# NavList:

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

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: Silicon Sea : Leg 69
From: Dan Hogan
Date: 2001 Apr 10, 10:12 EDT

```On 9 Apr 2001, at 21:13, Sam Chan wrote:
> ----- Original Message -----
>
> > >
> > > > 3)  What is the DR position?
> > > > --  ------------------------
> > >
> > > S = 10.7 C = 288.9 T,
> > >
> > >   20/05/2000 12:13:00 ZT = 20/05/2000 16:13:00 UT
> > >                           -18/05/2000 22:20:00 UT
> > >                          ========================
> > >                             1 day     17:53:00
> > > or 41.88 hrs
> > >
> > > Dist = 41.88 * 10.7 = 448.1 nmi
> > >
> > > D.Lat = 448.1 * cos 288.7 = 145.2 = 2d 25.2'
> > >
> > > DR Lat = 9d 50.5' + 2d 25.2' = 12d 15.7'
> > >  dmp = mp(12d 15.7') - mp(9d 50.5')
> > >      = 147.0
> > > D.Lo = 147.0 * tan(288.9)
> > >      = -429.4
> > >      = -7d 09.4
> > > DR Lo = -53d  00.0 - 7d 09.4'
> > >       = -60d 09.4
> > >
> > > DR  12d 15.7'N    60d 09.4'W
> > >
> > > Actually this is too far off to make any sense.
> > > We seem to be well past Barbados.
> >
> > It is off. If you are calculating use a Traverse Table interpolation may
> > be accumlating an error. A better method would be to use one of the
> > Sailings methods for DR. For sight reduction at least using a Law of
> > Cosines formula. If you insist on working it "longhand" download
> > 4mulas.zip from the web site.
> >
>
> Dan,
>
> I find no problems with the details and results from Mike Wescott. The
> calculation he shows is the Mercator sailing as shown in Cotter, the
> Elements of Navigation and Nautical Astronomy, Chapter 13. I got the same
> result doing the calculations by hand. I entered the same problem into the
> Starpath Star Pilot and got 12d 15.3N, 60d 09.6W.
The answer I got for Q3. DR= 13d 16.4'W  59d 32.3'W. The program I use
corrects for ellipsoid eccentricity and is a Rhumb-line formula. Mike's
working is OK, but I think the elapsed time is off.
In setting up the problems I work some backwards from the answer I want.
> For question 7, I got an answer of CTS 338 C with a SMG of 10.9 Kt. The
> Star Pilot gave an answer of CTS 324.4T and SMG of 10.9. Applying the Var
> 15dW (add), and a Dev 1dE (subtract), I arrive at a CTS of 338.4 C. I
> double checked the Star Pilot with a plot on the Mo Board and also arrived
> at a CTS of 324T.
Using several checks I keep getting 332.0d. But the DR being off would
account for the difference.
Dan Hogan WA6PBY
Catalina 27 "GACHA"
dhhogan@XXX.XXX
http://www.wa6pby.com
```
Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)