NavList:

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

HOME

Re: Moon sight?
From: Stan K
Date: 2014 Oct 5, 18:20 -0400

FYI, the refraction formula in the back of the Nautical Almanac was the Bennett formula,

    R (in arc minutes) = 1/tan (H + 7.31/H + 4.4),

through the 2004 Almanac. In 2005 it changed to

        R (in arc minutes) = 1/tan (H + 7.32/H + 4.32).





Interestingly, the altitude correction tables at the front of the Almanac reflected this change in 2004.  Not changing the formula in 2004 was apparently just an oversight.



FWIW, this change only affects the refraction value for apparent altitudes of about 2º or less.



Stan

-----Original Message-----
From: Bill B <NoReply_BillB@navlist.net>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Oct 5, 2014 4:52 pm
Subject: [NavList] Re: Moon sight?

On 10/5/2014 12:31 AM, Sean C wrote:
> Would you please explain to me the formula you use for Refraction and
> where you got the figures? The 7.32 and 4.32.

Sorry about the my answer. Off base as I commingled two of my projects
in my memory.

The  7.32 and 4.32 are very similar in value to those used in Bennett's
simple formula on page 106 of Meeus's "Astronomical Algorithms," but the
formula's differ.

Bennett's formula is:

R (in arc minutes) = 1/tan (ho + 7.31/ho + 4.4)

ho = apparent altitude = Ha

Bennetts's formula from Wikipedia
http://en.wikipedia.org/wiki/Atmospheric_refraction

R=cot(ha + 7.31/ha+4.4)

As Bennett's is referred to as a "simple empirical formula," and Bennett
gives two methods of refining the results, I would surmise the NA
version is more rigorous.

Bottom line, this does not answer you math/physics question of how the
constants and formula's were derived. Some of the list Guru's may help
out there.

What is NavList?

NavList is a community devoted to the preservation and practice of celestial navigation and other methods of traditional position-finding. We're a group of navigators, navigation enthusiasts and hobbyists, mathematicians and physicists, and historians interested in all aspects of navigation but primarily those techniques which are non-electronic.

To post a message, if you are already signed up as a NavList member, start a new discussion or reply to any posted message and use your posting code (this is a simple low-security password assigned when you join). You may also join by posting. Your first on-topic messsage automatically makes you a member, and a posting code will be assigned and emailed to you for future posts.

Uniquely, the NavList message boards also permit full interaction entirely by email. You can optionally receive individual posts or daily digests by email, and any member can post messages by email (bypassing the web site) by sending to our posting address which is "NavList@NavList.net". This functionality is similar to a traditional Internet mailing list: post by email, read by email, reply by email. Most members will prefer the web interface here for posting and replying to messages.

NavList is more than an online community... more about that another day.

© Copyright notice: please note that the rights to all messages and posts in this discussion group are held by their respective authors. No messages or text or images extracted from messages may be reproduced without the explicit consent of the message author. Email me, Frank Reed, if you have any questions.

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

NavList:

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

Compose Your Message

NavList

What is NavList?

Join / Get NavList ID Code

Retrieve a NavList ID Code

Email Settings

Custom Index

Add Images & Files
Name or NavList Code:	Email:

Name:
	(please, no nicknames or handles)
Email:

NavList ID Code: