Newsgroup: it.scienza.astronomia
Titolo: Una ciliegina.... sulla torta?

On December 15/16, Saturn's satellite Tethys will
occult a 9th-magnitude star for observers in Europe,
Siberia, northeastern China, Korea, and Japan; it
provides an opportunity to measure any thin
atmosphere of the icy moon, as well as its position
very accurately with respect to the star.
Jean Lecacheux at Paris Observatory writes:
The Saturnian satellite III (TETHYS) will occult a 9.1
magnitude K0-spectral type TYC-2 star during 40 sec. next Sunday
evening (Monday morning in eastern Asia). The star is
XZ 7356 = BD +21 deg. 931 = PPM 94676; it is not in the
SAO Catalog.  It's J2000 position is given below, but
Saturn is of course the most convenient reference - see
the diagram of the planet and satellites on Jan Manek's
Web site with link provided below.
The initial path, first computed by D.Herald last year,
crossed s.China, Himalayan countries, Middle and Near
East, and the n.African deserts. After that the Tethys
occultation remained almost forgotten during one year,
apparently eclipsed within the agenda of many observers
by the top-priority Pluto events of last summer.
But surprisingly Jan Manek published 4 days ago an
updated prediction, showing that the shadow of Tethys
probably will make a central hit upon west Europe,
from Baltic republics to Atlantic coast of France,
Russia, and also Japan and northeastern China.
Details are in Manek's web page :
http://sorry.vse.cz/~ludek/mp/updates/1215tet.html
The diameter of Tethys is ~1060 km, and the projected
ground path will be ~1280 km wide (NB: Manek quoted
530 and 640 km erroneously, but his calculations and
the world map in his page are correct).
The lateral uncertainty of the prediction might reach
+/-1000 km. So any country from n.Europe to s.Europe
(and even in n.Africa) could be concerned, plus many
regions in the former Soviet Union.
In consequence any observer in Europe, northern Asia
and Japan should be ready to observe !
Especially those who gained experience observing the
stellar occultation by the satellite III of Uranus
(Titania) in 2001 september are urged to contribute
again.
Bruno Sicardy of Paris Observatory wrote to us the
following message this afternoon :
 >******  Occultation by Tethys on 15 Dec. 2002 around 19:00 UT ********
 >
 > - Star coordinates (TYC 1310-2435-1, V=9.2, B=10.5) taken from Vizier:
 > 
vizier.u-strasbg.fr/cgi-bin/VizieR-5?-out.add=.&-out.add=
 > _RAJ2000,_DEJ2000&-source=I/259/tyc2&recno=281751
 > with proper motion taken into account on 15 December 2002:
 >
 > ra = 05 41 33.8871
 > dec= 22 03 31.214  (J2000)
 >
 > with typical errors of 20 mas ~ 120 km at Tethys.
 >
 > - Tethys ephemeris: JPL Horizon with no corrections.
 >
 > Two maps
 > show the track of the shadow on the Earth,
 > with assumed radius of Tethys 524 km. The dots on the central line
 > are plotted every minute and the shadow is moving from right to
 > left on the diagram.
 >
  - these maps have been posted in the item about this
    occultation near the top of 
http://iota.jhuapl.edu
 >
 > Note that the track is shifted by about half a Tethys radius to
 > the South wrt to the predictions by Jan Manek at the Web site
 > given above, Maybe due to a slightly different star position?????
 >
 > Bruno Sicardy,  12 December 2002
***********************************************************************
The coming occultation might be important to verify
again that Tethys has no significant atmosphere.
It is well known that stellar occultations are very
sensitive probes of atmospheric traces at the surface
of occulting bodies. The more is far an occulting
body, the more is efficient the method. At saturnian
distance sensitivity usually can approach one
microbar.
But please remember that only electronic detectors
with photometric ability, good time resolution, and
of course operated in excellent observing conditions,
can measure such a pressure upper limit.
Indeed only well-calibrated immersion and emersion
light curves contain the needed refractive signature.
Elementary visual timings, or uncalibrated video with
heavy noise, permit to refine path location (thus
saturnian system astrometry) with a few kilometers
accuracy, but have no potential for atmospheric
sensing.
[However, Enceladus is nearby, providing a good
  calibration, so maybe videos that are not very
  noisy might be useful for this event]
Former observers of the 2001 Titania event should
note that observing the Tethys occultation will be
a lot more difficult.
There are four main reasons :
1- The target star will be fainter by 2 magnitudes,
    i.e. +9 instead of +7.
2- The glare from the nearby planet (and principally
    from its rings) will be by far more penalizing.
3- A faster occultation (only 40s) than Titania's one.
4- Due to the brightness of Tethys (magnitude +9.8),
    the amplitude of the occultation will be only 1.2
    magnitude (i.e. the flux will drop from level 1 to
    level 0.33 at immersion, then rise to 1 again at
    emersion), whereas Titania was almost negligible
    during the totality phase.
Favourable characteristics are
1- Continental path through very populous countries
    (so all the virtual potential of IOTA-ES and EAON
    should be mobilized from East Russia to Atlantic
    coasts).
2- Occurrence at pleasant hour (19h UT, late enough
    after sunset, early enough before midnight).
3- No need to move (thus favouring large telescopes
    at fixed locations).
4- Altitude above horizon better for Europeans than
    during the past Titania event.
5- Easy comparison of the target with the bright
    nearby satellites Rhea (magnitude 9.4) or
    Enceladus.
                       ******
Meteo forecasts at -72 h give anticyclonic conditions
in Russia, Belarus and Baltic republics, also chances
of clear sky in central Europe (Poland, Slovakian
republic, e.Hungary,...), then a secondary cloud
break in n.Italy, s.France and ne.Spain.
Clear sky to everybody.

 occult a 9th-magnitude star for observers in Europe,
Deve essere abbastanza spettacolare come evento, pur essendo "piccolo",
spero solo che domani sera non ci sia la nebbia che c'č adesso o qualcosa di
peggio; oramai quest'anno ci ho fatto il callo :-((
ciao
Roberto
--------------------------------
Inviato via 
http://usenet.libero.it

 spero solo che domani sera non ci sia la nebbia che c'è adesso o qualcosa
