[time-nuts] finding time astronomically.

Chris Albertson albertson.chris at gmail.com
Mon Jan 23 16:32:53 EST 2012

On Mon, Jan 23, 2012 at 1:08 PM, Jim Lux <jimlux at earthlink.net> wrote:
> On 1/23/12 12:29 PM, Chris Albertson wrote:
>> 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?
> An interesting approach, because it could conceivably get "magnification"
> without using lenses or mirrors.  Imagine the shadow tip of a 2 meter long
> stick, and I have the camera positioned so that I only see about 20cmx20cm.
>  (of course, the shadow isn't that well defined, because the angular extent
> of the sun is huge)
> A similar scheme if i use a pinhole to project an image of the sun, and
> image that, instead.

This is why I sugested using the sun. It is easy.  I know fisrt hand
that using camera pointed upward requires months and years of effort
and it is unlirly you will find one person who knows enough to pull it
off as a solo effort.

But a wire or better a slit that sweeps an image across a photo diode
is far simpler.

Yes the sun is huge angular extent but you measure the entire light
curve and fit a function to the curve to find the center of the fuzzy
shadow.   Also you can collect data every clear day for years and over
time see how close you can get.  I bet "pretty good".

You don't want a pin hole or you'd be adjusting the aim every day

To get better data you can have multiple slits so you get three or
five light curves, say 15 minutes apart every day.

The hard part will be the "simple" things like designing the
instrument so dirt and bird poop does not block the photocell or slit
and rain doe not get into the electronics.   And build it sturdy
enough that it can last outdoors in the sun and rain for many years
with zero maintenance and not cost much.
>> 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.
> iPhone cameras (and most webcams, etc.) seem to have a FOV about 45 degrees,
> so one pixel is around 0.1 degree.  At 4 minutes time per degree, that's
> about 24 seconds per pixel.
> (It's not a monochrome sensor, either, so although it's NxM pixels, that
> doesn't mean that you could actually resolve a planet to that scale,
> depending on color, and how the image is processed)
>> 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
>> solution.
> tricky on a iPhone type camera, since star images are one pixel at best.  On
> the cameras I've seen that were designed to do this, they have a cleverly
> designed optical system that blurs the image.  (and another scheme uses a
> camera with a multi pinhole mask in front, to render the image in multiple
> places across the sensor.
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Chris Albertson
Redondo Beach, California

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