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
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...
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
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
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
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
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.
For radiant positions and more detailed descriptions of showers, see the IMO 2008 Meteor Shower Calendar at: http://www.imo.net/calendar/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!!
-------------------------------------
NAMN email: namn@namnmeteors.org
NAMN website: http://www.namnmeteors.org
Mark Davis, meteors@comcast.net
Goose Creek, South Carolina, USA
Coordinator, North American Meteor Network
Wayne T Hally, meteoreye@comcast.net
High Bridge, NJ
Writer, NAMN Notes
Lew Gramer, dedalus@alum.mit.edu
Homestead, Florida, USA
Coordinator, Public Outreach
Owner/Moderator, 'MeteorObs'
Kevin Kilkenny,
namnfireball@earthlink.net
Staten Island, New York, USA
Coordinator, Fireballs and Meteorites
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