[time-nuts] finding time astronomically.

Chris Albertson albertson.chris at gmail.com
Mon Jan 23 15:29:47 EST 2012

On Mon, Jan 23, 2012 at 12:02 PM, Jim Lux <jimlux at earthlink.net> wrote:
> This chat of zenith cams, etc. is interesting.
> How well could you do with something like the camera in the iPhone4 facing
> up. The front camera is VGA resolution.
> Say you're on another planet?

You can use a stick pounded into the ground and wait until the shadow
has minimum length.   But I assume we need better accuracy?

If you use a camera, accuracy will be limited by your knowledge of
where you are aiming the camera.  If you are off by one degree then
the error is about 1/360 times the length of the day on your planet.
  So finding the time is really about discovering where you have aimed
the camera.    This is best figured out at night when you can see
stars.    You can actually aim the camera at random, so long as you
measure the aim point and don't let it move.

That said, I think if you were to leave a cell phone in a fixed
position, un-moved all night you can likely get to 1/10th of a pixel
angular resolution.     So what is the angle subtended by  one pixel
on your phone divide that by 10 then multiply by one day.        A
total guess is "about 1 mSec" if you use a full night's data.  Just be
warned that reducing the data is not simple there are many steps
involved just one of then is matching your data to a good star catalog
and this implies having a good catalog.

You really can get to 0.1 pixel. You fit a function to the "fuzzy
blob" image of each star and then maybe 100 pixels contribute to a

Chris Albertson
Redondo Beach, California

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