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From: TerryMoselaol.com
Date: Sat, 6 Nov 2004 16:32:00 EST
Subject: LEONIDS LATEST

Hi all,

Latest information from the IMCCE indicates that observing parameters for the 
'early' Leonids are marginally better for Irish observers than indicated in 
my last email. That's because the radiant lies a few degrees further N & W than 
the normal Leonid radiant, and so will be a few degrees higher in the sky at 
the time of the predicted early peak. But even so, it's still pretty poor! 
From Belfast, the new radiant will be just about 2 degrees above the horizon at 
the predicted peak at 23.30, instead of a few degrees below it! Even that's 
likely to greatly reduce the number of meteors which could be seen. And for 
observers further S & W, it will be even worse, as the radiant will still be on, or 
below your horizon at that time!

The new radiant's position is predicted to be: R.A. 9h 53m, Dec +25 deg.

So a lot depends on how long any enhanced activity lasts - if it's still 
going on several hours later when the radiant is higher, then we should indeed see 
something. 

(There's also a prediction of some enhanced activity on Nov 19 at 21.49 UT, 
but as the radiant will be well below our horizon at that time, we won't see 
it.)

However, there seems to be some confusion among some observers about meteor 
showers & streams, meteor rates, & meteor observing generally!

Perhaps this info may help:

MAIN SHOWER, vs. FILAMENTS:
These specific predictions by David Asher et al (mainly J. Vaubaillon, F. 
Colas, P. Jenniskens, E. Lyttinen & M Nissinen) relate to particles contained in 
finite 'filaments' or narrow 'streams' emitted from the comet at certain 
perihelion passages, which modern techniques allow to be accurately modelled for 
gravitational perturbations, so that Asher et al can say if, & when, the Earth 
may pass through one of the filaments.
   But these 'extra' bursts of activity are superimposed on the general, 
normal, shower activity, which continues on an annual basis, even though rates do 
decline away from the dates of the parent comet's perihelion. This 'normal' 
broader Leonid peak is predicted for approx 09.00 on 17 Nov, at a Solar 
Longitude of 235.3. Since that's after sunrise for us, we'll see highest rates of the 
normal maximum just before dawn that morning, i.e. about 05.00 - 06.40 on 17 
November; maybe 15-40 per hour under good conditions

ZHRs and OBSERVED RATES:
   The ZHR is the Zenithal Hourly Rate, which is the rate which would be seen 
by an experienced observer, in a totally clear, very dark (L.M. = 6m.5) sky, 
and with the radiant in the zenith!  Even if we had the sky conditions, we can 
never have the latter condition in Ireland for the Leonids, as the radiant 
never gets higher than about 55 degrees, just before dawn (about 06.40). 
However, if the radiant is above about 50 degrees, the number of meteors seen will 
not be significantly reduced from the ZHR, all other conditions being equal.
   BUT, at low radiant altitudes, the observed rates are greatly reduced! The 
correction factor for radiant altitude is given by 1/sin a, where 'a' is 
radiant altitude in degrees. So for the radiant altitude of 2 deg at 23.30, the 
reduction factor is 28.6! (i.e. roughly 1/30th of the ZHR!) Even at 5 deg 
altitude, the reduction is a factor of 11.5, and at 10 deg it's 5.8. So with a low 
radiant, we just won't see many Leonids!
  You can see that at the very low altitudes of the radiant during the time 
in question, the observed rates will be drastically reduced. And that's not 
allowing for the effect of low altitude haze, which will reduce observed rates 
even further.

   Also, the sky Limiting Magnitude has a significant effect, which will vary 
slightly from one shower to another, depending on the 'Population Index', or 
the relative proportions of bright vs fainter metors in the shower. Obviously 
if a shower contains mainly faint meteors, observed rates are going to be more 
adversely affected if one is observing in a bright sky.  The correction 
factor for sky L.M. is: 'r' to the power of (6.5 - L.M.) [I can't do supercripts in 
email....] So if your LM during an observing session is say 5m.6, the 
correction factor is 'r' to the power of (6.5 - 5.6) = (0.9). We don't know the 'r' 
value for this new stream, but if it's say 2.9 (like the Lyrids) the correction 
factor would be 2.9 to the power of 0.9, = 2.6, so you would see only 1/2.6 
of the number of meteors compared with a sky with LM of 6.5. 

Adding those two factors together means that if you are observing in an 
average sky, with the radiant low down, you'll see only a tiny fraction of the ZHR!

SHOWER DATE LIMITS:
If you look up any of the reference books, e.g. Neil Bone's book, or the 
Handbook of the British Astronomical Association, or WGN (publication of the IMO), 
you'll see dates quoted for the start & end of a shower. For the Leonids, the 
HBAA gives "Nov 15 - 20". For the Perseids it gives "Jul 23 - Aug 20". BUT, 
these are the extreme limits of shower activity, when the shower activity is 
just noticeable above the background of sporadic meteors. The BAA defines the 
limit as when the shower activity reaches 1/4 of the sporadic background rate. 
So if the sporadic rate is 8 per hour, then when the shower rate reaches 2 per 
hour it has started. Another way of looking at it it is that at that time, on 
average, 1 in every 4 meteors will be a member of that shower. 
   So DON'T expect to see significant numbers of meteors near the beginning 
or end of a shower!

PROSPECTS FOR LEONIDS, FOR IRELAND, 8-9 NOV:
To sum up: I'll stake my reputation, such as it is, that observers in Ireland 
will see very few Leonid meteors before 00.30 on the night of 8/9 November!  
If the enhanced activity continues on to say 01.30 or 02.00, then we'll see a 
few more, but not that many. I would be delighted to be proved wrong, but I 
don't think I will be. However, observers much further east than Ireland, where 
the radiant will be approaching the zenith at about 23.30 UT, may well see 
lots of faint Leonids if the sky is clear.

But don't be too disappointed if we don't see much - there's a far better 
shower coming in December: the Geminids, on Dec 12 & 13!

Remember, if you want to be a meteor observer, rather than just a 'watcher', 
you need to note the following: the exact time of start & finish of each 
observing session (usually broken into spells of about 1 hour or so); the duration 
of any breaks from observing during that session; the sky limiting magnitude 
to the nearest 0.1 magnitude (obtained from visibility of stars in various 
selected parts of the sky); the percentage of sky obscured by buildings, trees 
etc; the average percentage of sky obscured by cloud, if any, during each period; 
and your observing location & altitude, if much above sea level. 
  To convert your observations to a local personal ZHR, you then apply 
corrections for: the actual duration of each watch / 60 mts, the mean radiant 
altitude from your site during each watch, any partial sky obscuration, and the sky 
limiting magnitude compared with the ideal of 6m.5 via the correction factor 
'r', which varies for each shower.

But if you don't want to do all that, just watch & enjoy!" But do send in 
your reports, giving at least your observing times & an estimate of the sky 
conditions, even if you don't see anything: that in itself is useful information.

Clear Skies,

Terry Moseley

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