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From: TerryMoselaol.com
Date: Fri, 23 Sep 2005 16:59:06 EDT
Subject: Super lecture, Birr Accom., Asteroid Occultation

Hi all,
1. We had a really fascinating & challenging lecture from Prof Chandra  
Wickramasinghe in Belfast on Wednesday evening. Some very interesting ideas,  
expertly presented. We had our biggest ever attendance for as astronomical  
lecturer, and they weren't disappointed. And the questions to  Chandra continued 
until we had to leave the building!
  We video'd the lecture, and hope to have some  DVD copies to sell at cost 
price at the Whirlpool Star Party on 7-9  October.
2. Talking of which, I have been advised by a member that B&B No  37 on the 
Birr list I circulated recently (Somerset  House), should be avoided at all 
costs! I can't say more in case I get into  legal trouble....  Please let me know 
of any other bad experiences, if any,  of any other properties. Or 
conversely, of any really good ones. But then you  would want to keep those to 
yourselves, wouldn't you?
3. There's a good opportunity for observers in Ireland to try to  measure the 
size of an asteroid, on Tuesday 27 September. This is for serious  observers 
only, so if you're the armchair type, you can skip the rest!
Yes you can, if you live in  the Eastern half of Ireland, and there are clear 
skies on the evening of 27  September!
The asteroid 565 Marbachia  will occult a brightish star that evening, at 
about 21.21 - 21.22 BST, and by  timing the event accurately we can work out the 
size of the asteroid. And if we  get lets of good observations, we can even 
work out its shape! The star is in N  Aquarius, below Delphinus (SE of Aquila).
To start at the beginning:  The asteroid (Minor Planet) will pass in front of 
a star, TYC 5182-00548-1,  which is magnitude 9.5 (visual), at about 21.22 
BST. Since the asteroid is  magnitude 15.4, you won't see it (unless you have a 
monster telescope!),  but you can easily see the sudden disappearance of the 
star as the asteroid  passes in front of it and cuts off its light. Since we 
know the rate of motion  of the asteroid, the duration of the occultation gives 
its diameter. 
The chart on the link below  shows the direction of motion of its 'shadow' on 
the Earth, which as you can see  is almost N - S, together with timings in 
minutes along the track. I have  done a more detailed and accurate plot of the 
actual latitude &  longitude predictions on Ordnance Survey maps. The centre 
line runs  N-S through or close to the following locations as follows: 
Portrush (2km W of  line)
Aghadowey (2km  W)
Magherafelt ( (1 km  E)
Coagh (on  the line)
Armagh (2km  W)
Crossmaglen (1km  E)
Johnstown (near Naas -  almost on line)
Ballymore Eustace (on  line)
Kiltegan (almost on  line)
Kilmore  Quay
Gt Saltee Island (Just  0.2km W of line - no observers there, but just to 
give you the  line!)
The path width is assumed  to be 29km, so if you observe within 14km of that 
line, then in theory you  should see an occultation. BUT - the outer two lines 
(dotted) on the chart show  the 1-sigma uncertainty in the line of the track! 
So in fact you might see an  occultation even as far West as Athlone or as 
far East as Belfast! If there is  an update on the track before the 27th, I'll 
alert you.
 At 21h 22m  BST the shadow should be over South Ulster, by 21.25 it will be 
over  Portugal.
The star chart on that link  is useless, so unless you have your own 
software, email me & I'll send you a  proper jpeg chart showing the star clearly, in a 
recognisable star  field!

The asteroid is assumed to  have a diameter of 28km, giving an assumed 
apparent diameter of 0.019"  (arcseconds). But that has never been directly 
measured, and is based solely on  its brightness, and assumptions about its albedo 
(reflectivity). If it's darker  than we assume, then it will be bigger than that, 
and vice  versa.
Based on that assumed  diameter, the maximum duration of the occultation will 
be 4.7 seconds, on the  centre line. If there are observers at various 
positions along and  across the track, each timing the start and end of the 
disappearance  accurately, we will get a series of 'chords' across the asteroid, 
showing its  size and shape. Some, as you probably know, are quite irregular, even  
'dumb-bell' shaped. So we just don't know what to expect.
HOW TO OBSERVE: Any  well-timed observations will be useful, even just visual 
with a stopwatch. But  you should aim for at least 0.2 second accuracy, and 
0.1 second is better.  However, even 0.5 second accuray is nbetter than 
nothing! Most stopwatches operate to at  least 0.1s accuracy. You simply start 
watching the star (having CAREFULLY  identified it beforehand!) a few minutes before 
the predicted time, and start  the stopwatch as soon as you see the star 
disappear, and then stop it again on  the 'lap-time' function as soon as it 
reappears. Then stop it again on the 'end'  time function against an accurate time 
signal, as soon as possible thereafter.  That will give the start and end of 
the occultation, and the total elapsed time.  Some wristwatches even have a good 
stopwatch built in. And for all I know, the  latest mobile phones do too! 
Check & practice well beforehand! Write down  each time before clicking the 
button again on the stopwatch, or you'll lose  it!
If you haven't a stopwatch,  you could try using a portable tape-recorder 
(dictaphone) with new fresh  batteries: start it a few minutes before the 
occultation is due, then say loudly  & sharply 'Out' as soon as the star disappears, 
and 'Back' as soon as it  reappears. Then record a time signal accurately on 
the tape asap before you stop  it. Run the tape again asap while the batteries 
are still strong, and take the  two timings relative to the time signal. This 
is not as accurate, but is better  than nothing.
(Don't worry at this stage  about your 'Personal Equation', i.e. the time it 
takes you to react to the  event, and press the stopwatch, or speak. Just 
record your raw times, and if you  get something useful I'll tell you how to allow 
for your P.E.  later.)
Even better is to record it  by video through the telescope with an accurate 
timer built in or  superimposed on the recording. Afterwards you check the 
start and end time  against an accurate time signal to give real time. This gives 
a permanent  record, and is less prone to human error.
1. Locate the star a few  nights previously to make sure you can identify it 
easily. Use the attached star  charts. I have given some suggestions for 
'star-hopping' from Aquila for those  of you without GoTo or setting circles.
2. Practice using the  stopwatch with the various timer functions beforehand 
- you only get one chance  at the time, and it happens quickly! Remember, 
ideally you want to record not  only the duration, but when it actually started 
and  finished.
3. Likewise, if using a  dictaphone, practice beforehand. The sooner you 
record the time signal after the  event, the better.
4. If using a video, test  & practice beforehand. Tolis Christou at Armagh 
Observatory has some  experience of this technique, so you could ask him for 
advice if you plan to try  it.
5. Ask not be to  interrupted, on any account, at the time of observation - a 
moment's distraction  and you could miss the start or end. Remember that 
you'll have no warning - the  star will simply disappear instantaneously, and 
reappear  likewise.
6. Use a telescope big  enough so that you can see the star clearly: if it is 
at your limit of  visibility then you're not going to get useful timings. You 
should be able to  see stars at least 2 magnitudes, and preferably 3 
magnitudes, fainter than the  target star. If using video, the star should show 
clearly on the image all the  time, except during any actual occultation.
5. If you are not exactly  on the track, observe anyway! The dotted lines on 
either side of the central  track indicate the uncertainty in the position. 
You might even discover a  companion, as some asteroids have 'satellite' 
asteroids going around them, so if  you see another brief occultation, report it too, 
and time it as best you  can.
6. NEGATIVE OBSERVATIONS  ARE ALSO VERY USEFUL! If you observe carefully, and 
DON'T see anything, report  that too - it gives an upper limit to the size of 
the  asteroid.
7. Be set up 10 minutes  before the time, i.e. about 21.10, and start actual 
observing /  recording a few minutes before the expected time - say 21.20 at 
the  latest.
8. Make sure all batteries  in drives etc are in good condition!
9. Time signals: The  Accurist Speaking Clock is "123" on a landline phone 
(in UK), but I don't  know if you can access it from a mobile? Otherwise the 
clock on the TV and radio  is fairly accurate if you can see the second hand, or 
hear the actual pips.  The Ceefax clock is not quite accurate enough. And 
unless you have magically set  it up for high accuracy, the clock on your computer 
won't be accurate enough  either. A modern digital quartz watch will be 
accurate over a short period, so  if you set it really accurately against the 
'pips' or the Speaking Clock earlier  that evening, it should still be accurate a 
few hours later. Just do  your best.
There's an article on  this in the latest Astronomy Now (October).
Whatever you do - REPORT  YOUR RESULTS, either positive or negative. Even if 
you make a mess of the  timing, at least let me know if you saw an occultation!
Good  Luck.
Clear  Skies!


Last Revised: 2005 September 26th
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