[time-nuts] finding time astronomically.

Jim Lux jimlux at earthlink.net
Mon Jan 23 15:43:13 EST 2012


On 1/23/12 12:05 PM, Poul-Henning Kamp wrote:
> In message<4F1DBCC9.9040900 at earthlink.net>, Jim Lux writes:
>
>> How well could you do with something like the camera in the iPhone4
>> facing up. The front camera is VGA resolution.
>
> Very badly.
>
> The major trouble is actually not getting the light from the star,
> but making sure your camera/telescope/transit-circle has a known
> and stable geometric relationship to the planet Earth.
>

Say you had it in some sort of "fixture" to allow it to be placed 
repeatably with reference to your local earth position.

I can think of two general scenarios here.

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.

(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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