# NavList:

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

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Re: Silicon Sea: Leg 80
From: Bill Noyce
Date: 2001 Oct 17, 6:35 AM

```> OK, but Great Circle is not of much use since on a small sailboat it
is just
> about impossible to sail it. But you have an idea of how many days to
go. Try
> using Mercator/Rhumbline calculations, the distance is a bit far for
accuracy
> from Mid-Latitude.

Here's an attempt at Mercator sailing.  I used a spherical
approximation,
not an ellipsoid:  MP = 7915.7 * log10( tan( Lat/2 + 45d ) )

From   4d  9.6' N   MP =  250.2       111d 34' W
To    19d 30.0' N   MP = 1193.6       154d 45' W
dLat = 920.4' N  dMP =  943.4 N  dLo = 2591' W

TC = atan(dLo/dMP)  =  290.0d
Dist = dLat/cos(TC) = 2690.1 nmi

Compared to the GC distance of 2687.4, I'll agree it's not worth the
aggravation to try to save 3 miles.  I assume this is because we're
at a fairly low latitude.

If we were far enough from the equator for it to make a difference,
I get the impression that the normal approach is to plot a few
waypoints along the GC course, then sail the rhumbline from point to
point.  With waypoints plotted a day or two apart, we would get
essentially all the benefit of the shorter distance, while still
being able to sail a "constant" compass course for long stretches.
And we'd be adjusting our heading based on new fixes from time to
time anyway. I assume a wobbly approximation to a GC course is still
shorter than an equally wobbly approximation to the rhumbline...
```
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