[time-nuts] finding time astronomically.

J. Forster jfor at quikus.com
Mon Jan 23 16:35:50 EST 2012


The atmospheric issue is more differential refraction, than refraction per
say. A zenith pointing camera is likely the best choice. The zenith is the
direction of the least atmospheric depth also.

-John

===========


>> I can think of two general scenarios here.
>> If you planet has air you will need to know how it refracts st
>> One is where you "lay the iphone on the table" in a fixed position.  One
>> could use the internal accelerometers to determine "level", but I don't
>> think you could tell orientation, unless, perhaps, you can see
>> circumpolar
>> stars?  That is, by watching the movement of the stars/planets through
>> the
>> field of view over some hours, could you figure it out?  Or is there
>> some
>> fundamental ambiguity.
>
> No, you can point to any location and you can (in theory) figure out
> where it's pointing given that you have a large enough field of view
> to see many stars at the same time.   You can make a fixture easy
> enough, just some epoxy and a large boulder.      I used lag bolts
> onto my garage roof and it worked more than good enough.
>
> If you can choose, straight up is the best aim point.  Refraction is
> not much of an issue and there is less air to look through.  But
> looking at the equator means there is less field rotation and the data
> is easier to reduce.  We looked at the equator because we did not want
> to deal with image rotation.   Motion blur is minimize down there too.
>
> But if you want to know "absolute time" then you need more.  Looking
> at any random but fixed location will get you the period of the
> planet's ration to about a mSec with cheap equipment but to get
> absolute time you need to measure the aim point relative to the local
> meridian.   That is not as easy.  Star with a protrator and a plumb
> bob.    That is what I used.   But to refine that you need a good
> source of time and for the purpose of this exercise we don't have
> that.  Only the plumb bob which means "a few seconds of error".  maybe
> an precision level can do 10X better?
>>
>> (obviously, you can trivially see the moon/sun)
>>
>> The other scenario is where you get an inexpensive camera (webcam, or
>> perhaps some slightly better point and shoot) and build a precision
>> mount
>> (so you DO have accurate knowledge of sensor orientation and position)
>> Could
>> you, perhaps over time, do an insitu calibration?
>>
>> I suppose any of these techniques is going to have issues with the
>> uncertainty in when the image is actually captured (e.g. there's
>> probably
>> 10-100 ms you're not going to get away from).
>>
>>
>>
>>
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>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
>
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