NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by Lunar Occultation
From: Paul Hirose
Date: 2019 Jan 13, 15:16 -0800
From: Paul Hirose
Date: 2019 Jan 13, 15:16 -0800
On 2019-01-13 10:01, Robin Stuart wrote: > If you could confirm the TT of geocentric conjunction of the Moon and star (T*_0) in R.A. then I think we could draw some concrete conclusions. RA (true equator & equinox of date) at 2019-01-18 19:33:52.697 TT moon 84.6970085° (DE431) zet Tau 84.6970084 moon 84.6970064 (DE406) Out of curiosity I included the old DE406 ephemeris from ca. 1997. It's pretty good — the conjunction is only 11 milliseconds later. > For reference here’s the relevant data that comes to Skyfield. It is as it appears in the original Hipparcos catalog. > magnitude 2.970000 > ra_degrees 84.41118447 > dec_degrees 21.14259299 > parallax_mas 7.820000 > ra_mas_per_year 2.390000 > dec_mas_per_year -18.040000 > epoch_year 1991.250000 The zeta Tauri star catalog data I used above came from this SIMBAD page http://simbad.u-strasbg.fr/simbad/sim-id?Ident=zet+tau&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id where they have been adjusted to epoch J2000.0. Those data are based on the 2007 Hipparcos re-reduction of van Leeuwen. With the CDS VIZIER site I compared the original Hipparcos catalog (HIP1) to the re-reduction (HIP2). Both are at epoch J1991.25. The upper line matches Skyfield. RA dec px pmRA pmDec 084.41118447 +21.14259299 7.82 2.39 -18.04 HIP1 084.41118462 +21.14259289 7.33 1.78 -20.07 HIP2 Now test what effect the star catalog differences have on zeta Tauri RA at 2019-01-18 19:33:52.697 TT. 84.6970133 HIP1 84.6970084 HIP2 84.6970084 SIMBAD 84.6970084 SIMBAD sans radial vel. That shows the SIMBAD adjustment from J1991.25 to J2000.0 is accurate (declination was identical too) and radial velocity can be ignored. (There are only about 20 stars in the Hipparcos catalog where that's not true. All have high parallax and proper motion.) The two different catalog data yield apparent positions .059″ apart at present. (That includes the discrepancies in declination.) In order for the moon and zeta Tauri RA to match, using HIP1, TT = 2019-01-18 19:33:52.724 84.6970133 zeta Tauri 84.6970133 moon All computations with IAU 2006 precession, 2000B nutation, JPL DE431 ephemeris.
