[time-nuts] Re: Brain Burp re Noon and the Sun!

[Michael Garvey]

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 at lists.febo.com> 
Sent: Thursday, November 23, 2023 6:00 PM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Cc: Poul-Henning Kamp <phk at 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 at 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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