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Re: Longitude by Lunar Occultation
From: Paul Hirose
Date: 2019 Jan 12, 00:47 -0800

On 2019-01-11 11:15, Robin Stuart wrote: > A comparison of how far off the disk a star stands at a given time is really useful for comparison as its not computed by the package. The high precision values used as inputs to the > T*_0 (TT): 19h 33m 52.731s > Delta T: 69.52s > q*_0: -0.602975 > p*': 0.602172 > q'*: 0.070193 > Also > Star RA: 5.64646763 h > Star dec: 21.1511853° > GHA at T*_0: 2.24009589 h > These were generated by Skyfield code and if you disagree with any of them I would be very interested in hearing about it. Part of the message seems to be truncated, since the second sentence ("The high precision values...") is incomplete. The RA/dec appears to be geocentric apparent place, true equator and equinox of date (2019-01-18 and the TT given above). My position is .060″ different. That could be due to a difference in precession nutation model or star catalog. I used IAU 2006 / 2000B and the CDS SIMBAD catalog data, which comes from the Hipparcos star catalog re-reduction (van Leeuwen, 2007), adjusted to the epoch J2000.0. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=zet+tau&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id > A few comments are in order: The immersion/emersion times are rounded to seconds on the site. A 0.5s change in the immersion time changes the longitude by 40². The times depend on the value of the ratio of the Earth to Moon radii used and the geodetic reference ellipsoid. Do you know what is being used on the site? Occultation predictions at the IOTA site are calculated with the OCCULT program, whose documentation says it assumes 6378.137 km Earth radius and k = 0.2725076. Ellipsoid flattening is not given.

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