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
In article 44D7E964.4000604@pacific.net, Brooke Clarke
brooke@pacific.net 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
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--
Geoff Powell
In article 7ntf7JA6qD2EFwuF@g8kbz.demon.co.uk, Geoff Powell
geoff@g8kbz.demon.co.uk writes
Does anyone know how a "Danjon Astrolabe" works?
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=5&u=24b729e881416a4d4feb2da5a9d34
ed3&author=Bomford&title=Geodesy
It is considered one of the primary references for geodetic surveying,
albeit somewhat dated.
Geoff Powell
In message 7ntf7JA6qD2EFwuF@g8kbz.demon.co.uk, 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
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
Subject: Re: [time-nuts] Information on the Danjon Astrolab
Date: Tue, 08 Aug 2006 07:59:00 +0000
Message-ID: 53121.1155023940@critter.freebsd.dk
In message 7ntf7JA6qD2EFwuF@g8kbz.demon.co.uk, 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
In message 20060808.102323.-861060158.cfmd@bredband.net, 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
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
On Tue, 08 Aug 2006 08:33:32 +0000, "Poul-Henning Kamp"
phk@phk.freebsd.dk 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
On Tue, 08 Aug 2006 01:44:45 -0700, Rex rexa@sonic.net wrote:
On Tue, 08 Aug 2006 08:33:32 +0000, "Poul-Henning Kamp"
phk@phk.freebsd.dk 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.
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
Subject: Re: [time-nuts] Information on the Danjon Astrolab
Date: Tue, 08 Aug 2006 08:33:32 +0000
Message-ID: 53594.1155026012@critter.freebsd.dk
In message 20060808.102323.-861060158.cfmd@bredband.net, 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
On Tue, August 8, 2006 10:33, Poul-Henning Kamp said:
In message 20060808.102323.-861060158.cfmd@bredband.net, 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".
Next time you have a surprice visit from a strategic submarine, be sure to
ask the navigator for the local gravity anomalies!
Or check the deflection from one of the earth gravity models.
--
Björn