[time-nuts] Measuring sidereal/solar time?

Attila Kinali attila at kinali.ch
Fri Dec 30 05:42:38 EST 2016


On Fri, 30 Dec 2016 10:59:03 +0200
Anders Wallin <anders.e.e.wallin at gmail.com> wrote:

> out of curiosity, are there any amateur/semi-pro experiments that can
> measure the length of the solar or sidereal day to sub-millisecond
> resolution?
> To reproduce data like this:
> https://upload.wikimedia.org/wikipedia/commons/5/5b/Deviation_of_day_length_from_SI_day.svg
> 
> Something in the sky that goes "ping" every day - detected with a pointing
> accuracy of < 1ms/24h or <0.01 arc-seconds (!?). Or perhaps two
> satellite-dishes pointed at the sun and noise-correlation/interferometry??

I don't know of any such experiment already performed, but I am not up
to date on what's going on in the hobby astronomy community.

I am not sure whether sub-milisecond resolution is feasible, but
I think the "easiest" method would be to do a "modern" version of
an meridian telescope:

Using a camera fix mounted (ie not moving and if possible vibration isolated)
on a pedestal pointed at the sky, approximately looking south. A simple
webcam would be probably enough for first experiments, as long as you get
a good picture of the stars. A good compact camera which allows to use
a remote shutter with a proper lens and exposure control should be better.
Probably the best resource here are the people/websites that deal with
book scanning, as they tend to automate the whole picture taking process.
Using magic lantern (http://magiclantern.fm) with Canon cameras might
give additional features needed for the task.

>From the pictures taken, calculate the positions of the stars (by fitting
circles onto the bright pixels) and figure out which star is which (using
astronomical list of stars). For this step there is a plethora of open source
astronomical software available, but I don't know how well they fit the task
of figuring out what the position of the stars relative to the camera reference
frame. After that, it's just some simple math of calculating the difference
between the position of the stars and where you would have expecteded them
at the time when the picture has been taken.

Some usefull software projects are:
http://astro.corlan.net/gcx/
http://www.clearskyinstitute.com/xephem/
http://starlink.eao.hawaii.edu/starlink
http://astro.corlan.net/avsomat/index.html
http://rhodesmill.org/pyephem/

HTH

			Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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