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From: TerryMoselaol.com
Date: 31 October 2006 00:33:49 GMT
Subject: HANDS ON 'SCOPES, Orionids, Leonids,

Hi all,

1. The next IAA public meeting will be on Wednesday 1 November (Yes, we
are all saints!*). This will be a 'Hands-On Telescope Session'. Or
"Everything you always wanted to know about telescopes but were afraid
to ask". We'll have a selection of telescopes & binoculars of various
types, brands, sizes etc, so you can see what they are like, how to use
them, ask their owners what they think of them, and maybe even pick up a
bargain - or sell one! So if you have one you want to sell, bring it
along & see what happens.  It's at 7.30 p.m., Lecture Theatre 5,
Stranmillis College, Stranmillis Road, Belfast: Admission is free,
including light refreshments, and all are welcome.

2. Thanks to Prof Alan Fitzsimmons of QUB for passing on this
interesting info on a much enhanced display of Orionid Meteors, with
plenty of fireballs!  "ORIONID METEORS 2006:  J. M.
Trigo-Rodriguez, Institute of Space Sciences, Consejo Superior de
Investigaciones Cientificas (CSIC) and Institut d'Estudis Espacials de
Catalunya, Barcelona; J. M. Madiedo, Universidad de Huelva; A. J.
Castro-Tirado, Instituto de Astrofisica de Andalucia, CSIC, Granada; P.
Pujols Grup d'Estudis Astronomics, Barcelona; and A. Sanchez report that
an increase over normal Orionid meteor rates was observed between
Oct. 21.062 and 21.229 UT from two locations in Spain [Montseny
(Barcelona) and La Mayora] by the Spanish Meteor Network using all-sky
CCD cameras and digital video cameras that recorded the outburst with
typical limiting meteor magnitudes of +2 to +3.  The highest rates were
recorded between Oct. 21.051 and 21.099, with maximum zenithal hourly
rates (ZHR) of about 50 +/- 15 meteors/hour (12 meteors in the magnitude
range -7 to +3) at Oct. 21.080 (1h55m +/- 5m; solar longitude 207.44
deg, equinox 2000.00).  A nice display of unusually bright fireballs of
magnitude -7 to -5 was recorded between Oct. 21.104 and 21.188.  On
Oct. 21.9 UT, S. Nakano (Sumoto, Japan) reported a notable increase in
meteors, including the appearance of several meteors on his CCD frames
taken around that time for cometary astrometric purposes (field-of-view
1.1 deg x 0.74 deg), the meteors appearing to radiate from the direction
of Monoceros.  Nakano later forwarded the reports of K. Mameta and
K. Sumie Kobe (observing near Chikusa-cho, Shisou-gun, Hyogo-ken,
Japan), who reported rates of 150-350 meteors/hr during Oct.
21.75-21.83.  P. Jenniskens, SETI Institute, writes that this
outburst of Orionids (parent comet 1P/Halley) occurred over the span
Oct. 19-23, but continues at elevated levels now. He has received
visual reports from observers worldwide, including B. Lunsford (USA,
Oct. 20.4, ZHR = 24 +/- 3 meteors/hr; Oct. 21.4, 53 +/- 5; Oct. 22.4, 46
+/- 4), K. Youmans (USA, Oct.  21.3, ZHR = 53 +/- 5 meteors/hr), K.
Miskotte (The Netherlands, Oct. 22.1, ZHR =  52 +/- 8 meteors/hr), and
P. Jenniskens (USA, Oct. 23.3, ZHR = 39 +/- 5  meteors/hr).  The
normal Orionid rate is ZHR = 15-25 meteors/hr.  The peak time was at
Oct. 21.55 +/- 0.05 (solar longitude 207.91 +/- 0.05 deg,
equinox 2000.0), based on radio forward meteor scatter counts by I.
Yrjola (Kuusankoski, Finland).  Further to their report above, J. M.
Trigo-Rodriguez and J. M. Madiedo report from single-station astrometry
a geocentric radiant at  R.A. = 96 +/- 1 deg, Decl. +14 +/- 1 deg on
Oct. 21.1 (from 12 meteors).    Many meteors were bright (with a
magnitude-distribution index, or the number ratio of meteors in
neighbouring magnitude intervals, of 1.88 +/-  0.08) with trains, some
reaching mag -6.  In other years, negative-magnitude Orionids were rare
after Oct. 20.  A similarly broad outburst was observed on 1993 Oct. 18
(solar longitude 204.5 deg)." ENDS.  It seems to have been totally
cloudy in Ireland over that period - I haven't heard of any local
reports.  Just shows that any meteor shower can spring surprises on
us! Which leads nicely to -

3. BRIEF HIGH LEONID ACTIVITY for Nov 19. Dr David Asher of Armagh
Observatory calculates that we may have a brief period of much enhanced
meteor activity from the Leonids on the morning of 19 November. David
has a great track record of predicting activity from particular
filaments of material within the overall Leonid meteor stream, usually
getting the time of peak activity right to within 5-10 minutes, which is
remarkable!  The Leonids are associated with comet Tempel-Tuttle, and
are just about the fastest of all known meteor showers, as they collide
with the Earth almost head-on, with entry speeds of around 250,000 kph.
Thus they are very swift, and rarely last for more than 0.5 seconds
(although the 'trains' they leave behind can last for many seconds, or
even some minutes). The radiant, or point in the sky from which the
meteors appear to come, lies in the 'Sickle of Leo'. The normal Zenithal
Hourly Rate (ZHR: see below) for the Leonids is only about 10, but they
have also produced the greatest meteor showers - or 'storms' on record!
 The comet has a period of about 33 years, and much higher than normal
rates therefore tend to occur about every 33 years or so, when Earth
encounters the densest part of the stream of particles released by the
comet each orbit. For example, there were displays of about 250-300 per
hour in 1998, 3,700 p/h in 1999, and about 480 p/h in 2000. But the
greatest display of all in modern times was in 1966, when they produced
an astonishing rate of about 140,000 p/h for a brief period as seen from
Western USA!  We don't expect anything like that at all this year, but
David predicts that we might get a ZHR of about 120 during a brief
period at around 04.45 (+/- 10 minutes), on Nov 19.  What's 'ZHR'?
It's one of the most misunderstood terms in astronomy, for a start! Many
people see that as the quoted rate for a meteor shower, and are very
disappointed when the reality does not match the prediction! The ZHR is
defined as the number of meteors which would be seen in 1 hour, at the
shower's peak, by an experienced observer, in a very dark unobstructed
sky, and with the radiant in the zenith. And those factors are critical!
 If you observe from an average sized town you might see only 1/4 of
the rate you would see in a totally dark sky, where the limiting
magnitude is 6m.5. (Not 6m.0, 6m.5!) There are VERY few places in
Ireland where you can still achieve that! And if you observe from within
3 miles of the centre of Belfast or Dublin you might see only 1/10 of
the dark sky rate!  And if the radiant is low down, you will also see
only a fraction of the number compared with it being in the zenith: If
it's at an altitude of 30 degrees, you will only see about half of the
zenithal rate. If it's only 20 degrees up, you will only see about one
third of the zenithal rate. And at an altitude of 10 degrees, only about
1 sixth!  Finally, the peak may occur at a time when it's not visible
where you are: either the radiant is below the horizon, or it's in
daylight! For example the Perseids maximum one year might be on "August
12" - but it could be at 14.00 on August 12, which isn't much use to
us"! Since we could only observe on that date up to within about 10
hours before, or 8 hours after, we would miss the peak.  And those
factors multiply! If you are not observing at the actual time of the
peak, and from a town or city suburbs or with bright moonlight, and with
the radiant only about 20 degrees up, you might only see about 1/10 of
the quoted ZHR!  So, beware of the ZHR - don't expect to see many
meteors if you're missing out on those factors! Many so-called experts
go into print, or on the radio or TV, and quote the nominal ZHR for a
particular shower, and say that that's the number of meteors that the
average person will see. Almost certainly, they won't -nor even close
to it!  You'll see most meteors by looking at an elevation of about
45-50 degrees above the horizon, and about 40 degrees on either side of
the radiant. Leo, which will be quite high up in the SSE sky at that
time, will look a bit odd that night as the bright planet Saturn will be
closely to the West (right) of Regulus, the brightest star in Leo.
Saturn will be about a magnitude brighter than Regulus, and the radiant
will be about 8 degrees above and left of Saturn - roughly at the apex
of an isoceles triangle with Saturn & Regulus. Any meteor whose path
traced backwards appears to come from within a circle roughly the size
of the 'Sickle', centred on that point, will be a Leonid.  Fortunately
the radiant will be quite high up for us in Ireland at 04.45 - about
50-55 degrees up, so that at least won't be much of a problem. And the
Moon will be a very thin waning crescent, not yet risen, so that's OK
too. So, if it looks like being a clear night, set the alarm, get to a
nice dark location with plenty of warm clothes - and you might be
treated to a real display of celestial fireworks! BUT, as noted above,
meteor showers can spring surprises - and sometimes they fail to perform
as predicted, as well as sometimes exceeding expectations.  This will
be the last chance to see any enhanced Leonid activity for about another
30 years (according to the experts!), so take the chance if it's there.

Clear skies,

Terry Moseley

* A saint is of course just a sinner who hasn't been found out yet.


Last Revised: 2006 October 31st
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