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:

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

In article 44D8C0BB.1030608@pacific.net, Brooke Clarke
brooke@pacific.net writes

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

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...

/b

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From: Rasputin Novgorod priapulus@yahoo.com
Subject: Re: [time-nuts] Information on the Danjon Astrolab
Date: Tue, 8 Aug 2006 13:49:29 -0700 (PDT)
Message-ID: 20060808204929.10631.qmail@web50715.mail.yahoo.com

First of all, this ball of iron and siliconoxide that we call home, is far from
homogene. This causes local variations in gravity, simply because the local
ore may be heavier than the average crust. This does not only cause the gravity
to be stronger downwards, but also affect things sideways. As you get closer to
such an area your gravity force will point more towards that area rather than
towards the gravity center of earth. Thus, the gradient of the gravity force
will help to point you off center so to speak. Another fun little gravity play
is that ball of stone that made a wonderfull spectacle in the sky this evening,
called the moon. It's gravity will also pull things, such as water, and do that
sideways too.

Did that make sense?

Cheers,
Magnus

Rasputin Novgorod wrote:

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

In article fJo2gIAsON2EFwIu@g8kbz.demon.co.uk, Geoff Powell
geoff@g8kbz.demon.co.uk writes

"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