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

[Poul-Henning Kamp]

> 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

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