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(meteorobs) Re: NM South Delta Aquarid ZHR



To answer Jure A.'s question,

>Norman, in years 1976 the top hourly count is 24 and in 1981 this is 22,
what were the
>ZHR's at the time?

I have to add the rest of the data. This was be a ZHR based on LM6.5.
Location was at Bahia Honda in the Florida Keys, latitude 25N, with sky LM
at 7.5.  To correct down to LM6.5 I am using a 16% loss of meteors per
half-magnitude as determined from my  sporadic rates in various skies.  So
losing a magnitude of sky condition costs 32% of the South Delta Aquarid
meteors which have the same average magnitude as sporadics.  The
multiplicative factor for limiting magnitude is thus 1.00 - 0.32 =  0.68,
the reciprocal of which is  1 / 0.68 = 1.47.

The formula to use is,

ZHR = observed meteors / (hours X LMcorrection X sin R), 

where R = radiant elevation.  From latitude 25N and radiant declination
-17, the South Delta elevation is 48 degrees ; not bad for the southernmost
major shower.  The highest rates did come when the radiant was culminating.
The sine of 48 degrees is 0.743, and the rate is based on exactly one hour.
Thus the highest count of 24 becomes,

ZHR = 24 (  1 X 1.47 X 0.743) = 22

Similarly, the other count of 22 corrects to ZHR 20.

I have generally felt that ZHR 20 is a reasonable  value for this shower,
and it is confirmed by the above calculations.  In 1981 half of the highest
tally came within just ten minutes ; we were treated to a
near-Geminid-strength run of South Deltas in a black Keys sky but not one of
them was brighter than 4th magnitude.  Lew was with me that year.


There was one error in the rate table ; the hours for the following night
are corrected to:

1973  July 25/26, hours 11-4 ; 7,3,4,2,3.


Kim's question,
...do the SDA's have a dependable rate year to year?
seems reasonably answerable in the affirmative.  I have much less data in
the past 15 years from July due to consistently poor summertime weather, and
a handful of nights gave me the impression of much lower SDA rates than in
the past. 

 The present year is mostly clear, naturally with a full moon.  In 1961 I
watched late July with a full moon in Miami, seeing only a handful of
brighter Aquarids. That was my first full summer of observing, fresh out of
ninth grade with plenty of clear nights during a drought and plenty of
observing time in a simpler world.  I just realized that was two complete
19-year cycles of moon phases in the past.  Every 19 years the phases and
dates almost exactly coincide.

Aquarid fireballs are indeed rare.  I missed a yellow  -6m South Delta in
1984 ; we were just quitting when someone yelled, at the same instant I saw
a yellow flash on the ground, then looked up and saw a 10-second train.  He
agreed with the -6m estimate.

Kim also points out the difficulty of classifying July meteors.  This is
indeed the roughest, most complex period of the year with so many radiants
active.  It took me about 5 years to get comfortable with it.  Speed alone
becomes meaningless for the shortest meteors. I take duration into account
as well to get some idea as to how fast a meteor is going.  Last week I saw
a meteor go only 1/6 degree in 0.3 second, so that is a fast one.
Converting that to degrees/second gives 5/9 of a degree per second which
would be very slow.  This meteor was obviously not slow.  It looked
distinctly different from a point satellite flash also.

Norman

Norman W. McLeod III
Asst Visual Program Coordinator
American Meteor Society

Fort Myers, Florida
nmcleod@peganet.com

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