FIRST DETECTION OF AN ASTEROID SHADOW OVER ARMAGH

Armagh Observatory, 16 January 2002:
Apostolos Christou and John McFarland have made the first detection, from anywhere in Ireland, of the shadow cast by a star of an asteroid as it passed over Armagh. Such events, known as asteroidal occultations, allow a precise fix to be made on the asteroid's position in space, and also an estimate of its size and shape. The event which took place during the early hours of 14 January, was noted by Dr David Dunham, President of the International Occultation Timing Association, as "the first asteroidal occultation observed from anywhere in Ireland."

Occultation astronomy is a relatively new branch of astronomy, growing rapidly in recent years following the launch of the HIPPARCOS spacecraft, which accurately measured the positions of over 100,000 stars from 1989 to 1993, and because of the availability of improved orbital predictions for asteroids and other solar system bodies.

Asteroids are mostly small, rocky objects, lying hundreds of millions of miles from the Earth between Mars and Jupiter. Occasionally, one of these objects will pass in front of a star as seen from certain locations on the Earth, blotting out the star's light for a few seconds. By timing such an event to a fraction of a second the position of the asteroid in space can be determined with a precision far better than even the best conventional imaging techniques. To get a feel for the accuracy, every second of timing precision corresponds to an accuracy of about 15 km in space at the asteroid: the equivalent of locating a penny with precision at a distance of more than 200 miles. On this occasion, the asteroid Amherstia, measuring approximately 76 kilometres across, was predicted to pass in front of the star designated HIP 36335 in the HIPPARCOS catalogue. Its shadow would sweep at some 35,000 miles per hour across Northern Europe, the Atlantic Ocean and USA, crossing Northern Ireland about 13 minutes after midnight on the 13/14 January. The observed occultation lasted approximately 5 seconds, and was accurately timed by Apostolos Christou at Armagh Observatory. His colleague, John McFarland, made an independent observation from Richhill, Co. Armagh, while international observers recorded data from as far afield as Florida, USA. These efforts were part of a coordinated campaign, involving both amateur and professional astronomers, aiming to catch as many of these occultations as possible.

Occultation astronomy is one of a number of serious observational projects for which coordinated networks of small instruments fare better than single, large professional-class instruments located at fixed sites and for which telescope time has to be carefully rationed.

Amherstia belongs to the rare metal-rich M-type asteroid class. It lies some 265 million miles from the Sun and has an orbit with a period of 4.4 years in the main asteroid belt between Mars and Jupiter. It was discovered in 1903 by R.S. Dugan while working at Heidelberg, and named by Dugan after the college he attended, Amherst College in Massachusetts, USA.

FOR FURTHER INFORMATION CONTACT: Dr Apostolos Christou or John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; e-mail: aac@star.arm.ac.uk and jmf@star.arm.ac.uk; URL: http://star.arm.ac.uk/.

See also:
Sky and Telescope January events
Occultation Updates
Occultations 2002

Last Revised: 2002 January 16th WWW contact: webmaster@star.arm.ac.uk

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