NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Modern Lunars
From: Paul Hirose
Date: 2016 Sep 27, 00:46 -0700
From: Paul Hirose
Date: 2016 Sep 27, 00:46 -0700
On 2016-09-25 22:08, Antoine Couëtte wrote: > Also are my Apparent Equatorial Coordinates from the Bureau des Longitudes Server the same as the ones you are obtaining on your side ? Maybe this should be the very first point to check again together this time. I am using the following ones (*before* performing the +0.50"/-0.25" correction earlier mentioned): > *Moon, 1855-09-07T08:05:00.00, i.e. 08h05m00.000 s TT : RA = 08 h 10 m 10.16146 s , DEC = +25 *°* 17 ' 31.0209 ", Distance = 0.002701090 UA,* My problem with the IAU 1976/80 precession / nutation model has been fixed. 08 10 10.16146 +25 17 31.0209 --- Bureau des Longitudes 08 10 10.1690 +25 17 31.006 .10 JPL HORIZONS 08 10 10.165 +25 17 31.01 .14 me DE406 1976/80 08 10 10.196 +25 17 30.96 1.31 me DE406 2006/00B 08 10 10.195 +25 17 30.97 1.27 me DE422 2006/00B The error column is the separation angle (arc seconds) with respect to the Bureau des Longitudes position. Apparently the Bureau des Longitudes still uses the IAU 1976/80 precession nutation model. JPL does too, but it's corrected with the pole offsets (from the IERS) in recent epochs. I don't know what corrections they apply in the 19th century. I used my lunar program (IAU 2006/00A precession nutation model) to calculate the 4th and 5th lines. However, in the 3rd line I used a different program to calculate the Moon's geocentric apparent place with IAU 1976 precession and 1980 nutation. It agrees closely with the BdL and JPL positions. I think the position discrepancies are almost entirely due to different precession / nutation models. It's unfortunate that two gold standards, the BdL and JPL, continue to use IAU 1976 precession, which was known to have accuracy problems by the 1990s. The IAU 2000 model was basically a simple rate correction to the 1976 model. In the 2006 model (the current one) we got something completely new and much better. It's even fairly good for archeoastronomy. My lunar program says 7h08m13.903s Greenwich apparent sidereal time, within a millisecond of Kermit.
