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. July’s Southern Showers...
2. The Perseids Perk Up...
3. A Multitude of Minor Showers...
4. Magnitude References...
5. For More Information...
As meteor rates begin to pick up in the late summer, a number of showers become active in the area surrounding the Antihelion (ANT) radiant. The alpha-Capricornids (CAP) overlap with the Antihelions on the western edge, but stand out due to their slow velocity. South of the ANT, the Southern delta-Aquarids (SDA) and Piscis Austrinids (PAU) also peak toward the end of the month. All of these showers provide low rates for northern hemisphere observers, due to their low elevation at this hazy time of the year.
The highest ZHR (~20) is owned by the SDAs - the NAMN has recorded about 270 per year recently. The CAP’s ZHR is only 3 but since it’s radiant is about 6 degrees higher, comparable to the Antihelions, just over a hundred of each (ANT and CAP) are seen each July. Finally the PAU radiant, much lower than the others (nearly 20 degrees below the CAP radiant) are rare events in North America, averaging only 8 meteors per year.
With all these radiants packed so close together, this is a good time to break out an alignment cord to help separate the shower members. For those not familiar with the technique, all you need is a string or cord long enough to be held between your outstretched arms. When you see a meteor, you then place the cord over the path with one end held over the intersection of all these radiants. Once you get the hang of it, this allows you to clearly differentiate meteors from shower radiants only a few degrees apart.
It’s good practice as well for later in the year when the Northern and Southern Taurids are close to each other, as well as in December when there are several showers near the Geminids. Once you practice it a bit, it only takes a second or two to align the cord and trace back to the southern shower radiants. The benefit to meteor data is that instead of just counting the Aquarids as a whole, accurate data is collected for each of the individual showers.
Another tool to tell them apart is the speed. The SDAs are the fastest at 41 km/sec, while PAUs are 35, the Antihelions about 30, and the CAP meteors slowest at 23 km/sec. In the case where the alignment cord points at two or three radiants, if you’ve seen enough of them, you should be able to tell which shower the meteor came from based on the angular velocity.
Of course, in the midst of these showers, the ANTihelion radiant continues on it’s eastward march at about a degree per day. In past years this included the Northern delta-Aquarids and Northern and Southern iota-Aquarids. All these showers had rates at or below the visual detection threshold and are within the ANT radiant, which is 15 degrees from north to south, and 30 degrees wide from east to west along the ecliptic. They should be reported to the IMO and NAMN as ANT meteors.
For all the showers, with the full moon on the 18th, I give positions on the 15th and 22nd when there is more than an hour of dark sky time: in the morning on the 15th, in the evening for the 22nd. Some locations may be able to get a full dark hour one day closer to the full moon. For days in between those listed you can interpolate, or contact me for an Excel spreadsheet with daily positions for all showers.
Antihelion (ANT) 30 km/sec Date Position RA (degrees, then Hr:min), Declination July 1: RA 292 (19:28), Dec -21 July 8: RA 298 (19:54), Dec -19 July 15: RA 305 (20:20), Dec -18 July 22: RA 312 (20:48), Dec -16 July 31: RA 320 (21:20), Dec -14 Alpha Capricornids (CAP) 23 km/sec Date Position RA (degrees, then Hr:min), Declination July 3: RA 283 (18:54), Dec -16 IMO Start of activity July 8: RA 287 (19:06), Dec -16 July 15: RA 292 (19:29), Dec -14 July 22: RA 299 (19:54), Dec -12 July 30: RA 305 (20:20), Dec -10 IMO Peak Southern delta Aquarids (SDA) 41 km/sec Date Position RA (degrees, then Hr:min), Declination July 12: RA 327 (21:46), Dec -19 IMO Start of activity July 22: RA 335 (22:18), Dec -18 July 31: RA 341 (22:43), Dec -16 Peak July 28-31 Piscis Austrinids (PAU) 35 km/sec Date Position RA (degrees, then Hr:min), Declination July 15: RA 330 (22:00), Dec -34 IMO Start of activity July 22: RA 336 (22:22), Dec -32 July 28: RA 341 (22:44), Dec -30 Peak
There is one other item of note in this area. Sirko Molau’s IMO video data summary shows another short lived radiant from July 29th to August 3rd. It may be related to the CAP stream, since the meteors have similar speed (a bit slower) and is nearby. The radiant is located at RA 298 (19:50) and Dec -04, in southern Aquila. If you see any of these, for this year at least, report them as SAQ.
This most popular shower for the public and casual observer begins in mid July. While this year the peak in August will be squeezed by the Full Moon, as the rates slowly rise during late July we have an opportunity to collect some good data. Some care must be taken to use the correct radiant for any date you observe though. During July the radiant is moving more than a degree per day through Cassiopeia into Perseus. IMO video data suggests a start date as early as July 12th, so I will use that as my start date in the list below.
The Perseids are of course material from the comet 109P/Swift Tuttle, and we’ll discuss them more next month as we approach the August 12th peak. The important thing for July is that these meteoroids hit us much closer to head-on than any of the southern showers so traverse the sky much faster. This also leads to brighter meteors in general.
There are a few days from that start date before the Full Moon, and once skies darken again in the third week of July there should be more than one or two Perseids per hour. By the end of the month, they should become the most active source of meteors in the sky.
Perseids (PER) 59 km/sec Date Position RA (degrees, then Hr:min), Declination July 12: RA 000 (0:00), Dec +51 Start of activity? In Cassiopeia July 22: RA 015 (1:02), Dec +52 July 27: RA 025 (1:39), Dec +53 Cas-Per Border July 31: RA 030 (2:01), Dec +54 Cas-Per Border
Many minor showers are allegedly active in July. Here I will discuss a few. Space limitations don’t give me the chance to cover them all here, but I’ll bring up the ones that have been reported to the NAMN, that used to be on the IMO’s Working List, and some that may have a chance for some visual activity in the average year.
The first is the alpha Cygnids (ACG/ACY). I’m not sure what to make of this shower. It has been the largest minor shower source reported to the NAMN in the last 4 years, with 60 meteors. However, it’s hard to explain how this shower, as listed, could possibly exist. The ACG shower used to be on the IMO Working List, with RA of 315 (23:00) and Declination of +48, very near Deneb at the tail of Cygnus. Now come some of the problems. The activity period is shown from July 1st through September 30th from this same radiant!
As far as I know, it is physically impossible for a radiant to show no motion in the sky during 1/4 of the earth’s annual orbit. You would expect a shift in RA close to 90 degrees during that time. It is not in Gary Kronk’s Meteor Shower list, and does not show up on the expansive IAU list either. From what I have been able to determine, the original source is from British Astronomical Association data prior to 1995. Still, without an explanation for the impossibly fixed radiant for 3 months, I am very skeptical. Hopefully, I can obtain some original data from the BAA that shows this “1 or 2 an hour” rate for the late summer and early autumn. The peak is supposed to be July 15th.
So, for this year, I suggest that anyone reporting these meteors make an extra effort to determine how close the projected path of any ACGs comes to Deneb. If you can translate that information into RA and Dec, even better; if you need help, feel free to contact me. A plot or photo of the meteor would help determine the validity of the shower. Since the radiant supposedly doesn’t move, I won’t list dates and radiant. Just use the radiant listed above. I can find no speed listed for these meteors either, which makes me suspect that these are sporadic meteors assigned to a shower that might not exist. Since that’s the case any angular velocity estimates that can be supplied would be very useful. Perhaps we can help determine whether it’s real or not!
Nearby (but not the ACG’s) is the omicron Cygnids (OCY), as long as our eyes are looking that way. This shower was never on the IMO list, but does show up on the IAU database as shower #182. The reported ZHR is 2.5, so might be visible. Meteor speed is 37 km/sec, medium fast. The peak is July 19th, practically on the full moon night, so it isn’t a good year. The short activity period is from the 17th to 22nd. Radiant location is 304 (20:16) +48, only about 10 degrees from Deneb, trailing behind the tail of Cygnus.
Next up is the July Pegasids (JPE). This shower was on the old IMO Working List, and is IAU shower #175. The listed ZHR is about 3 per hour, though the IMO reports that activity has been low to non-existent for the last 2 decades at least, thus it has been dropped from the current working list. These are exceptionally fast meteors (70 km/sec) similar to Leonids. The NAMN has recorded about 4 per year recently. The activity period is from July 6th to 14th, so will all be before the full moon this year. The peak night’s radiant (July 9) is RA 340 (22:39), Dec +15. It’s not far from that position on the other nights. This is on the edge of the Great Square of Pegasus, closest to the Circlet of Pisces.
There are also 2 showers in Phoenix, but aren’t really targets for north American observations, and again, due to space limitations, I won’t go into detail. Contact me if you desire more information on the PHE and NPH showers.
The brightest object (other than the Sun and Moon) for most of the month is Jupiter, glowing at magnitude -2.7 in Sagittarius. It’s bright enough to be a distraction, but what a lovely one, especially if you use your scope in morning twilight.
Vega, almost directly overhead is magnitude 0 (zero).
Altair in Aquila and Antares, the heart of Scorpio, are both just a touch brighter than 1st mag, while Formalhaut, near the PAU radiant is mag +1.2, just a bit dimmer.
The corners of Pegasus’ Great Square are +2.0, +2.5, +2.5, and +2.8, so cover the range from +2 to +3.
The small diamond shape of Delphinus has stars of +3.6, +3.8, +4.3, and +4.4 so give a good feel for a 4th magnitude meteor.
The Circlet of Pisces (5 stars) below Pegasus are +3.7, +4.1, +4.3, +4.5, and +4.9.
Equuleus, to the left of Delphinus consists of 4 stars of magnitude +3.9, +4.5, +4.6, and +5.1.
For radiant positions and more detailed descriptions of showers, see the IMO 2008 Meteor Shower Calendar at: http://www.imo.net/calendar/2008
For those who wish to look for other minor showers not listed here, or want daily radiant positions for all the showers, drop me an e-mail at my meteoreye address below, and I’ll send you my current Excel spreadsheet.
Feel free to contact us for questions and comments!!
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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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