# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Chronometer Suggestions
From: Gary LaPook
Date: 2009 Jan 6, 15:45 -0800
 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 wrote:From: RBEmerson Subject: [NavList 6951] Re: Chronometer SuggestionsTo: "NavList" Date: Tuesday, January 6, 2009, 3:03 PM```For laughs and grins, I ran some fix data from another thread herethrough 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-08Ic = 31.5' OFF, He = 0.0 ft (or meters ;-) )Body WT HsCapella - 03:52:46 - 39-35.7Vega - 03:54:45 - 32-06.9Altair - 03:56:11 - 22-37.0That results in a fix of either(Watch Time without change)49-30.6 N 122-57.3 Wor(Watch Time + 1 minute)49-30.6 N 123-12.3 WNote there is no change in latitude, only in longitude. Adding oneminute gives a fix 9.75 nm west of the un-adjusted fix.Cheers,RickOn Jan 6, 4:32 pm, Alexandre E Eremenko 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 andwatch accuracy is actually required for practical navigation.  Can we predicthow many miles off one would be for every second of time error?>> > Bill>>```

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