# 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: Lu Abel
Date: 2009 Jan 06, 14:58 -0800
Frederico:

Thanks, you actually caught an error I made in my comments.   The 1 nm for every 4 seconds applies to an observer on the equator with a body directly east or west of him.  That's because the speed of the earth's surface due to rotation is 1/4 nm per second at the equator.   But as I move towards the poles, my speed drops proportionally to the cosine of my latitude (because my distance from the earth's axis is proportional to cos(L)).   So, for example, at 60 deg latitude, my speed is only 1/8 nm per second.

Lu

Federico Rossi wrote:

Lu,

If I’ve understood well, this error doesn’t depend on your latitude on earth, i.e. it’s a maximum of 1 nm for every 4 seconds (for bodies due east or west) whether you are on the equator or far from it, does it?

Federico

I
nviato: martedì 6 gennaio 2009 20.05
Oggetto: [NavList 6941] Re: Chronometer Suggestions

Irv and Bill:

It's a MAXIMUM of 1 NM for every 4 seconds, not a minimum.

If the body you're sighting is directly north or south of you, even a fairly significant time error would result in a very minimal shift in the LOP produced by the body (the extreme example is Polaris).   On the other hand, if the body you're sighting is directly east or west, then it's Geographic Position is moving by 1 NM every four seconds and any LOP developed from that sight would be off by 1 NM for every four seconds of clock error.

Lu Abel

Irv Haworth wrote:

Minimum of 1 NM for every 4 seconds..( a quick answer)..

Irvin F Haworth

Sent: January 6, 2009 5:05 AM
Subject: [NavList 6931] Re: Chronometer Suggestions

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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