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

Neville Michie namichie at gmail.com
Wed Jan 25 17:29:06 EST 2012


On 26/01/2012, at 2:49 AM, J. Forster wrote:

>> Finding your location without GPS is not all that difficult.
>> You need a quality theodolite, but even an ordinary one will read to
>> 1 second of arc.
>
> 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.


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

>
>> 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.
>> 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.
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

> -John
>
> ==============
>
>> You observe the sun at noon to find the local time and set your local
>> clock. You then
>> wait for an event like an eclipse of a planets moons to establish the
>> relationship
>> between your local time and the time at a known site.
>> A theodolite has a telescope that can be "plunged" i.e. used upside
>> down and this
>> technique is used to get a very accurate level from a striding level.
>> No pool of mercury
>> is needed.
>> The setting up of a theodolite uses sitings  and reversed sitings to
>> set the vertical level.
>> The main error is the atmospheric refraction which scatters
>> individual observations,
>> so many repeated observations are needed. The local time observations
>> need to be
>> repeated for good accuracy.
>> A sextant is a less accurate instrument that has the main redeeming
>> feature that when
>> reading it you superimpose the image of a star or the sun with the
>> image of the horizon.
>> Although the image seen may be rolling around, the position of the
>> sun on the horizon
>> is rock steady and is adjusted by the thimble for coincidence. The
>> elevation is then
>> read off the vernier. A theodolite needs a solid base to work from
>> and would be useless
>> on a ship.
>> cheers,
>> Neville Michie
>>
>>
>>
>>
>>
>> On 25/01/2012, at 12:52 PM, Jim Lux wrote:
>>
>>> On 1/24/12 3:19 PM, J. Forster wrote:
>>>> Is the USNO almana/ephemeris still published in hard copy every
>>>> year? That
>>>> had moon timing, etc.
>>>
>>>
>>> You can download pieces from the Astronomical Applications website
>>> at USNO.
>>>
>>> Or you can buy a copy of the Nautical Almanac for about $20 from a
>>> variety of sources.  You could also download the pdf (but printing
>>> it would cost you more than the $20)..
>>>
>>> Amazon has it, for instance.
>>>
>>> http://www.usno.navy.mil/USNO/astronomical-applications/
>>> publications/naut-almanac
>>>
>>> will find it, but the GPO version is more expensive than the
>>> commercial versions..
>>>
>>>
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>>
>>
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>
>
>
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