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Frank Reed wrote:

 > As long as  you don't mind waiting around for the stars  to come out (and
> given your  assumptions of an almanac and a good clock set to  GMT or some known
>  zone time), you can get your position "by eye" within two or  three  degrees
> easily and up to ten times better with careful  land-based  observations simply by
> looking at the zenith --no sextant  required.

 Agreed, but I thought the original question was  whether you could fix your 
position anywhere in the world with celestial without having an initial DR or 
EP (I  guess technically since it wasn't specified originally that could be 
interpreted  to mean either with or without a sextant...).

 > If I  look  straight up and see Vega, for example, within a degree of the zenith at  0600
 > GMT  on  July 1 (Greenwich Date!), then I can write down my position immediately.  The
 >  declination of the zenith is the observer's latitude.

 Your point is definitely  well taken, however two practical questions for you:

 1) Without a sextant (or some other reasonably  accurate measuring device), 
where exactly is the zenith? Dunno about you, but my  neck isn't exactly 
calibrated in degrees - and if I recall correctly, that's the  main reason 
that we measure up from the horizon instead to find altitude (and it  gets 
converted to co-altitude "automatically" either with tables or using a  
mathematical formula so arranged to take that measurement into  account).

 2) Assuming that an  observer can find their zenith accurately, what if no 
celestial objects happen  to transit their GP at that particular time of the 
year?

 My initial best-guess for the easiest way to solve  this without a clue to DR 
position would probably be a LAN shot, but that also  assumes a current 
almanac and a semi-accurate timepiece.

--
GregR


----- Original Message ----
From: Frank Reed 
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Sent: Tuesday, June 6, 2006 2:32:20 PM
Subject: Re: Reality check

Greg:
"But as a practical matter (assuming  that we've got a current almanac and
can get a reasonably accurate time  reading), even if we had no idea at all
about a DR position couldn't we at least  narrow down ..."

As long as you don't mind waiting around for the stars  to come out (and
given your assumptions of an almanac and a good clock set to  GMT or some known
zone time), you can get your position "by eye" within two or  three degrees
easily and up to ten times better with careful land-based  observations simply by
looking at the zenith --no sextant required. If I look  straight up and see
Vega, for example, within a degree of the zenith at 0600 GMT  on July 1
(Greenwich Date!), then I can write down my position immediately. The  declination of
the zenith is the observer's latitude. So for the case of Vega in  the zenith,
my latitude must be close to 39 degrees North. The right ascension  of the
zenith is the observer's sidereal time. Of course navigators don't use RA  and
SidT anymore, so for modern almanac data, you use the equivalent rule: the  GHA
of the zenith is the observer's longitude (west). For Vega in the zenith,
the SHA is about 81 degrees and the GHA of Aries is about 9 degrees at 0600GMT
on this date, so my longitude is 90 west.

Declination of zenith =  Latitude
GHA of zenith = Longitude

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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