# 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: Suitable Sextants - Mirrors
From: Joel Jacobs
Date: 2005 Oct 13, 12:19 +0000
Sorry Frank, I disagree.

The idea that the boat and the crew are moving in unison to the affects of motion is incorrect. The centrifical forces can cause the vessel to go one way, and the individual the other.

The procedure of rotating the sextant is to have the celestial body subtend an arc in which its lowest point just kisses the horizon. At that moment, the sextant is perpendicular, and the reading is taken. To do that correctly, the object will sweep across the field of view as an arc, and not be centered "at all times" as he suggests.

Joel Jacobs

--
http://www.landandseacollection.com

-------------- Original message from Frank Reed <FrankReedCT@AOL.COM>: --------------

> Joel Jacobs wrote:
> "Your analysis though interesting, fails to take into account that a
> sextant's mirrors are not used in a static state, and hence size does make a
> difference. Consider that the platform is moving directionally, and rolling and
> pitching all at the same time."
>
> But since the mirrors, telescope, and other components of the sextant are
> all experiencing the same motion, this really isn't relevant to mirror size.
> They're either big enough to fill the field of view, or they're not --no matter
> how much pitching and rolling there is.
>
> And:
> " There also is motion introduced by the user when he rocks his arm to align
> for perpendicularity."
>
> Why ! would that depend on mirror size? Many people mistakenly rock the
> sextant about the axis to the horizon in which case the Sun or star's image
> sweeps
> across the field of view from one side of the horizon glass/mirror to the
> other. When "rocking" for perpendicularity, the sextant is supposed to be
> rotated about an axis that points to the Sun or star. And when this is done
> correctly, the Sun or star remains centered in the field of view. For objects
> that
> are low in the sky this is nearly the same motion as the "incorrect"
> procedure. But for objects at high altitudes, it's very different. If the Sun
> or star
> is in the zenith, "rocking" the sextant amounts to spinning about vertically.
> And in every case, the Sun or star stays centered in the field of view so
> mirror size does not appear to be relevant.
>
> Just so there's no misunderstanding, I agree wi! th you that a sextant should
> have (or at least potentially accept) a telescope with a relatively large
> aperture and the mirrors should be compatible in size with that --large enough
> to fill the field of view. I don't agree that it is because the mirrors
> "collect more light" but this theoretical issue is really a minor concern
> compared
> to your experience in actual use.
>
> -FER
> 42.0N 87.7W, or 41.4N 72.1W.
> www.HistoricalAtlas.com/lunars
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)