|October 2004 Meeting Report|
The guest speaker was Mr John McFarland, from the Armagh Observatory and his lecture was entitled “Some Astronomical Cover-ups: Occultations”.
John began by explaining what an occultation is and giving some illustrations of different types of occultation and mentioning their scientific merits. He described the relatively recent European Space Agency’s HIPPARCOS ( High Precision Parallax Collecting Satellite) astrometric space mission and the subsequent upsurge in occultation studies following the publication of the very accurate positions in the HIPPARCOS and TYCHO star catalogues. This was the first space mission devoted entirely to astrometry.
He went on to outline the history of stellar positional astronomy commencing with the work of Hipparchus of Nicaea, through the naked-eye position measurements of Tycho Brahe, and work of the nineteenth-century meridian instruments, culminating in the HIPPARCOS satellite which had increased the positional accuracy by a factor of 200 times over any previous work. He also outlined the future plans for the next generation of astrometric space missions, the ESA GAIA mission (Global Astrometric Interferometer for Astrophysics).
Also explained was how the amateur astronomical community could participate in occultation studies, taking as his/her prime example asteroidal occultations. An asteroidal occultation occurs when a minor planet passes in front of a star, blocking its light from reaching the Earth. By accurately timing the duration of these events, it is possible to gain useful information on the asteroid’s size, shape, orbit, composition, density, any duplicity and origin. It is also possible to derive information about the occulted star, for instance, its angular diameter and whether it is a single or double star or a member of a multiple system.
Outlined was the equipment that could be used for timing the events. For visual work: GPS receiver (to determine longitude, latitude and altitude of observing site), digital radio-controlled clock (giving time to a hundredth of a second), a stopwatch (reading to 0.01 sec), and a sturdily mounted pair of binoculars or telescope. Even better would be a video camera having a time-stamped mode to record the event for later study. Timings should be to the nearest tenth of a second, if possible, remembering to subtract the observer’s reaction time. Also discussed was the report form, which could be completed and returned to Jan Manek of the European Asteroidal Occultation Network (see http://www.euraster.net/ ).
John gave an in-depth account of some of the work already carried out by the occultation team set up at Armagh Observatory in the autumn of 2001, including the first successful event observed on the island of Ireland on 14 January 2021 when (516) Amherstia occulted the star HIP 36335.
Descriptions were made of planetary occultations of stars and the valuable information that they can yield. In 1904, Anton Pannekoek published a paper on a method of calculating the temperature of a planet’s atmosphere by studying the diminution of the star’s light as it is gradually immersed by the planet’s atmosphere. Highlighted was the occultation of a star by the planet Uranus on 10th March 1977 that resulted in the accidental discovery of Uranus’ narrow ring system from the photoelectric observations made by astronomers on board the Kuiper Airborne Observatory.
The talk was concluded by mentioning some interesting planet/planet occultations that have occurred during the last 500 years and the prospects for observing other such events this century.
A lively question and answer session followed which judged the popularity of the lecture.
If you have any questions about this lecture or would like to learn more, try posting your question on the EAAS Forum where are members and others will do their best to answer.
Thanks goes to Mr John McFarland for sending us this report.