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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