I typed too fast and have now added to the confusion. I should have use "SHA" in the second paragraph of my last post. Please ignore that one and read the corrected paragraph here.
This points out a fact that many may have not have noticed. Any
stellar fix always gives you your latitude and the time only needs to
be accurate to the nearest month or so depending on the level of
accuracy you are looking for. You can think of the fix being in terms
of latitude and SHA, the SHA doesn't change but the longitude changes
in step with the change in GHA of Aires. the only thing that ties the
fix to longitude is time which fixes the relationship between GHA Aires
and longitude. Every star fix (the sun and moon change declination so
this is not true for them) moves westward at the rate of 15º 02.5'
(15.041º) per hour but the latitude doesn't change. This may be more
obvious if you work with H.O. 249, volume 1 for selected stars.
gl
--- On Tue, 1/6/09, Gary LaPook <glapook@pacbell.net> wrote:
From: Gary LaPook <glapook@pacbell.net>
Subject: [NavList 6955] Re: Chronometer Suggestions
To: NavList@fer3.com
Date: Tuesday, January 6, 2009, 3:45 PM
You did that the hard way. All you had to do was multiply 15.041' (since we are talking the rate of change GHA Aries, 15º 02.5' per hour; 15.041º per hour) change of GHA of all the stars shot for one
minute of time by the cosine of 49.5º to calculate the distance the fix moved westward of 9.74 NM which is 15' (approx) of longitude at your latitude. Note the longitude changed by 15'.
This points out a fact that many may have not have noticed. Any stellar fix always gives you your latitude and the time only needs to be accurate to the nearest month or so depending on the level of accuracy you are looking for. You can think of the fix being in terms of latitude and GHA, the GHA doesn't change but the longitude changes in step with the change in GHA of Aires. the only thing that ties the fix to longitude is time which fixes the relationship between GHA
Aires and longitude. Every star fix (the sun and moon change declination so this is not true for them) moves westward at the rate of 15º 02.5' (15.041º) per hour but the latitude doesn't change. This may be more obvious if you work with H.O. 249, volume 1 for selected stars.
gl
--- On Tue, 1/6/09, RBEmerson <pavilion@pinefields.com> wrote:
From: RBEmerson <pavilion@pinefields.com>
Subject: [NavList 6951] Re: Chronometer Suggestions
To: "NavList" <NavList@fer3.com>
Date: Tuesday, January 6, 2009, 3:03 PM
For laughs and grins, I ran some fix data from another thread here
through StarPilot, with and without adding a minute to the timing.
The raw data is:
AP 49-16.1 N 123-07.1 W (in British Columbia, Canada)
All times GMT on 27-Nov-08
Ic = 31.5' OFF, He = 0.0 ft
(or
meters ;-) )
Body WT Hs
Capella - 03:52:46 - 39-35.7
Vega - 03:54:45 - 32-06.9
Altair - 03:56:11 - 22-37.0
That results in a fix of either
(Watch Time without change)
49-30.6 N 122-57.3 W
or
(Watch Time + 1 minute)
49-30.6 N 123-12.3 W
Note there is no change in latitude, only in longitude. Adding one
minute gives a fix 9.75 nm west of the un-adjusted fix.
Cheers,
Rick
On Jan 6, 4:32 pm, Alexandre E Eremenko <ereme...@math.purdue.edu>
wrote:
> Bill,
>
> Of course. 1 sec of time = 15" =0'2.
> (Same accuracy as a good sextant
> measurement).
>
> Alex.
>
> On Tue, 6 Jan 2009, William Sellar wrote:
>
> > As a beginning celestial navigator, I am wondering how much time and
watch accuracy is actually required for practical navigation. Can we predict
how many miles off one
would be for
every second of time error?
>
> > Bill
>
>
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