[BAA 00306] Asteroid occultation Wed/Thur 12/13 September 2007

[Rick]

PROMINENT OCCULTATION BY ASTEROID (120) LACHESIS ON 2007 SEPTEMBER 12/13

BAA electronic bulletin No. 00306 http://www.britastro.org/

The 13th magnitude asteroid (120) Lachesis is predicted to occult the 11th magnitude star GSC 1826-0245 in the early hours of Thursday September 13, i.e. the night of Weds/Thurs September 12/13.  The prediction indicates that the occultation will occur over the UK between 03:44 and 03:45 UT.  At the time, the star will be in Taurus at around 62deg elevation, 148deg azimuth, and therefore well placed for observation.

Lachesis has a listed diameter of 174km and so the shadow track crosses a large swathe of England and Wales from the far south west to the north east. In more detail, the main track crosses north Cornwall, most of Wales, parts of the north Midlands, Cheshire, Merseyside, Lancashire, Greater Manchester, Yorkshire, Cumbria, Durham, and the north east (using pre-1996 county nomenclature).  A small selection of major conurbations included are Swansea, Aberystwyth, Wolverhampton, Liverpool, Manchester, Preston, Blackpool, Lancaster, Sheffield, Leeds/Bradford, York, Scarborough, Sunderland, Middlesbrough, Newcastle-on-Tyne.

The magnitude drop during occultation will be 2.3 (about a factor of ten in brightness) and the maximum duration on the centre line will be about 22 seconds.  The formal uncertainty is expected to be small with calculated probabilities of a positive event of 90-95% on the centre line, 50% at the shadow edges and 16% at the one standard deviation error lines.  (Despite recent improvements in prediction, observers should be aware that occultation tracks may shift by 2-3 standard deviations.)  The following links give more details.

Track map and basic star charts:-

http://www.astrosurf.com/eaon/Cartes/09%20September/13%20lachesis.gif

For the more adventurous, here is a link to Google maps allowing detailed examination of the track, a list of sites with distances from the centre line, and much much more:-

http://www.poyntsource.com/New/Global.htm

The undersigned would welcome any serious attempts at timing this event (weather permitting!).  Identification of the correct star is crucial but planetarium software containing the GSC makes the task easier.  The star should be observed for at least 2 minutes either side of the above times i.e. from 03:42-03:47 UT (04:42-04:47 BST).  The actual timings of both disappearance and reappearance are required, preferably to an accuracy of 0.1 second.  Observations may be visual, video, or with a CCD in 'drift scan' mode.  A specimen report form (which requests other details) is here:-

http://astrosurf.com/eaon/Report%20form.htm

Please send any reports direct to the undersigned.  Negative reports from within or close to the predicted track are also valuable to help pinpoint the actual track.

If you miss this one, there is another good event involving (444) Gyptis on the night of October 13/14.  The track traverses the whole of the west side of GB from north to south, with a maximum duration of 29 seconds.

Acknowledgement is given to Steve Preston for the updated prediction, the providers of the above web sites, and Google Maps.

Good luck!

Andrew Elliott
Assistant Director (Occultations), ARPS