3/19/99 Lachesis Update


IOTA/IOTA-ES occultation update for
            (120) Lachesis / TYC 6752 00211 event on 1999, March 23
                   visible from Canada, the USA, and Mexico

Summary

On the night March 22/23, 1999 the 9.9 mag star TYC 6752 00211 will be
occulted by the 178 km. diameter asteroid 120 Lachesis. This update is
based on USNO/Flagstaff astrometry for the asteroid and the star position
suggested by D. Dunham after looking at the binary pair catalog data.
The calculation shows the path shifted by about 0.5 path widths to the
north-west and about 15 seconds earlier compared with Goffin's nominal
prediction.

The event at one glimpse:
 * date and approx. UT time of event: March, 23, 1999 @ 08:05 UT
 * magnitude of target star: 9.9
 * magnitude drop [mag]: 2.8
 * estimated maximum duration [s]: 38.8
 * path description: Path runs low in the horizon over Central Canada at
about 07:58 UT, Crossing into the USA (Denver is on the southern edge of
the path at ~08:00:30) and the Southwester USA (Salt Lake City - 8:02,
Las Vegas - 8:04, and LA - 8:05.5, are in the northern edge of the path
while Phoenix- 8:04, is on the southern edge). The shadow goes over San
Diego/Tijuana and Ensenada, Mexico at ~08:05.5 before heading to the
ocean.

The occultation path:
 * approximate width [km]: ~195
 * uncertainty [path widths]: ~0.5 (+~1.0)
 * uncertainty in time [s]: ~30
 * map: attached
 * remarks: The first path uncertainty given is derived from the asteroid
orbital elements calculations average residuals (+-0.05"). The star is a
member of a binary and its position may be uncertain by an additional
path width (~0.1")

Data for the target star:
 * name: TYC 6752 00211
 * constellation: Libra
 * J2000 position [h,m,s; o,',"]: 14h 56m 20.922s; -24* 40' 23.24"
 * position source: D. Dunham
 * estimated accuracy ["]: 0.1"
 * Johnson V mag [mag]: 9.90
 * B-V [mag]:
 * remarks:

Data for the minor planet:
 * general information:
     mumber, name: (120) Lachesis
     approx. diameter [km]: 178
 * orbital information
     orbit source: calculation Sada (using CAA)
     source of used astrometry: USNO/Flagstaff (R. Stone) + TMO (B. Owen)
     number of used observations: 32 (22 USNO + 10 TMO)
     number of rejected observations: 4 USNO (2/12/98, 2/13,3/13,3/16/99)
     time covered by the observations: 1998 02 14 - 1999 03 14
     rms residuals ["] (RA,DE): 0.05" (combined)
     estimated positional accuracy at epoch of event ["]:
     remarks: Was a bit picky with choosing residuals.

Data for the event:
 * UT date and time of least geocentric approach: 08:05:20 UT
 * approx. V mag of minor planet at event [mag]: 12.6
 * geocentric parallax of minor planet ["]: 4.094
 * magnitude drop [mag]: 2.8
 * estimated maximum duration [s]: 38.8
 * apparent motion of minor planet ["/h]: 10.6
 * angular distance to moon, phase of moon [deg,%]: 149, 37%
 * remarks:

Centerline path coordinates:

Occultation of TYC 6752 00211 by 120 Lachesis on 1999 March 23.
Orbital elements from 32 USNO and TMO observations (2/14/98-3/14/99).
Star position suggested by D. Dunham.
      Altitude
  E. Longitude    Latitude     U.T.    star  sun
   -154 00 00   + 29 39 02    8 13 50    1   -47
   -152 00 00   + 28 48 20    8 13 47    2   -49
   -150 00 00   + 28 06 10    8 13 40    4   -50
   -148 00 00   + 27 31 55    8 13 29    6   -52
   -146 00 00   + 27 05 02    8 13 15    8   -54
   -144 00 00   + 26 45 09    8 12 58    9   -55
   -142 00 00   + 26 31 57    8 12 38   11   -56
   -140 00 00   + 26 25 13    8 12 15   12   -57
   -138 00 00   + 26 24 47    8 11 50   14   -58
   -136 00 00   + 26 30 32    8 11 22   15   -59
   -134 00 00   + 26 42 25    8 10 52   16   -60
   -132 00 00   + 27 00 26    8 10 19   17   -60
   -130 00 00   + 27 24 38    8 09 45   18   -60
   -128 00 00   + 27 55 08    8 09 08   19   -60
   -126 00 00   + 28 32 05    8 08 30   20   -60
   -124 00 00   + 29 15 44    8 07 50   21   -60
   -122 00 00   + 30 06 24    8 07 07   21   -59
   -120 00 00   + 31 04 34    8 06 23   21   -58
   -118 00 00   + 32 10 50    8 05 38   21   -57
   -116 00 00   + 33 26 02    8 04 50   21   -56
   -114 00 00   + 34 51 20    8 04 00   21   -54
   -112 00 00   + 36 28 30    8 03 09   21   -52
   -110 00 00   + 38 20 14    8 02 15   20   -50
   -108 00 00   + 40 31 15    8 01 19   19   -48
   -106 00 00   + 43 11 10    8 00 19   17   -45
   -104 00 00   + 46 47 45    7 59 11   14   -41

Calculator: Sada