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(meteorobs) Re: Request for comments: "Hot Falling Stars of Summer"
A lot of interesting information came from Malcolm and Pierre. I agree with
most of what they had to say. Further thoughts follow, from Malcolm first..
>Reliable in my dictionary means trusty, dependable. It was the shower
>that beginners or casual observers would select (not least because of
>holidays and warmer nights). You knew that you'd always see a good
>display and occasionally a stunning one. 50 m/h including sporadics
>and minor showers is a good display compared with a typical night.
Exactly what I was thinking. But I am also including the frequent tone of
popular writers in calling the Perseids "the most reliable" in the sense of
"best of the year." For a long time that was also true of S&T's meteor
column -- sometimes referring to the Geminids as the "wintertime equivalent
of the Perseids."
>Given dark skies at Perseid maximum, I can't recall a mediocre
>display. This was before 1989...
Agreed here fully also. My poor Perseid years have all come after 1989.
>The 1972 maximum was
>memorable with over 600 meteors in about 5.5 hours.
I do recall hearing about the very good 1972 Perseids from Europe. This
fits right in with the activity curve that has evolved from my own
observing. 1969 was a big one for me, and it shifts westward about 6 time
zones per year.
Hence, Europe was the place to be in 1972, and also in 1980 after the good
1977 shower here.
> In some ways
>the second peak made the Perseids unreliable in the sense we didn't know
>what to expect in the way of a peak ZHR.
I don't know what to say about a second peak yet. Nothing unusual has
happened away from my best years that I have been able to see.
>If you want to extend the
>definition of "reliable" I'd prefer "consistent" rather than "best".
I wrote a little piece for Meteor News in 1981 on this very point. This
extended definition made the Lyrids and Geminids "the most reliable of the
year." The following year with the mini-storm from the Lyrids changed things.
>Norman how does your perception coefficient change with shower speed?
>Are you relatively poorer at seeing fast-moving meteors compared with
>the average?
I haven't tried measuring perception vs. speed of meteors. But I do see the
historically quoted peak rate of 25 from the Orionids, still quoted at ZHR
25 today, and these being swift, I wonder whether this would really matter.
Norman earlier:
>> I would have seen about 86% of those meteors in sky LM6.5, borrowing my own
>> correction factor determined from observed sporadic meteors empirically,
>Does this allow for the brighter/smaller mean magnitudes/population indices
>of the Perseids compared with sporadics?
No, this percentage was determined by direct comparison of sporadic rates vs
sky conditions. For the years 1971 to 1977 I used sporadic rates completely
removed from the major-shower periods to eliminate any uncertainty about
shower meteors getting mixed in. For this purpose all meteors seen were
considered to be sporadics as I don't get many hours with as high as 2
members of a minor shower appearing. Using 10-day periods binned together,
then using the same local hours within those periods, I found that sporadic
rates in sky LM6.5 were 86% of the rates in LM7.0 sky, with LM6.0 sky at
61%, and LM5.5 sky at 50%. I haven't derived error bars for these. These
would represent the maximum loss of meteors I would experience at
less-than-optimum conditions, as sporadics are fainter than most of the
major showers. Orionids and Delta Aquarids average the same brightness as
sporadics.
This work represents the first attempt in the U.S. at examining how meteor
rates are affected by various sky conditions. Mark Adams shortly afterwards
used a magnitude method on his data, a completely different technique, and
derived correction factors remarkably similar to mine. Our combined
results showed that Olivier's (founder of the AMS) correction factors were
just a guess; he assumed a 10% loss in rates for each 0.5m loss in LM in
preparation of the AMS rate catalogs. We found out the loss per half
magnitude is more on the order of 15% . Mark's method also can be used on
the individual showers much more easily than my method. I haven't worked
out shower corrections yet so am using the sporadic factors as a reasonable
ballpark estimate.
>> My long-term baseline for an average Perseid shower, back to 1964, is
>> 40/hr observed at the max. The popular quoted peak rate before 1977 was
>> usually 50.
> the ZHRs quoted during the early
>1970's were between 60-70. This is in line with the 40/h observed.
>There appeared to be a trend of improving Perseid ZHRs of about 80 in
>the late '70s and early '80s, and then a fall back, until the second
>peak came along.
40/hr getting corrected up to 60-70 for ZHR represents stronger correction
factors than I use.
>In the early 1970s, the nominal Geminid ZHR was around 55.
It was already higher than that once I got to see the Geminids in dark
skies. In 1972 my top rate was 70, and in 1974 it increased to 75. These
are in LM7.0 sky. After some correspondence with S&T in 1983-84 on another
matter, I fed them updated Geminid information which they began using. They
were still using the very old 50/hr max Geminid rate. In 1979 I had one
freak Geminid rate at 103, the highest I have ever seen from any shower
excluding the 1966 Leonids. The one standout year since then was 1985 when
I peaked at 90-95/hr. All other dark years after 1979 have been very stable
with peak rates of 80-85/hour. The Geminids have no distinct peak as I see
almost the same max rate every dark year.
>> years. The 1990's have all been below my average until 1997. None of the
>> recent outbursts have occurred when I was watching.
>Must be a conspiracy. (-:
The 1993 Perseid max should have occurred here, in my well-established
4-year cycle. But it shifted earlier and came over Europe instead. Then in
1994, when I should have seen at least the beginning of the peak, it shifted
westward into my twilight and became much briefer. You be the judge !!
>You can't draw many conclusions about peak rates
>from one or two observers at one location.
For very short peaks, this would be true. I have enough years of observing
to trace out a good general activity curve for each shower based only on
what I have seen. This especially holds for the Geminids with a wide
observing window of 8 to 10 hours at good levels. The Perseid peak is a
quarter-day wide so it appears once every four years at peak levels (but not
guaranteed to perform). Using results from elsewhere, it appears the return
of Swift-Tuttle has caused the Perseid maximum to shrink to only an hour at
very high rates, drawing those good rates from the surrounding hours and
leaving poor rates to be seen away from the peak. The first of those
enhancements, in 1991, occurred in the Far East as I would have expected.
A few items from Pierre's post next :
>In past years, I have had a successful trend of seeing the Perseids at or
>near peak during clear nights. Those years were 1991, 1992, 1994, 1996
>and 1997. Nearly all produced a dramatic increase of high activity during
>the last couple hours before dawn. Only 1996 failed to do that. It
>produced consistently "moderate" rates all night and was somewhat a
>disappointment.
This could be due to a last-hour perception surge -- more investigation
would be worthwhile. Most observers excel at some particular time of night.
Pierre could be like Felix Martinez, who observed with me from 1976 to 1979
much of the time, then only occasionally since then. For a 5-hour watch,
the Felix perception would go roughly 1.1,1.2,1.3,1.4,2.0. We would start
off about the same, then he would gradually increase, then have a major
surge the final hour blowing me away.
In 1973 from the Keys Bill Gates (not from Microsoft) determined our
relative hourly perceptions for 5 of us. Mine was almost level throughout
the night -- Bill called me "definitely the best of the group for staying
awake." All the others had strong and weak hours. One guy started off
well, then slumped permanently. Gates himself would start off "in a
stupor," as he put it, only at perception 2.8 !! The third hour he peaked
at perception better than 5.0, killing us all at seeing meteors. Then he
fell back to 3.0 by the end, still a substantial slaughter.
>All observations are from areas near Ottawa, Ontario,
>Canada (-75.4 W; Lat: 45.3 N) except for 1997 which is 2 hours drive
>north-west from Toronto.
Pierre is located at the optimum latitude for Perseid rates. Using a graph
prepared by Mark Adams, Perseid precentage-of-optimum-latitude-rate vs.
other latitudes, I find that for latitude 26N the Perseid rate is still 92%
of the highest possible rate based on radiant elevation. So I don't miss
that many Perseids being this far south. The long summer nights here offset
the lower radiant elevation substantially.
>1996
>From my suburban backyard (LM of 5.8), the shower turned out to be much
>weaker with rates no better than 31/hr. Other family members came out to
>see, and were not very impressed.
That was more than I saw in sky LM7.3. My best was only 28/hour, the same
night that Joan wasn't impressed either. She saw about the same number.
Losing 1.5 magnitudes of sky costs me 50% of my meteors, so that would be
14. Adjust that to Pierre's latitude gives 14 / 0.92 = 15. He actually
saw 31, so his perception is 31/15 or 2.07X mine.
>1997
>The best one yet! Activity rose from "average" early in the evening to
>"EXCELLENT!" in the early morning (according to my vocabulary). Under
>dark limit. mag. 6.6 skies, I recorded an amazing 121 Perseids between
>7:39-8:40 UT. There were times when 5 Perseids would appear within the
>minute! My voice kept talking in the recorder as fast as it could as
>more meteors flashed overhead. Total 379 Perseids that night.
Far beyond anything I have seen. Let's correct my 68 peak rate in sky LM7.3
to the same location as Pierre. Rounding our sky difference to half a
magnitude, I would have seen 86% of 68, or 58 Perseids at LM6.6. Then
moving from my latitude to his most favorable latitude, 58 / 0.92 = 63
Perseids for my corrected rate. But he saw 121 Perseids, or 1.92X my
perception. No doubt in my mind, Pierre has twice my perception for at
least the final hour of the night. The other years he listed appear to
yield similar results. Consistency here is excellent -- observers with
erratic perception are much tougher to calibrate.
-----
One last thought. The sky correction formula derived from theory being
using outside the U.S. produces a much stiffer loss of meteors, around 30%,
per half-magnitude loss in limiting magnitude. I tried this out on last
year's Perseid rate of 68 and got a ZHR of only 51.
1.8 ^ (6.5 - 7.3) * 68 / sin 57o = 51
The better-than-standard sky is working against me in this regard.
My own correction factor gives a ZHR in sky LM6.5 of 70, a very reasonable
value; calculated from 86% of 68, then divided by sin 57o, or 68 X .86 /
.83867 . (Latitude 26N). Using the stronger correction lowers my apparent
perception into the toilet. It is actually close to normal, based on
long-term Orionids at ZHR 25, Lyrids 15, Leonids most years 15, Taurids 15.
I see those rates in reality as the radiants of all these showers pass near
my zenith. Also, a good many people I have observed with have fairly
similar rates to me : we can't all be below normal.
Norman