When we think of the more famous meteor showers, we often think of events such as the August 'Perseids' or the December, 'Geminids'.  However, this year, the annual return of the 'Leonid' Meteor shower affords us the opportunity to view a meteor shower which looks set to rival even its more famous cousins.

The 'Leonid' meteor shower derives its name from the constellation of Leo, from which on the night of November 17th, a steady stream of meteors or 'shooting stars' appear to emanate.  The reason for this regular and predictable occurrence is the fact that at the same time each year, the Earth encounters a stream of debris left behind by the Comet 55/P Temple-Tuttle.  Temple-Tuttle returns to the inner solar system every 33 years, re-seeding the inner solar system with fresh supplies of 'meteoroids'.  The last return of this comet was in 1995 and the following year saw significantly heightened activity from this shower.  The diagram above shows the position of the radiant which is over 50° above the south-eastern horizon by 4am.  It is worth noting that the radiant of this shower doesn't rise at all until after midnight so there is little to be seen before this.

The main enhanced activity has now passed and as such this year's regular event should allow us to see no more than a few meteors per hour from dark locations.  However work by David Asher (Armagh Observatory) and Robert McNaught (Australian National Observatory) suggests that we are likely to encounter an intense filament of meteoroids on the morning of November 19th at 4:45am.  This filament was created when the parent comet returned in 1932.  If Asher and McNaught are correct then it is possible to see up to 150 meteors per hour (ZHR, see jargon buster below) for a very short period around this time.  For further details of this event see Armagh Observatories, 2006 Leonid page.  It is expected that many of the meteors from this localised outburst may be fainter than normal leonids.  That in itself, is an observation worth making.

The diagram opposite is centered on the location where the Earth intercepts the 1932 filament.  Basically the closer an observer is to the centre of activity the more meteors should be seen.  The line going from the bottom left to top right is the night/day terminator.  Therefore favoured locations are to the left of this line.  Thus western Europe and west Africa are particularly favoured. Reproduced with the kind permission of David Asher.

 

Jargon Buster - Zenithal Hourly Rate (ZHR)

The Zenithal Hourly Rate is a figure which describes the intensity of a meteor shower.  The figure denotes the theoretical maximum an observer with perfect conditions should see.  A ZHR assumes that the observer has perfectly clear skies (with a limiting magnitude of +6.5), a 360° unobstructed view of the sky and that the radiant of the meteor shower is directly overhead (at the zenith).  In practice this is virtually impossible.  Any observer can expect to see significantly less meteors per-hour than the ZHR suggests.

To best observe any meteor shower you first need to find a location as far from sources of light pollution as possible.  Although some meteors will be seen from urban locations, the numbers will be significantly below those seen from very dark skies.  Allow time for your eye's to get used to the dark.   Once properly 'Dark  adapted' you will then see many more meteors than you otherwise would.  The author has spoken to observers on past occasions, who went outside for 5 minutes in a light polluted environment and saw nothing.  To best observe such a spectacle you need to make an investment in time and effort.  Be sure to dress warmly (warm head and footwear are most essential) and don't forget to bring along a thermos of your favourite hot beverage.