. I think it was reversed for two of the exposures and then after development read on what amounted to a coordinate measuring machine. Then a number of corrections could be made depending on the geometric relation between the 4 points. The Dent Meridian Instrument or as he called it the Dipleidscope uses a couple of mirrors behind a clear glass and when the two reflections coincide the star is on the meridian. The manual gives a number of ways of aligning it, one of which depends on already knowing the time. See: http://www.pacificsites.com/~brooke/Dent.shtml But it's intended for visual use, not automated. Have Fun, Brooke Clarke Geoff Powell wrote: In article [EMAIL PROTECTED], Geoff Powell [EMAIL PROTECTED] writes Does anyone know how a Danjon Astrolabe works? snip Source for this - the book Geodesy by Bomford. Sorry, no ISBN or publisher (I'm typing from memory) Geodesy Bomford, Guy Clarendon Press, Oxford 2nd Edition (1965) (Out of Print) 3rd Edition (1971) ISBN 0198519192 (Out of Print) 4th edition, from 1980. ISBN 0-19-851946-X Or you can try this Biblioquest search http://www.biblioz.com/main.php?action=5u=24b729e881416a4d4feb2da5a9d34 ed3author=Bomfordtitle=Geodesy It is considered one of the primary references for geodetic surveying, albeit somewhat dated. -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
In article [EMAIL PROTECTED], Brooke Clarke [EMAIL PROTECTED] 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 hope. 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 ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Information on the Danjon Astrolab
Rasputin Novgorod wrote: ofcourse they have a different local gravity and the Wait a minute. The center of gravity ~is~ the center of gravity. You can't have two, or multiple centers. The strength of gravity varies from place to place, but that doesn't change the direction. If I'm wrong; please explain... You are wrong. Here's my attempt at an explanation. The direction of gravity -- the direction of the plumb line -- is everywhere normal to the local gravitational equipotential surface. (The particular gravitational equipotential surface that corresponds to mean sea level is called the geoid.) The geoid is is not exactly an ellipsoid of revolution, although such as ellipsoid (such as the WGS-84 ellipsoid, for example) is used to define the location of the graticule - the grid of meridians of longitude and parallels of latitude that we use for a horizontal datum. For many surveying purposes, a different reference surface, the geoid, is used instead of the ellipsoid. The direction of the plumb line (the down direction) is normal to the local gravitational equipotential surface -- that is, normal to the geoid if you're location is at sea level. The normal to the ellipsoid andthe normal to the geoid do not point directly at the earth's center of mass (the geocentre). They might point at the geocentre if the earth did not rotate on its axis, but the earth's rotation, and the centrifugal force that we observe as we rotate with it, causes the geoid to fit more closely to an ellipsoid than to a perfect sphere. The angular difference between the normal to the ellipsoid and the plumb line (the normal to the geoid) is called the deflection of the vertical, and is typically a few seconds of arc. (The deflection of the vertical was discovered during the 19th century by British surveyors during the great survey of India. The same survey organization, also discovered that Mount Everest was so tall, and named it after their leader.) If the earth was a perfect sphere (which it could only be if it didn't rotate on its axis), and was perfectly uniform in its mass density, then all plumb lines would pass through the geocentre. But the earth does rotate, and it has mountains and seas with different densities of the underlying rocks, so the geoid is not spherical, and the plumb lines do not generally pass through the geocentre. -- James Maynard Salem, Oregon, USA ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] Information on the Danjon Astrolab
Hi: After seeing a reference to Splitting the Second by Tony Jones on Tom's web page I read it and found that the Photographic Zenith Telescope was replaced by the Danjon Astrolabe. But Google provides almost no information about it. A patent search has turned up a number of optical systems, mostly for spacecraft, that use star separation for navigation, and if used at a know location on the earth could be used for timing. For these see my Stellar Time Keeping web page: http://www.pacificsites.com/~brooke/StellarTime.shtml Does anyone know how a Danjon Astrolabe works? Have Fun, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
