# NavList:

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

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Re: sign conventions and units.
From: Dan Hogan
Date: 2001 Apr 23, 11:51 AM

```On 22 Apr 2001, at 21:16, Paul Hirose wrote:

> Though I'm an American, I prefer "east is positive" for longitudes. It
> makes figuring the LHA of a star simple, since you just add everything:
> GHA Aries, SHA star, and assumed longitude. This is also a good
> convention to use internally in software which uses geocentric
> rectangular coordinates. Normally in such a system the x and y axes lie
> in the plane of the equator with the x-positive axis directed to the
> Greenwich meridian and the z-positive axis directed north. With this
> layout, calling east longitude positive is most natural.

>From my experience the labeling of E or W +/- has more to do with the
algebraic formula used than convention.

> In astronomical work I've seen longitude measured east to 360. The
> Astronomical Almanac gives coordinates of the major observatories like
> that. However, in "Astronomical Algorithms" Jean Meeus grumpily refuses
> to conform, saying astronomers used the opposite convention for more
> than a century.

Again most astronomers use RIGHT ASCENSION for "longitude", RA is a pain to
use for a sailor.

[Snip]
>
> With respect to minutes and seconds vs. decimal, obviously minutes are
> convenient for a navigator plotting on a chart. Aviation seems to use
> decimal minutes exclusively. (One air-launched weapon I saw several
> years ago accepted target coordinates to .00001', roughly the width of
> Saddam's mustache.) Surveyors and astronomers use seconds a lot. Purely
> from the standpoint of efficient packing, decimal degrees are better.
> E.g., 125.4767 and 125 28' 36" have the same number of digits, but the
> decimal form has 2.8 times finer resolution. If we use George's system,
> expressing the angle in "milliturns", it's 348.5464, with 2.8 times the
> resolution of decimal degrees.

Degrees and seconds probably came from the early days of navigation when
there was nothing else. But dd mm.m works very well with modern calculators
and computer programs.

> Mentally figuring reciprocals, right angles, and 45s becomes child's
> play with this system. Sadly, I've never seen grads used, though my HP
> calculator can deal with them.

Yeah, just try to get an old salt to change his methods.
Cheers
-Dan-
```
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