# NavList:

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

### Compose Your Message

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: Computaion of table of offsets.
From: Andrés Ruiz
Date: 2007 Oct 29, 16:21 +0100

 Considere this figure: Earth’s normal section to the plane of the CoP.

Tan(90-H) = R(CoP)/R(Earth)

A spherical Earth hás R(Earth) = 60*360/2PI = 21600/2PI

Then R(CoP) = R(Earth)/ Tan(H)

We can discern between the radius of the CoP in his plane, and the angular distance that is the radius of the CoP on the surface of a sphere: R = 90-H

Gary, In this table you can see that for high altitude CoP are near the same

 H zd = 90-H 360/(2*PI)/TAN(H) 10 80 324.94 20 70 157.42 30 60 99.24 40 50 68.28 50 40 48.08 60 30 33.08 70 20 20.85 71 19 19.73 72 18 18.62 73 17 17.52 74 16 16.43 75 15 15.35 76 14 14.29 77 13 13.23 78 12 12.18 79 11 11.14 80 10 10.10 81 9 9.07 82 8 8.05 83 7 7.04 84 6 6.02 85 5 5.01 86 4 4.01 87 3 3.00 88 2 2.00 89 1 1.00 90 0 0.00

Andrés Ruiz

http://www.geocities.com/andresruizgonzalez

-----Mensaje original-----
De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de glapook{at}PACBELL.NET
Enviado el: lunes, 29 de octubre de 2007 10:01
Para: NavList
Asunto: [NavList 3691] Computaion of table of offsets.

In the discusssion of the St. Hilaire method the subject of the table

of offsets used to approximate the curved LOP came up. This is table 4

in Bowditch (1975 ed), table 19 in the online Bowditch and is also

printed in each volume of H.O. 229. Bowditch gives the formulas for

this calculation and one of the formulas gives the radius of the

circle of postion. The formula for this is: R = 3438' cot altitude.

Ruiz gives a similar formula: R = (60*180/pi) cot altitude which

reduces to the other formula given in Bowditch.

My question is why do you use this formula? Why isn't the radius of

the circle of position simply  60 times the zenith distance? This is

the formula we have always used for plotting high altitude circles of

positions around the GP.  Should we use the Bowditch formula for this

purpose also.

gl

--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~---

File:

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)