Text 16239, 69 rader
Skriven 2011-04-03 15:59:22 av Stewart Arnett (3:772/100)
  Kommentar till text 16160 av Robert Bashe (2:2448/44)
Ärende: Navigation
==================
Hello Robert.

02 Apr 11 10:14, you wrote to Björn Felten:

 RB> Björn Felten wrote to Michiel van der Vlist on Friday April 01 2011 at
 RB> 11:17:

 MV>>> One problem with astronavigation is that you must have a clearly
 MV>>> visible horizon. That may be difficult on a dark night flying
 MV>>> above cloud.

 RB> That's why people normally don't use sextants at night. Incidentally,
 RB> sextants only tell you your latitude, not your longitude. You need a
 RB> chronometer for the latter.

Incorrect you can use lunars, take the elevation of a celestial object combined
with the angular distance between it and the moon to give you your position lat
and long and to find the time without a Chronometer. You need a known star and
the moon to arrive at an accurate date and time the moon follows a circle in
the sky relative to any star this circle takes 28 days to complete giving a
change of 12 degrees per day or 30 minutes for every hour between your choosen
planet/star and the moon with the required almanac you can find the date and
time from the position of the moon on this circle or as it appears straight
line and the altitude of the moon at that time will give you your longitude and
latitude. It is of course a reqirement that this methode of navigation is
carried out on clear nights and is ideal for arial navigation from above the
clouds, it is difficult and is something I haven't done at sea for about 35
years. But I corrected a chronometer this way in 1972 while coming to NZ after
it seemed to have lost time and my position moved dramaticaly using timed
longitude.

 RB> Just a second! Did Michien write "flying"? And is Bjorn talking about
 RB> ships? What's the subject? Airplanes or ships?

 BF>> That problem can be solved with a bowl with oil or some other
 BF>> thick fluid that reflects good. The horizon is exactly between the
 BF>> star and the reflection of it. Most sextants have a special set of
 BF>> mirrors for this.

This is correct and is one way to find an artificial horizon.

 RB> Interesting idea. But you still need the longitude. I figure most
 RB> people use GPS for location determination nowadays anyway.

Correct, it is a requirement to have 2 GPSs for cat 1 classification, I have
three on the yacht, one reads out in decimal one chart plotter and one plugged
into the back of the laptop and now it think of it there is one on my cell
phone.

 RB> The main problems people had were before chronometers were invented
 RB> and ship captains had to navigate "by the seat of their pants" - they
 RB> knew their latitude, not not their longitude and that was a real
 RB> problem.

Most people stopped using lunar time lines arround 1780 when chronometers
became reliable and affordable. Captain Joshua Slocum never used a chronometer
on Spray his clock was a broken alarm clock boiled in linseed oil to make it go
he found his way round the earth using Lunars, when ever he took a sight that
is.

 RB> But I still don't know whether you two are talking about airplanes or
 RB> ships. There's a big difference.

in elevation ;-)

Stewart

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