CONCLUSION: My thanks to those members who tried to observe the occultation of the +5.7 magnitude star in Taurus by asteroid 431, Nephele. Unfortunately the weather did not clear early enough for most of us throughout the UK to attempt the timing of the event. It would seem however, that according to observations coming in from the eastern USA, the path of Nephele's "shadow" was even further north of the latest predicted path. It seems very doubtful therefore that anyone in the UK would have obtained anything other than a negative result for this particular asteroidal occultation.

IMPORTANT UPDATE (27 October): As we report below, during the early morning of 2002 Nov 03 UT, the 95 km diameter asteroid (431) Nephele will occult the 5.9 mag star ZC 593 (SAO 93721) in the constellation Taurus for observers along a path crossing Russia, Scandanavia, Great Britain, and the eastern USA.
Where an occultation takes place, the combined light of the asteroid and the star will drop by 7.1 mag to 13.0 mag (the magnitude of the asteroid) for at most 8.4 seconds.
Originally it was thought that the path of Nephele's "shadow would cross the North of England and Northern Ireland, but recent updates based on UNSO/Flagstaff astrometry and other sources, together with the Hipparcos Star Catalogue suggest that the path is shifted further towards the North and will now miss mainland Britain altogether. There is a chance though, that observers on the Shetland Islands to the north of Scotland may possibly see something of the event given clear skies!

ORIGINAL REPORT (24 August): The best chance of seeing an asteroidal occultation from the UK takes place in the early hours of Sunday morning, 3 November when the magnitude +13.2 asteroid 431 Nephele may occult a magnitude +5.9 star numbered 593 in the Zodiacal Catalogue ( SAO 93721).

ZC 593 is relatively easy to find if you have a star atlas. It lies between the Hyades and Pleiades open clusters in the constellation of Taurus, the Bull. The star’s celestial co-ordinates ( Epoch 2000) are: RA 04h 00m 48.793s, Declination +18deg 11m 38.32s. At the moment, the “shadow” of the 97.7 km. diameter asteroid is predicted to cross central to northern U.K. from E to W, and observers who witness the occultation should expect to see a drop in the brightness of the star in the order of 7 magnitudes. In other words, for most of us fortunate enough to see the event, the star will blink out of sight. The duration of the occultation is expected to be anything from 0 to a maximum of 9 seconds. A map, showing the most up to date prediction for the occultation path may be seen below. It has to be remembered though, that predictions involving asteroids are very difficult to make because their orbital elements are often not known with sufficient accuracy. Also, the smaller asteroids are prone to be pushed off course by a variety of gravitational effects as they journey around the sun. I therefore encourage all members to observe ZC 593 ( SAO 93721), (see finder chart) no matter where you are in the UK. Like winning The National Lottery, not many people in this country have ever witnessed an asteroidal occultation because of the difficulties outlined above; but — you never know — this time it could be YOU!

The event, should it occur at your site, may be observed with any optical instrument from binoculars upwards. The moon is New, the following day so there will be no interference from moonlight.

What do you need if you want to do some very useful work in this observational area and do not have sophisticated CCD equipment to record the event?

The answer is: a stopwatch which has the ability to give a split/lap time and an accurate time source such as a radio controlled clock or transmitted time signal.

You should set up your equipment in good time, at least half an hour before the event (From 01h UT on November 3) and make sure that you can located the star. Incidentally, it is very good practice to have located ZC 593 to your satisfaction on a previous night. Check that you know exactly how to use the split time/lap function on your watch. Practise starting the watch, obtaining a split time, then stopping the watch a number of times.

From 01h 30m onwards keep watch and be very alert, especially from 01h 40m to 01h 50m UT, stopwatch finger ready!

If the star blinks out START the watch immediately. When it blinks on again press the split/lap button and record the time interval during which the star was occulted. (The stopwatch should still be running!)

If you get this far — CONGRATULATIONS!

The last thing you must do is to STOP the watch against an exact time signal as shown on your radio controlled clock or given by an accurate time signal and make a note of the total elapsed time since you started the watch when the star winked out.

By deducting the total elapsed time from the exact time you stopped the watch (as shown on the radio controlled clock), you will have the exact time the occultation started and by adding to this the split/lap time you recorded ( of up to 9 seconds on this occasion ) you will then have the exact time the occultation came to an end.
If you do get a positive result please send your timings to me with any description of what you actually saw the star do as it went out (Was there, for example) any unusual effects, fading, uneven fading, fluctuations in the stars brightness, not due to atmospheric effects, etc.). Even if you see absolutely nothing please do send me a report. This is useful too as it will enable me to know where the occultation was NOT seen from.

I will forward any results we may obtain to E.A.O.N (European Asteroidal Occultation Network) as they may be of extreme value in helping to work out more accurately the shape and dimensions of asteroid (431) Nephele. Incidentally, the asteroid was named after the wife of mythical King Athamas of Boeotia. It was discovered on December 18, 1897, by A. Charlois, from Nice in France.

Jon Harper

Email: occultation@popastro.com

Address in Popular Astronomy

 

 

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