NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bowditch: Distance to visible horizon
From: Lu Abel
Date: 2012 Dec 6, 11:38 -0800
From: Lu Abel
Date: 2012 Dec 6, 11:38 -0800
I personally doubt that there would be a 3% change in the value of the earth's radius over the past 50 years; I believe it was pretty well established a century ago, even to the extent of knowing that the pole-to-pole diameter of the earth is less (by 8 miles, if I recollect correctly) than the equator-to-equator diameter. So my guess would be that the change is due to revised estimates of refraction.
As I mentioned in a previous post, it's hard for me to get excited about a small change in the last decimal digit. That leads me to also ask the following: It seems to me that refraction depends on a whole lot of factors -- temperature, atmospheric pressure, and even atmospheric moisture. Is there a set of "standard conditions" on which the magic 1.17 value is calculated? How does it vary with changes in temperature, atmospheric pressure, etc??
As I mentioned in a previous post, it's hard for me to get excited about a small change in the last decimal digit. That leads me to also ask the following: It seems to me that refraction depends on a whole lot of factors -- temperature, atmospheric pressure, and even atmospheric moisture. Is there a set of "standard conditions" on which the magic 1.17 value is calculated? How does it vary with changes in temperature, atmospheric pressure, etc??
From: Greg Rudzinski <gregrudzinski@yahoo.com>
To: NavList@fer3.com
Sent: Thursday, December 6, 2012 7:35 AM
Subject: [NavList] Re: Bowditch: Distance to visible horizon
Here is the explanation for table 8 from a Bowditch Volume II .A refraction factor is probably causing the differences. Also the value of the Earth radius may be different in the older Bowditch editions.Greg Rudzinski[NavList] Re: Bowditch: Distance to visible horizon
From: Bill B
Date: 06 Dec 2012 08:16
On 12/6/2012 4:56 AM, Gary LaPook wrote:
> That's interesting, the 1962 and 1975 editions give it as D=1.144 and
> the 1938 edition gives it as D=1.15.First place I would look is the refraction values in the formula used to
derive distance to horizon.Bill B----------------------------------------------------------------
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