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(meteorobs) RADIANT RISING, No. 1
RADIANT RISING
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Produced by: New Jersey Astronomical Society
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With this inaugural, electronic issue of RADIANT RISING, comes the
announcement of the association of the Meteors Section of the New Jersey
Astronomical Association (NJAA) with the North American Meteor Network
(NAMN). In addition to coordinating meteor observations, these two groups
will also jointly produce RADIANT RISING. This publication will replace the
NAMN Newsletter beginning with the next edition. It is hoped that this will
provide a higher quality electronic publication, while at the same time
preventing the duplication of effort of these two amateur groups. Comments
and suggestions are welcomed.
THE ETA AQUARIDS:
Possibly the best Southern Hemisphere shower due to the long period with
high rates, the eta-Aquarids are a more challenging target for North
America. North of 45 degrees latitude, they are practically invisible.
However, this year provides a unique opportunity to see them and contribute
to meteor science. This is due to the fact that the New Moon occurs just
before the peak, as well as the increase in skilled observers brought on by
the NAMN's efforts.
OBSERVING WINDOW 1997:
Since the eta-Aquarid radiant rises just before twilight in the east, even
though the moon is near new as May begins, until it is rising after twilight
there is still some interference. If you are going to observe, it will have
to be early in the morning as well. The official period of activity is from
April 19th until May 28th. Of more concern for North American observers is
the period where the ZHR is above 10 or so, since with the radiant at 30
degrees elevation only about half of them can be seen even under excellent
(LM = +6.5) conditions, while at an elevation of 10 degrees only a sixth can
be seen. This is from approximately April 28th until May 16th. Due to the
moon being in the east before sunrise, the effective period starts on May
2nd or 3rd. The IMO's data suggests a peak near 60/Hr on May 6th, while the
DMS list a more conservative 37/Hr...in any case the peak is broad with
nearly a week of rates higher than 20.
The best NAMN Observer location is that of Norman Mcleod in Southern FL. Not
only is the latitude below 25 degrees, but there are over 9 hours between
nautical twilight in the evening and morning. Compare this with the South
Carolina and Southern California sites with about 8.5 hrs, New Jersey with
less than 8, 7.5 for Boston, and just over 7 for Portland or the Great White
North's latitude.
This means that while in Florida you can observe until close to 6AM, in
Ottawa it's over before 4! This, and the radiant's low elevation even by
twilight, are the factors which make North American observations such a
challenge. So why should we bother?
THE ZENITHAL HOURLY RATE, AND HOW WE GET THERE:
When meteor shower rates are compared, we must be able to use observations
taken under widely varying conditions. For this reason, all observations can
be corrected, or normalized, to a standard set of circumstances. This is the
ZHR. It is defined as THE NUMBER OF SHOWER MEMBERS THAT A SINGLE OBSERVER
WOULD SEE IN ONE HOUR NET OBSERVING TIME, IF THE RADIANT WERE AT THE ZENITH,
THE LIMITING MAGNITUDE WAS +6.5, AND THE SKY WAS CLOUDLESS.
The formula is ZHR =(n/Teff)*F*c*z where
n= number of shower meteors observed; and
Teff = effective observing hours (Total time minus breaks and meteor recording
time)
These give you the observed hourly rate.
We then correct thusly:
F= correction for obscuration to Field of View
c = correction for limiting magnitude; dependent on characteristics of the
shower
z = correction for radiant elevation
In addition, for observers who have enough experience and history, a
correction can also be calculated for each observer's individual perception
coefficient.
Two factors have great importance for eta-Aquarid observations: the
variables z and c. The radiant correction is based on the geometry of how
much sky is visible where the shower's members can appear in the sky. When
the radiant is on the horizon, at least half can clearly not be seen, since
they're below the ground. Then there are theories covering how many are not
seen due to the larger amount of air the light must pass through, etc. What
is needed now is lots of good, high quality data to allow accurate
corrections for low elevation radiants. That is what the eta-Aquarids are; a
shower with reasonable rates for over two weeks, visible from a low radiant
elevation in North America. This year the moon allows us to take a long hard
look. The critical information which allows the radiant elevation to be
determined is the time and location recorded on your form.
The other information which requires careful attention is the limiting
magnitude, and the individual meteor magnitude statistics. Both of these are
used to determine the correction due to limiting magnitude. The Limiting
Magnitude polygons # 14, 5, 6, and 21 are the most likely to be useful.
Please count any of these that are visible each half hour, especially close
to twilight. For more Northern observers, area 21 will probably not come
into view before twilight, but count it if you can see it.
Stars you can use as magnitude references for estimating the meteors include
Deneb (CYG), and Altair (AQL) which are just above and below Mag +1
(remember using them for Hale-Bopp magnitude estimates not too long ago?).
a-Andromeda, on the corner of the Great Square of Pegasus is +2, Cygnus'
lower wing and head are +3 as well as the most southern of the stars in
Aquarius' arrowhead. Delphinus has three +4's, and Equuleus has three +5
stars. These last two constellations are just above the radiant's position,
and should be useful for judging fainter meteors.
RADIANT POSITION AND MOVEMENT
For finding and plotting the eta-Aquarids, the radiant is nearly at the
center of Atlas BRNO chart #6. It moves from near beta-AQR on April 20, to
eta-AQR by the peak nights near May 6th. By the end of activity in late May,
it is in the Circlet of Pisces. For plotting purposes, if you hold chart 6
upright, map 7 is to the left, map 1 above and to the left, map 3 above and
to the right, and map 9 is to the right. Those are the 4 maps that will be
underneath map 6 in my pile!
Date Radiant Position
RA (deg/Hr & M) Decl
Apr 20 323 (21h 32m) -7
Apr 25 328 (21h 52m) -5
Apr 30 332 (22h 08m) -4
May 05 337 (22h 28m) -2
May 10 341 (22h 44m) 0
May 15 346 (23h 04m) +3
May 20 350 (23h 20m) +5
May 25 355 (23h 40m) +7
Assumed radiant diameter for counting should be between 15 and 20 degrees;
if you are plotting, please use the radiant to meteor distance chart in the
IMO handbook. (Table 8-3)
Once again, we should emphasize that attention to detail is very important
to ensure that the large correction factors required are as accurate as
possible. Please carefully check your times and limiting magnitude counts,
and try and get a little practice refreshing your skill at estimating meteor
magnitudes. Try and note the time when you can first see the radiant above
the horizon.
The eta-Aquarids are swift meteors (66 kps) and should stand out against
sporadic activity, as well as from the Sagittarids, which like most ecliptic
showers have geocentric velocities near 30 kps, only half as fast.
Get plenty of sleep before heading out, and say hi to all the particles of
Halley's comet you see each night. I hope we have many clear nights across
the continent.
REVIEW OF 1996 NAMN OBSERVATIONS
The moon was not our friend last year, wiping out most of the peak. 6
observers counted 40 eta-Aquarids in 73.69 hours Teff. Bob Lunsford and
George Zay each caught 17 in their 30 hours apiece from INRISC (It never
rains in southern California), while Norm McLeod only saw 3 in nearly 8
hours, all very early, during the Lyrid peak. Mark Davis (SC), Richard
Taibi (MD), Lew Gramer (MA), and Wayne Hally (NJ) each managed just one
clear night, combining for 3 meteors in about 5 hours. With the moon more
cooperative, and a little help from mother nature, perhaps we can exceed
that this year. The highest counts, on May 4th to 6th were only near 2 per
hour, due to moon's interference, it should be better this year.
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