NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2012 Dec 14, 15:28 -0500
Hi John
The altitude of the body is dependent on your position and time. For example, the altitude of sirius is different from the altitude when I observed it 3 Hours ago. If I switch my position on the globe without shifting the time, similarly, the altitude changes. This was my meaning.
Once the altitude for each body is known, without refraction, we must determine how that body is to appear with refraction. This would only be valid for objects above (say) 25 degrees, else we get into abnormal refraction problems. Since both must be above this, we are limited to 130 degree distances as a maximum.
Finally, we must clear the distance using (for example) corner cosines, similar to a lunar distance problem.
++++++++
And now back to the initial question, which was Bauer's table of distances. This does not include refraction as altitudes are not a table entry, nor the distance as adjusted by corner cosines. It seems to be a nice starting point, to get within a few arc minutes as a starting point for beginning sextant practice manipulations.
As a method of calibrating the arc of your sextant, it fails on the accuracy issue.
Or did I miss something? That's always a possibility! ;-)
Kind Regards
Brad
Brad,
You ask a question that is deeper than it might first appear.
As you know, the "normal" refraction correction for the observed altitude of an single body depends only on its altitude, not on your position or time.
Now it might not be obvious, but the result of refraction on the observed arc distances between two bodies (like star-star distances, or moon-sun distances), can be expressed in terms of these two normal refraction corrections. In lunar distance sights, using these two normal refraction corrections for the individual bodies to correct the lunar distance is called "clearing the lunar distance." See chapter 8 in my book, particularly figures 8.2 and 8.3 (Expanded Edition is best). Figure 8.3 shows that the key to the "clearing" problem is that the relative bearing angle between the two bodies is constant -- is the same for the observed and the corrected star-star distance. None of this depends on the observer's position or on the time.
Hope this answers the question,
JK
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------
