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Antoine Couette wrote:
> Lunar occultation of the Star currently designated as k CNC (kappa Cancri).
> 
> This star is listed as follows in the following star catalogs : BSC 
> #3623 / FK5 # 1238 / SAO # 98378 / HD 78136.
> 
> Its 2000.0 Mean elements are as follows : R.A.=09h07m44.819s *** 
> µRA=-0.0014s/y *** Delta=N10°40'05.44" *** µDelta -0.01 "/y no parallax 
> quoted.

SIMBAD says the HD number is 78316. Parallax is practically zero: only
.0067".

"Your k CNC position has been derived from the Hipparcos Catalog. From
the NGC catalog, I find a position which is almost 1" apart in RA, which
would already account for a possible error of 2 seconds of time in my
results. This difference in RA between our both results is well above
anything I could expect. Would you then indicate the relevant k CNC
Hipparcos data (most preferably referred to 2000.0 rather than to
1991.25), or at least be so kind as to tell me where I can find them on
the net. BTW, what is k CNC Hipparcos Ref # ?"


To get the Hipparcos catalog data, begin at the SIMBAD site:
http://simbad.u-strasbg.fr/simbad/sim-fid

Enter kap cnc in the Identifier box.

(Greek letter codes:
http://simbad.u-strasbg.fr/guide/chA.htx
Constellation codes:
http://vizier.u-strasbg.fr/vizier/VizieR/constellations.htx
Most of the codes are obvious, but there are a few exceptions, e.g., pi.
not pi for the letter π.)

SIMBAD will give you all the designations for kappa Cancri. It also
displays coordinates in several systems, including the ICRS at epoch
2000. The proper motions and parallax in milli arc seconds are displayed
too.

To see the Hipparcos catalog entry, copy the HIP number from the Web
page and go here:

http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=hipsearch

Paste the number into the Hipparcos Identifier box. Note that Hipparcos
catalog gives the proper motion in RA in milli arc seconds per Julian
year (365.25 days) in great circle units, i.e., the RA proper motions
does not tend to increase for stars near the poles. SIMBAD uses the same
convention. This is important because many older catalogs simply give
the rate of change of RA per year.


"Now, since you seem using DE405 full theory which (and unless I am
mistaken) does include Libration, I would have thought that you would be
using its exact Libration values which accordingly would enable you to
compute the exact apparent topocentric difference between both
geometrical and gravity centers."

Actually, I used DE406, which omits nutations and librations. My program
can use the DE405 ephemeris, but I don't know how to apply libration
corrections, so the program ignores that data. In all the computations
below, I will apply the simple fixed offsets to longitude and latitude.


"IMMERSION at UT=22h43m33.4s"

My value = 22:43:36.7 UT1, with -1.1 s delta T.

"EMERSION at UT=23h59m31.4s"

My value = 23:59:36.0 UT1.


"In [Meeus] 'Astronomical Tables of the Sun, Moon, and Planets',
Copyright 1983, ISBN 0-943396-02-6 and Edition 1983 (?) which I own and
which is a GREAT book, we can find on pages 5-8 and 5-9 the following
example :

"Occultation of Aldebaran, 1997 October 19, at Palomar Mountain
Observatory, for which we have :

"Lat = N +33°.3562, Lon = + W 116°.8640, Altitude + 1 706 m (above
ellipsoid),

"delta T = +68s

"For IMMERSION, Jean Meeus finds UT = 07h44m41s, and
"for EMERSION, he finds UT = 09h02m44s"

I get 07:44:40.5 and 09:02:43.0.


"I am using delta T = 62.8 s, which I am guessing/feeling is much closer
from your own value and FER's On Line Computer value, and I reworked
this example for 3 different altitudes :"

In my program, delta T is a user input, so I can give it any value.


"Alt = 0000 m / 0000 ' UT = 07h44m48.2s UT = 09h02m51.8s"

My values: 07:44:48.2 and 09:02:51.3

"Alt = +1760 m / 5597 ' UT = 07h44m47.3s UT = 09h02m51.2s"

My values: 07:44:47.3 and 09:02:50.7

"Alt = +4000 m / 13123 ' UT = 07h44m46.2s UT = 09h02m50.3s"

My values: 07:44:46.2 and 09:02:49.8.

In each case we agree perfectly (less than .1 s and .1") at immersion,
and disagree by .2" at emersion. The lunar distance is changing at
23.6"/minute at emersion, so .2" arc = .5 s time.


Tomorrow I will compute the coordinates of kappa Cancri in several 
reference systems (barycentric ICRS, geocentric ICRS, etc.) to help you 
figure out why our coordinates are different.

-- 

   

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