The Leonid Meteors 2006
Time of maximum on November 19th
Expected meteor rates
Trail encounterThe Leonids are the debris of Comet 55P/Tempel-Tuttle. Every 33 years or so, the comet returns to the inner solar system and releases material that forms into a new dust trail. The Earth's passage right through the centre of trails is associated with the most spectacular meteor displays (studies show that, as well as how close to the centre of the trail you are, the strength of the display also depends on how far along a trail's length you are). For example, in 2001 November and again in 2002 November, the Earth passed very near the centre of the trail released at the 1866 return (i.e. 4 revolutions of the comet ago), as well as near other trails, producing meteor storms.
In 2006, on November 19th, the Earth passes very close to the centre of the trail created at the comet's 1932 return (i.e. 2 revolutions ago), as can be seen in the schematic plot by David Asher. More detail of the Leonid stream's cross section encountered by the Earth in 2006 can be found from Jeremie Vaubaillon's computer modelling. The encounter with the 1932 trail will lead to enhanced meteor activity: a nice outburst, but below storm level.
Time of maximum on November 19thThe calculated peak time of the outburst is 04:45 UT (Universal Time = Greenwich Mean Time). It will probably not last very long (i.e. meteor activity will rise and fall quite sharply) and so to see it you need to be in the right part of the world. You need it to be night-time, and you need the radiant (the point in the constellation Leo where the meteors radiate from) to be above the horizon.
In the following diagram, created by Robert H. McNaught (Australian National University), the regions shown are facing the radiant. However, the right hand half of the map is in daytime and the three bands down the middle are (right to left) civil, nautical and astronomical twilight. (The dashed lines denote moonrise and "lunar civil twilight", but in any case, the moon, being close to new, will not be a problem for this year's Leonids.)
This map is also available in PostScript format
The closer you are to the centre of the map (without being so close that it is daytime), the better, as Leo will be higher in your sky. So regions well over to the west of Europe and the west of Africa are favoured, although the outburst can also be seen from some east parts of North and South America.
The peak time 04:45 UT is for the geocentre. Approximate corrections for various locations are as follows:
E Brazil -7 mins W Africa -2 mins S Spain 0 mins UK +4 mins NE Canada +5 mins Scandinavia +6 mins
This topocentric correction is described by McNaught and Asher (1999); see paper 332 of Armagh Observatory Reprint Series. These adjustments (a few to several minutes) are not enough to affect when you should observe, but if there are accurate observations it will be scientifically interesting to assess observed times of maximum. Overall, the prediction of the peak time should be accurate to +/-10 mins and more likely +/- 5 mins. You may like to start observing an hour before the peak, or more for keen people, less for the extremely lazy (and all depending on what your local conditions allow).
Expected meteor ratesThe closest parallel is a Leonid outburst in 1969. Based on this, McNaught and Asher (1999), paper 330 of Armagh Observatory Reprint Series (note that this is a different 1999 paper from the topocentric correction paper described above) predicted the peak ZHR (Zenithal Hourly Rate = number of meteors visible to someone who has perfect observing conditions; real rates are usually somewhat smaller) for the 2006 outburst as 150. We seem to have inadvertently publicised the value 100 many times in the last few years; note that 150 was the intended value. In fact, owing to rounding errors, to uncertainty in the exact ZHR seen in 1969, and to other sources of uncertainty, it cannot really be said that 150 will be right and 100 will be wrong. In fact an empirical correction for how much the trail diffuses during each revolution (this correction means the data are fitted better) reduces the value of 150 to around 120. We can adopt 120 (or, less precisely, 100 to 150) as the best estimate. For comparison, "meteor storm level" is often conventionally regarded as ZHR above 1000.
Apart from the encounter with the 1932 Leonid trail, there is a background to the Leonid shower for more than a week, at rather low rates (e.g. ZHR of 10 or so). Serious enthusiasts can observe for at least a few nights before and after the outburst, starting when Leo rises, in the middle of the night, until morning twilight.
Meteors in the outburst are expected to be quite faint, as in 1969; this makes it even more important than usual to be well away from artificial light pollution.
Other linksArmagh Observatory main Leonid page
International Meteor Organization
Last Revised: 2006 August 22nd
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