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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