[time-nuts] establishing your position w/o gps

J. Forster jfor at quikus.com
Wed Jan 25 18:07:42 EST 2012


>> Ordinary? You mean something like a Wild T-2 or Kern DKM-2. Even then
>> getting close to 1 arc-second requires a lot of care.
>
> A wild T1 reads directly to 6 seconds, but with repetition will get 1
> second.
> Unlike digital instruments you need a little bit of skill and
> persistence to get the best measurement from an analogue instrument.

Assuming youcan do that w/o bias.

A T-2 ius a 1 ard second instrument, a T-3 is 0.1 arc-second. I've never
seen a T-4 in the flesh.

>>> You observe circumpolar stars at night to obtain a true azimuth.
>>> (North and South) and also the latitude by the inclination of the pole.

That means observations over more than 18 hours. It'll take you most of a
year, unless you are above the artic circle.

>> Not quite so straight forward. You have to have accurate siderial
>> time and an almanac. Polaris is only near the pole, not at it.
>
> No need for time, you follow the azimuth of the star until it turns
> around and then again until it turns back. Half the difference gives you
> the azimuth of the pole very accurately.

See above.

> Fit your observations to a
> parabola to get a good result.
> Works best in Winter when the sun is down for more than 12 hours. A
> good technique as refraction errors cancel.

In practice, the "seeing" is nowhere near 1 arc-second for 2-3" aperture
'scopes.

>>> On a time photograph these stars draw circles around the pole, the
>>> centre of the circle
>>> is the celestial pole and its elevation above the horizon gives the
>>> latitude.

And to do that you need a sub-arc second telescope mount. You just can't
mount a camera on a tripod.

>>> You can also use an almanac and a calendar to determine your latitude
>>> by observing stars
>>> with the theodolite.
>>
>> Not so easy. At the celestial equator the stars are moving in Hour
>> Angle at 15 arc-seconds per second.
>>
>
> As I said, analogue measurements need some skill and perseverance.

That's an understatement. I've done it, both for North lines and to adjust
a 24" telescope.

> If you added more modern technology you could track your theodolite/
> telescope with a clock so you would get a longer period to adjust/
> observe the observations and set your clock.
>
> Neville

The "modern technology" just makes the angle readout direct.

 -John

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