(meteorobs) NAMN Notes: May 2008

Sun May 4 17:36:48 EDT 2008

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NAMN Notes: May 2008
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NAMN Notes is a monthly newsletter produced by the North American Meteor
Network and is available both via email and on the NAMN website at:
http://www.namnmeteors.org

Contents:

1. (ETA) The eta-Aquarids Greet the Dawn...
2. (ELY) This Newer Shower is Worth Looking for...
3. (ANT) Antihelion Showers in May...
4. The Nights Get Shorter...
5. Lyrids Squashed by the Moon...
6. For More Information...

1. The eta-Aquarids Greet the Dawn...

This year, the eta-Aquarids are one of the few moon free showers.
However, watching this shower takes some effort, since it is only
visible in the few hours just before morning twilight. That means
observing in the early morning hours. The radiant rises after 2 AM, and
by the time nautical twilight occurs, the radiant is not very far up in
the sky. In the southern US, it just reaches 30 degrees, an elevation
which would give hourly rates half of the ZHR (~ 70 this year?) under LM
+6.5 skies. That’s not too likely since the sun is only 12 degrees below
the horizon at the time. For the Canada/US border, at nautical twilight
the radiant is only about 12 degrees elevation so rates would be
expected to be far lower. Still, with the peak occurring at the New
Moon, it is one of the best years ever to see this shower.

The eta-Aquarids are one of two streams associated with Halley’s comet,
the other being October’s Orionids. Why do we get two showers from this
comet? The answers are:

1. Unlike many other comets/meteor streams, Halley’s Comet has a
perihelion well inside earth’s orbit. It’s perihelion is closer to the
Sun than Venus, at .591 AU (Venus’ is .781 AU).
2. Other meteor streams with perihelia inside of Earths only intersect
the Earth on the inbound or outbound leg, but the “Halleyid” stream is
aligned perfectly so it hits us both ways.

According to the IMO, the peak is expected around 18UT on the 5th, so
Monday morning would be expected to have the best rates; however other
sources suggest a later peak is possible. There is some data that
suggests a 12 year periodicity which could lead to higher rates for the
next few years, which is why the IMO lists the ZHR as 70, an increase
over the normal rate of about 60. The radiant is just below the “water
jar” asterism on the north side of Aquarius. This shower began last
month and continues until nearly the end of the month, on the other side
of the full moon. These are very swift meteors (66 km/sec) but often
appear slower due to the low elevation of the radiant. Especially keep
an eye out for long earth grazing meteors just as the radiant rises.

Radiant Positions:
May 1 -	RA 333 (22h 12m), Dec -03
May 5 Peak - RA 337 (22h 28m), Dec -01
May 15 (Before Full Moon) - RA 345 (23h 00m), Dec +03
May 23 (After Full Moon) - RA 351 (23h 26m), Dec +06
May 28 (End of Activity) - RA 355 (23h 42m), Dec +08

2. (ELY) This Newer Shower is Worth Looking for...

While you are waiting for the ETA radiant to rise, another minor shower,
the eta-Lyrids (ELY) can occupy your time. This radiant is much higher
in the sky, between Vega and the closest wing of Cygnus. Active from
possibly as early as May 3rd to as late as May 17th, the peak occurs
sometime between the 9th and 11th.

This shower was proposed before it was observed, based on the orbit of
the parent object. Comet C/1983 H1 (IRAS-Araki-Alcock) was discovered 25
years ago this week as an object that was anomalously bright in IRAS
(the InfraRed Astronomy Satellite) images on April 25th, and
independently by two amateurs on May 3rd. Jack Drummond of the Stewart
Observatory first pointed out that meteor activity was possible from
this comet. Even though it is a long period comet (964 years) the orbit
passes within about 0.006 AU (6 times the distance to the moon.)

It has been a generally weak shower, with a ZHR of only 3, but long
period showers have produced surprises in the past, so it’s worth
keeping a look out. It has also been detected in the IMO’s video
observations, so is a real shower. At 44 km/sec, these will be medium
fast meteors. The radiant rises shortly before sunset and is nearly
overhead by dawn’s early light. Since the radiant position and activity
periods are a matter of current research, plotting would be useful, as
well as watching throughout the suspected periods.

Radiant Positions with Source:
May 3, IMO Visual Start of Activity - RA 281 (18h 44m), Dec +44
May 8, IMO Visual Peak - RA 286 (19h 4m), Dec +44
May 10, IMO Video Start - RA 291 (19h 22m), Dec +43
May 11, IMO Video Peak - RA 291 (19h 25m), Dec +43
May 12, IMO Visual End - RA 290 (19h, 20m), Dec +44
May 17, IMO Video End - RA 295 (19h 42m), Dec +44

3. (ANT) Antihelion Meteors in May...

Activity from the Antihelion source picks up a bit this month. The
center of the wide radiant is located in eastern Libra at the start of
the month, passing through northern Scorpio before the Full Moon, and
ends the month in southern Ophiuchus. Remember, the radiant is 30
degrees wide along the ecliptic, and 15 degrees “tall”. This encompasses
many minor (each with nearly undetectably low rates) showers in Libra,
Scorpio, and Ophiuchus that were individually counted previously. These
are generally medium slow meteors with speeds near 30 km/sec.

Radiant Positions:
May 1 - RA 233 (15h 32m), Dec -19
May 15 (Last Morning with Moon - RA 247 (16h 28m), Dec -22
    Free Hours before Full Moon)
May 31 - RA 263 (17h 31m), Dec -23

4. The Nights Get Shorter...

As we approach the Summer Solstice, fewer hours are available for meteor
observing. True darkness (between astronomical twilights, when the sun
is 18 degrees below the horizon) decreases a lot the further north you
go; in fact at 50 degrees north latitude, May 31st is the last night
with any astronomical twilight at all. Nautical twilight (Sun 12 degrees
below the horizon) brightens the sky quite a bit, but as long as you
accurately record the limiting magnitude, such observations are useful.

      Date    Latitude   AT-AT   NT-NT
      5/1        50      4:15    6:28
      5/1        40      6:37    7:54
      5/1        30      7:41    8:48

      5/31       50      0:18    4:30
      5/31       40      5:14    6:46
      5/31       30      7:15    8:03

5. Lyrids Squashed by Moon...

The Full Moon wiped out most of the Lyrid observations this year. So
far, only 40 have been reported to the NAMN in a mere 11.33 hours of
observing time during the activity period. Total Teff for April was only
19.95 hours. The total Lyrids from previous years:

2004   127 Lyrids,  31.76 Teff in April
2005    76 Lyrids,  30.97 April Hours
2006   106 Lyrids, 127.73 April Hours
2007   384 Lyrids,  82.02 April Hours

Next year will be more moon friendly, so hopefully we’ll get back to
seeing more than 100 in a season!

Speaking of the Moon, it is New on the 5th, with moon free morning hours
continuing until the 15th or 16th, depending on your latitude. Full Moon
occurs on the 19th, and evening moon free hours resume on the 22nd or
23rd. By the end of the month we are close to June’s New Moon, so
observations can be conducted without worry depending on where the sun is.

Jupiter is the brightest planet, rising with Sagittarius in the east at
magnitude -2.4. Saturn is in Leo at magnitude +0.5, next to Regulus
glowing at +1.4. Mars sets in the evening with Gemini at magnitude +1.3.
Two 0 magnitude stars with different colors are in the sky, blue white
Vega, and yellowish Arcturus. Deneb (in Cygnus, +1.2) and Altair (in
Aquila, +0.9) are guides for first magnitude meteors. The “water jar” of
Aquarius has one  star of +3, and 3 of +4, and from east to west, the
tiny stars of Sagitta are +3.4, +3.8, and the pair on the right are +4.4.

6. For More Information...

For radiant positions and more detailed descriptions of showers, see the
IMO 2008 Meteor Shower Calendar at:  http://www.imo.net/calendar/2008.
All three showers mentioned above have maps giving the radiant positions
for 2008.

To learn more about the IRAS satellite that discovered the parent comet
of the eta-Lyrids, see:  http://irsa.ipac.caltech.edu/IRASdocs/iras.html

Feel free to contact us for questions and comments!!
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NAMN email: namn at namnmeteors.org
NAMN website: http://www.namnmeteors.org

Mark Davis, meteors at comcast.net
Goose Creek, South Carolina, USA
Coordinator, North American Meteor Network

Wayne T Hally, meteoreye at comcast.net
High Bridge NJ
Writer, NAMN Notes

Lew Gramer, dedalus at alum.mit.edu
Homestead, Florida, USA
Coordinator, Public Outreach
Owner/Moderator, 'MeteorObs'

Kevin Kilkenny, namnfireball at earthlink.net
Staten Island, New York, USA
Coordinator, Fireballs and Meteorites

Back issues of NAMN Notes can be found online at the NAMN website and in
the MeteorObs archives at:
http://www.meteorobs.org by selecting 'Browse Archive by Month'

To subscribe to the meteor email list:
Contact Lew Gramer at: dedalus at alum.mit.edu

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Here's to 'Clear Skies' for May...

May 2008 NAMN Notes
Written by Wayne T. Hally & edited by Mark Davis
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