[time-nuts] establishing your position w/o gps
namichie at gmail.com
Wed Jan 25 08:34:02 EST 2012
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.
You observe circumpolar stars at night to obtain a true azimuth.
(North and South)
and also the latitude by the inclination of the pole.
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
You can also use an almanac and a calendar to determine your latitude
by observing stars
with the theodolite.
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
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
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
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.
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.
> will find it, but the GPO version is more expensive than the
> commercial versions..
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