A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2010 Apr 3, 02:37 -0700
Dear Frank and Dear Peter,
Frank, you mentioned in [NavList 12671]:
Today, oblateness and refractional flattening are as easy to include as any other aspects in the lunars clearing process so by default they are "on" in the clearing tool on my web site. You can try turning them off with various geometries to assess their relative importance.
I would like to thank you again for these 2 "options" you have made available for us to use on your On-Line Computer. I really liked using them yesterday morning as I subsequently reported their results in [NavList 12656] Date: 2 Apr 2010 01:56 in reply to an earlier comment from Dave.
These results triggered the subsequent inquiries from Peter in [NavList 12662] , to which I replied in [NavList 12666].
Now, I see your view point on vocabulary issues and certainly I cannot be of any "authority" here since the Language of Shakespeare is not my native language and - whatever my continued efforts until the Very End - I will never be able to master it as well as you all do.
Although I am more than well aware that in the Astronomy and Navigation Communities exactitude on terms has never been given the required attention by our predecessors (see DANJON's ASTRONOMIE GENERALE), I am just surprised that :
- for the effect of the Earth Flattening - which is already the official IAU term specifically used to "tag" this concept - you have chosen to stick to the word "Oblateness" , while
- for the effect of the irregular Sun or Moon oblateness you are currently using the word "Flattening".
So ... and even if it might have been used here or there earlier, is your on-line Computer current wording the optimal one ???
The most important is to recognise both effects which can be easily taken in account with the computing power at hand to-day as Frank rightly underlines.
If you are to compute the Lunar parallax with a FIXED value of the Earth radius, you may very well use its mean value (obtained for Lat = 30° N or S as I can remember) as it will decrease the possible maximum value of your neglected term which in such case will just reach and NEVER exceed 0.1 arc minute under any "extreme cases" in the sense given in [NavList 12666]. Since Parallax is an effect which is dependent of the LATITUDE of the Observer , two cases can happen :
- If you compute your Moon parallax with a fixed radius value equal to the Equatorial value, you will have a parallax error of 0.0' at the Equator, reaching 0.1' at 30° N/S Latitude and reaching a full 0.2' at the poles, but with no consequence on the Longitude determination at the poles as rightly indicated by Frank,
- If you compute your Moon parallax with a fixed radius of equal to the radius at N/S 30°, you will zero-ize your error at these latitudes but remain with a unaccounted for effect up to 0.1' at the Equator or at the Poles in "extreme cases".
If you use a fixed value for the Earth radius, it is mainly a matter of personal choice for you as there is no such error free choice. I would recommend here that you update earth radius value as a function of the Observer's Latitude and then ... you are fully done with this parameter.
Regarding the body shape distortion of the Sun and Moon apparent disks when close to the Horizon, I have not personnaly verified the accuracy of the "Ellipse method approximation" earlier quoted by Franck as being sufficiently accurate for our purposes. I am not using this approximation. Our end results obtained by independent methods and software are in excellent agreement with only remaining differences (most probably) due to slighty different Refraction algoritms. Moreover, such check would somewhat be probably irrelevant given the well known dispersion on Refraction observed at low altitudes. So this kind acccuracy check is not in my top priority list.
Apart from that, no comment from my side on the other remarks from Frank to which I fully subscribe.
Best Regards to all
Antoine M. "Kermit" Couëtte
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