British Astronomical Association
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British Astronomical Association
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Meteor Section |
Meteor observers look to the autumn months as the main ‘season’ for good activity. Background sporadic rates are at their highest for the year during September-October, when the Apex of the Earth’s Way is highest in the sky. The Apex marks the direction in which the planet’s orbital motion is carrying us, and when we are facing towards this we are looking in the direction from which randomly-distributed meteoroidal debris lying in the ecliptic plane is being swept up. Sporadic rates for an attentive observer at a dark location may be as high as 12-15/hr in the early morning hours at this time of year.
Throughout September and October, the sporadic background is augmented by a trickle of activity from the Piscids, with multiple weak radiants close to the ecliptic plane. Piscid meteors are slow and seldom bright. Poorly-defined peaks are thought to occur around September 8, September 21 and October 13: the first and last of these are favoured by the absence of moonlight in 2003, but rates from the shower are unlikely to be higher than a couple of meteors per hour.
Most attention in the coming months will surely focus on the more conspicuous activity of the Orionids and Leonids.
Active October 16-30
Radiant 06h 24m Dec +15o
Produced by debris shed from Comet 1P/Halley, the Orionids have a diffuse radiant northeast of Betelgeuse, roughly midway from that star to Gamma Geminorum. The radiant is below the horizon until late evening. Watches after midnight local time are likely to be most rewarding as the radiant climbs. The table presents Orionid radiant elevations for a central-UK latitude of 53oN:
Local Time Radiant Altitude
22h
8.5o
23h 17.6o
00h
26.5o
01h 35.1o
02h 42.8o
03h 48.7o
04h 51.7o
05h 51.3o
06h 47.4o
Orionid meteors are among the swiftest, a consequence of the stream’s retrograde orbit. Meteoroids from the Orionid stream enter the atmosphere at 66 km/s. As a result, the shower has a high proportion of events leaving behind persistent ionisation trains.
Past observations indicate no particularly great abundance of bright meteors among the Orionids - most appear to be in the magnitude +1 to +4 range, though occasional brighter examples are seen.
The shower has a broad peak from about October 20-22, with sub-maxima occurring within this interval, apparently reflecting the filamentary nature of the meteor stream. The precise timing of maxima appears to vary from year to year, dependent on which individual filaments are encounter: given 1P/Halley’s numerous perihelion passages, the stream is richly populated with filaments.
Peak observed rates may approach 15-20 meteors/hr, corresponding to a sky transparency- and radiant altitude-corrected Zenithal Hourly Rate (ZHR) of about 25. Activity is noticeable from October 16 right through to the end of the month and possibly into early November. Further sub-peaks may occur in the declining phase - October 27-28, for example, has been a productive night in past years.
In 2004, moonlight conditions favour the earlier parts of the shower, including the peak period. The Moon is at First Quarter on October 20-21, at that time setting around 21h 30m local time for UK-based observers. By October 22-23, moonset will have crept back to after midnight local time, and on subsequent nights the window of dark sky is pushed ever further into the morning hours. It should be possible to obtain productive observations from October 16-17 through till about October 24-25, but the Moon will be a more or less night-log nuisance thereafter.
The Orionids are a reasonably productive visual shower, but their high velocity and relative lack of bright events renders them a more difficult photographic target. Nonetheless, observers may wish to attempt photography. A standard 50 mm or wide-angle 28 mm lens at f/2.8 is recommended, taking time exposures of 10-15 minutes on ISO 400 film, with good aiming directions being towards Auriga or Taurus.
Active Mid October to Late November
Radiants N RA 03h 44m Dec +22o
S RA 03h 44m Dec +15o
Observers carrying out watches for the Orionids should find activity supplemented by low rates from the Taurids, with radiants west of the Hyades (southern branch) and Pleiades (northern branch). Produced by debris from Comet 2P/Encke, the Taurid stream is ancient and very spread out, resulting in a log span of activity with low rates. The peak period is quite extended, through the opening week of November, though during this time observed rates are usually never much better than 5-8 meteors/hr.
Taurid meteors are notably slow (geocentric velocity 30 km/s) and the shower has acquired a reputation as a source of bright meteors. BAA Meteor Section results from the 1980s suggest that although bright and impressive Taurids can be seen, fireballs (by established definition, meteors of mag. -5 or brighter) are no more abundant than in other major showers. There is, however, a respectable proportion of sub-fireball Taurid meteors in the mag. 0 to -4 class, making the shower a sometimes rewarding target.
In 2004, the waning Moon will be retreating into the early morning sky during early November, and productive late-evening watches for the Taurids should be possible from about November 3-4 onwards. The major problem at this time of year is, of course, competing firework activity...
Active November 15-20
Radiant RA 10h 08m Dec +22o
The Leonids have been the major attraction for meteor observers in recent years, with a lot of attention surrounding the storms (nominally, intervals during which the ZHR reaches 1000) of 1999, 2001 and 2002. Overall Leonid activity has been elevated since 1994, as we encounter the richer ortho-Leonid ‘cloud’ in relative proximity to the parent comet 55P/Tempel-Tuttle. Now, six years ‘downstream’ from the comet (which was at perihelion early in 1998), the expectation is that the enhanced ‘background’ activity in the Leonids should soon dwindle to the (clino-Leonid) levels seen during, for example, the 1980s.
No storm activity is anticipated in 2004, but if we continue to encounter the ortho-Leonid cloud, the shower may produce a performance comparable to that of 1995, when observers enjoyed peak rates of around 30 Leonids/hr (corresponding to a ZHR close to 40) at the November 17-18 peak. Whatever happens, observations at this stage will be of value in helping to map out the extent of the ortho-Leonid cloud.
The Moon will be well out of the way for the 2004 Leonids. On maximum night (Nov 17-18), the 5-day old waxing crescent will have set by the time the radiant, in Leo’s ‘Sickle’ has risen. As with the Orionids, a late-night stint is required to catch the Leonids, with best viewing circumstances in the hours leading up to dawn.
Local Time Radiant Altitude
23h
6.4o
00h 14.7o
01h 23.6o
02h 32.6o
03h 41.4o
04h 49.4o
05h 55.6o
06h 58.8o
Leonid meteors are extremely swift (geocentric velocity 70 km/s), and as a result many leave lingering persistent trains. Those associated with the brightest meteors can last for several minutes, and may even be captured on film with 20-30 second exposures at f/2 on ISO 400 film. While rates may, in all likelihood, be fairly modest, photographers hoping to catch Leonids on film may wish to try exposures as for the Orionids, with cameras aimed towards Gemini/Cancer or Ursa Major.
Observations of any of these showers will, of course, be welcomed by the Meteor Section. Notes on standard visual observing methods can be found elsewhere on these pages, and in the August 2004 issue of the Journal of the BAA.
The Leonids will be followed in December by an extremely favourable return of the Geminid meteor shower, for which detailed notes will appear closer to the time.
Neil Bone
Aug 23 2004
Page last updated: 01/03/2005