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(meteorobs) admin fwd: RE: ZHR's Nov. 17/18 from Safford (AZ)
note: "Tom Van Flandern" <metares@mindspring.com> is not subscribed to
meteorobs, please include his address in a reply.
----------------------------------------
Roger,
It is not unusual for meteor counts to vary greatly from ZHR
rates, and you do not say what corrections (if any) you applied. ZHR
represents what could be seen under theoretically ideal viewing
conditions. That depends on radiant altitude, sky transparency,
moonlight, artificial lighting, horizon obstructions, how many eyes are
looking (one observer cannot see the whole sky at once with any
effectiveness), visual acuity of observers, and a variety of other
lesser factors. Appreciating that you mentioned a limiting magnitude of
6.5 (continuously?), I wonder if the following rule of thumb applies. On
a truly clear, dark night, a visual observer should be able to see
roughly 2000 stars. Count stars in an area of sky chosen to represent
some known fraction of the whole sky to estimate the total number
visible. If you can see, say, 500 stars in the sky, you see 1/4 of what
is actually there, That applies to the meteors also, because most
meteors, like most stars, are faint. So correct your observed meteor
counts by allowing for that factor, and for the fraction of the sky you
are covering (including losses for horizon obstructions). If you were in
the east below about 40 degrees latitude, the correction for radiant
altitude would be relatively minor for you.
Although I have seen few reports from the east, I have several
from the west, all roughly consistent with 1200-2200 per hour when
properly corrected for the above factors. Only the raw, uncorrected
counts were sometimes as low as your figure. And those were observers
missing roughly half the meteors that went down instead of up from the
low radiant. Best wishes. -|Tom|-
-----Original Message-----
From: Roger Venable [mailto:rjvmd@knologydot net]
Sent: Tuesday, November 20, 2001 8:07 AM
To: dms-mail@yahoogroups.com; Carl Johannink
Cc: imo-news@yahoogroups.com; meteorobs@atmob.org; Marco Langbroek;
esko.lyytinen@minedu.fi; tvf@mindspring.com; rmn@aaocbn.aaodot gov.au;
dja@star.arm.acdot uk
Subject: Re: [IMO-News] ZHR's Nov. 17/18 from Safford (AZ)
Hi Carl.
You wrote:
>max. ZHR on Nov. 17/18 for the DMS-observers at Safford (AZ) was 2200
at
>10:35 UT (+/- 5 min.),
>based on data from Marc de Lignie and Marco Langbroek.
That's wonderful! But at the same time (1035 UT) for me on the east
coast
of the US, just before morning twilight (LM 6.45), I had a ZHR of about
600.
I had this same ZHR repeatedly for observing periods of a few minutes
duration around that time. That's quite a discrepancy.
-- Roger Venable
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