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(meteorobs) Leonids: 1966 rates revised down!



Marco correctly points out that the 1966 figure of 150,000 Leonids/hr is
not a ZHR, and I should have known better than to perpetuate this error.

Milon's "head sweep" technique MAY have been for more than 1.0 seconds,
but MAY have been less.  I feel you have to go with what they said.  It
wouldn't be possible to recreate this in tests without a substantial
injection of adrenalin and the IMO ethics committee may have some comments
on that.  However, even if the eye was open for exactly 1.0 seconds, the
observed number of Leonids (which is still largely a guestimate) would
have to be corrected for the fact that the meteors have a finite duration
and that the true rate per second would be less than the observed number.
But yes, correcting for the radiant altitude (which increases the rates)
and for a limiting mag of say +7.0 (which decreases the rates) one ends up
with a ZHR of around 100,000 (using a mag distribution index of 2.9 and
radiant elevation of 65 deg).  Depending on the effect of meteor duration
on the number seen in 1 second, one may end up with a ZHR of around, say
70,000.

Whereas the Milon "head sweep" is certainly not IMO best practice, I think
it is being dismissed too readily.  Jenniskens rightly pointed out the
discrepancy in the rates profile at the point where the normal counting
stopped and the "head sweep" started.  This change, due to the inability
to continuously count all the meteors, may have resulted in too low a
count before the change (meteors not being recorded) and perhaps too
high afterwards.  But we do have the photographs as evidence of the rates
of bright meteors.

With the peak rates lasting for under 15 mins, not many photographs were
taken during this period.  If we do the sums for say one photographed
Leonid every 2 seconds (assuming 1/sec is a statistical aberration),
with a photographic limiting mag of +3.0, a mag distribution index of 1.5
(and radiant elev fixed at 65 deg) one gets a "ZHR" of over 9,000 in the
camera field alone.  I would assume that the rates as seen by a visual
observer would be at least 3 times that of the camera field.  Thus even
with what I presume to be clearly underestimated numbers, the ZHR as
implied from the photographs is over 25,000.

Whilst I'm willing to accept that the true ZHR was closer to 70,000 in
1966 this is only about a factor of two different from the 150,000 (or
144,000 to be pedantic about the naive conversion from 40/sec to /hr)
whereas it is more than a factor of 4 different from the estimate of
15,000.

Going back to my initial point, I feel a ZHR of 15,000 for 1966 cannot
be sustained on the basis of the photographic evidence, but it would be
interesting to have someone do a formal analysis of these photos.

Cheers, Rob

Robert H. McNaught
(rmn@aaocbn.aaodot gov.au)

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