# 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: Reducing back sights
From: George Huxtable
Date: 2004 Aug 12, 22:26 +0100

```Dave Weilacher ended his last posting with-

>I don't mind being wrong nearly as much as I mind you being right yet again.

Bravo, Dave! For those not in the know, Dave and I have a history of
enjoying our disagreements, which goes back over many years.

In that latest posting, he has chosen a simpler example, with the Sun on
the Southern meridian, which he views with his sextant; either in the
normal way, brought down to the Southern horizon, or alternatively by
backsight, behind his head, brought down to the Northern horizon. This
time, he seems to leave refraction (parallax, too) out of the picture,
which is fair enough, for the purposes of the argument.

Dave says-

>Say it is LAN. I can see both the southern horizon and northern horizon
>just fine.  Using the southern horizon,  I get a lower limb sight of 60
>degrees exactly after correcting for dip and IC.  I get an upper limb of
>60 degrees, 32 minutes.
>
>Now what I know is that if I do a backsight by using the northern horizon,
>I need to end up with 60  degrees exactly for a lower limb reduction, as
>opposed to any other number.
>
>If I use the northern horizon and bring what looks like the lower limb of
>the sun in the eye piece down to the horizon, it is actually the top edge
>of the sun that I'm using.  The angle I'll get, after dip and IC, will be
>119 degrees 28 minutes.
>
>If I subtract this from 180, I get 60 deg 32 min.  By golly, thats an
>upper limb correction needed.

==============

And so it is, and so it should be! I agree with every word! Any
disagreement between Dave and me seems to be quite illusory.

The point is that, as Dave realises, in his words "it is actually the top
edge of the sun that I'm using." for his backsight  He kisses, to the
Northern horizon, "what looks like the lower limb of the sun", but is savvy
enough to appreciate that although it may look like the lower limb, in
reality, in those circumstances, it's the upper.

So why should he be surprised that after subtracting from 180deg, the
correction to apply is that appropriate to the upper limb, not the lower?
Where is our disagreement?

So we're both right, it seems to me. In terms of his statement-

"I don't mind being wrong nearly as much as I mind you being right yet again."

Dave doesn't need to mind being wrong (because he isn't) but he may have to
mind me being right, which may sum up as a negative result in that scale of
values...

George.

================================================================
contact George Huxtable by email at george---.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

```
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)