OBSERVING ALERT: Binary asteroid occultation, September 26

[Rick]

BAA electronic circular No. 00203

OBSERVING ALERT:  Binary asteroid occultation, September 26/27 (Monday evening)

The asteroid 90 Antiope will occult the star UCAC2 21873650 (GSC 6877 0218) on 2005 September 26 at about 20:23 UT. (21:23 BST)

This is an extremely rare opportunity to observe a binary asteroid system by occultation timing.  90 Antiope comprises two objects each about 85 km in diameter orbiting each other in close proximity.  The shadow track of BOTH components cross the United Kingdom and Ireland although it is unlikely that any one observing station will witness more than one occultation event.

Please observe from: 20:20 to 20:26 UT (about 9.20-9.26 p.m. local time in the UK).  The maximum duration is expected to be about 7 seconds with a drop in brightness of about 1.5-2.0 mag (i.e. a factor of 4-6).  Unfortunately  the star being occulted is about 12th magnitude (V=12.1) and lies in Sagittarius and so will be low in the sky (9-12 deg altitude) in the SSW direction.

Location of star:   R.A. 19:04:29,  Dec. -24:46:17

Details showing the two shadow tracks, etc. can be found at:

http://www.astrosurf.com/eaon/Cartes/09%20Septembre%2005/26%20antiope.gif

It is recommended that observers use a telescope of 20 cm or larger.  Do check beforehand whether you may have to travel to find a good location with a clear southern horizon.  Please attempt to time the event (disappearance and reappearance of star) using a stopwatch or other method. Video or fast CCD observations are particularly valuable.  Where possible calibrate your timing device against an accurate source (radio-controlled clock or telephone speaking clock).

Good luck with the weather.

Richard Miles
Assistant Director, Asteroids and Remote Planets Section