Brain Burp re Noon and the Sun!
Take a look at the Dipleidoscope :
(https://en.wikipedia.org/wiki/Dipleidoscope); they show up on eBay from
time to time. Accuracy is ~10 seconds. They date from the mid-19th
century.
Mike Garvey
Swampscott, MA
-----Original Message-----
From: Poul-Henning Kamp via time-nuts time-nuts@lists.febo.com
Sent: Thursday, November 23, 2023 6:00 PM
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Poul-Henning Kamp phk@phk.freebsd.dk
Subject: [time-nuts] Re: Brain Burp re Noon and the Sun!
You might say that a plumb bob will tell you straight down and go from
there.
The shadows cast by the sun are surprisingly fuzzy and that makes it very
hard to precisely time the shortest shadow.
I once tried photographing the shadow of a vertical pencil on a horizontal
sheet of graph-paper once per second, using a 6MP DSLR, triggered by
electronic timer.
Even with +/- 15 minutes of pictures and quite advanced imageprocessing, it
was not possible for me to nail the moment of shortest shadow better than
approx 40 seconds.
I was at the AO4RTC workshop at ESO a couple of weeks ago, presenting the
prototype RTC cluster we built for the ESO/ELT telescope.
Surprising, at least to me, was a couple of presentations from solar
observatories about how they use AO to cancel out atmospheric turbulence, in
order to get sharper pictures.
However, I have not been able to figure out any way that could be used to
improve the measurement of time. For one thing, with AO you also have to
figure out, where your telescope is actually looking at any one moment in
time.
Timing the stars at night is just /so/ much easier and precise.
(Not to mention much more aesthetically pleasing :-)
With the new GAIA catalogue as reference, and a moderately modern digital
camera, firmly bolted to a steady monument, it should be a trivial matter of
computing to determine both precise longitude, latitude and time.
(Unless you live somewhere like Denmark, with only 50 clear nights per
year.)
Poul-Henning
--
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.
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Thank you all for the answers to my questions about Local Meridian.
I can safely advise that I accept that solar noon occurs at exactly the same
moment in all locations that share your local meridian.
I had a similar struggle 60 years ago regarding Single Sideband
transmission. Once it clicks, it is so obvious!
Thanks again.
Clive
-----Original Message-----
From: Clive S Carver via time-nuts [mailto:time-nuts@lists.febo.com]
Sent: 22 November 2023 22:29
To: time-nuts@lists.febo.com
Cc: Clive S Carver clive@ancient-mariner.co.uk
Subject: [time-nuts] Brain Burp re Noon and the Sun!
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at exactly the
same moment in all locations that share your local meridian."
Is that correct? I thought that the Latitude of the locations also comes
into this?
My reason for doubt, based on reading elsewhere, is Axial Tilt, also known
as obliquity, which is the angle between an objects rotational axis and its
orbital axis, which is the line perpendicular to its orbital plane. The
rotational axis of Earth, for example, is the imaginary line that passes
through both the North Pole and South Pole, whereas the Earth's orbital axis
is the line perpendicular to the imaginary plane through which the Earth
moves as it revolves around the Sun; the Earth's obliquity or axial tilt is
the angle between these two lines and is about 23 degrees.
Thus to me, looking at the Earth from a distance, a Meridian will not be in
line with the Earth's Axis (other than twice each year) and this would cause
noon at two or more different Latitudes on the same Meridian to be at
different times.
I realise that there are other complex variables involved, ie The Equation
of Time.
I am in the UK so tend to think in terms of the Greenwich Meridian and am
considering the case of two locations on the Greenwich Meridian, but could
be any other Meridian.
(For twenty years as a Merchant Navy RO, I supplied the Radio Time Signal
for the Navigators to keep the chronometer accurate; I wish now that I had
taken more notice of their sextant and tables.)
Many thanks.
Clive
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Michael Garvey writes:
Take a look at the Dipleidoscope :
(https://en.wikipedia.org/wiki/Dipleidoscope); they show up on eBay from
time to time. Accuracy is ~10 seconds. They date from the mid-19th
century.
Yes, marvelous instruments, full manual here:
https://books.google.co.uk/books?id=9QlbAAAAQAAJ
But they are not "accurate", they are only "repeatable".
You set up the instrument, using either a known longitude ("from
the ordonance map") or an already calibrated chronometer, and
then it will tell you, with high repeatability when the sun
passes the instrument.
My experiment was trying to determine local noon without knowing
the longitude (or time) by measuring when the shadow was shortest.
--
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.