|ii) How many meteors will we really see, and when?
Predictions vary, depending on the researcher, the assumptions and methods
used in the analysis, and the data analyzed.
Every 33.2 years, the parent comet passes around the sun in its orbit, and
sheds debris. This debris takes on a slightly different orbit from the parent
comet - and the debris shed in each subsequent comet return is laid down in a
slightly different orbit. Over the years, we end up with a number of filaments
or trails of debris, each in slightly offset orbits. When the earth passes
through this area in space, it passes through a number of different trails at
The researchers call each trail by the number of revolutions around its orbit
that the dust has made since being shed. Some trails will produce more meteors
than others. Some will have less debris in them and be almost insignificant -
and the researchers may not quote meteor rates expected from those. The trails
that are quoted in predictions this year are the 4-rev trail (debris from 1866),
the 5-rev trail (debris from 1833), the 6- rev trail (debris from 1799) and the
7-rev trail (debris from 1767). The trails expected to produce the most meteors
are the 4-rev trail and the 7-rev trail.
The meteor rates per hour are quoted as a Zenithal Hourly Rate (ZHR). This is
the number of meteors, on average, that an observer would expect to see if they
were out under a dark country sky, and if the radiant, the area in the sky where
the meteors seem to come from, is directly overhead. Keep in mind that the peak
of the Leonids falls on full moon this year - so the fainter meteors will not be
as visible and the actual rates seen by observers will be lower than the quoted
The time is quoted in Universal Time (UT) which is also the time at
Greenwich, England. For observers in North America on Eastern Standard Time
(EST), subtract 5 hours. For other time zones, adjust accordingly.
Note: Before we get into the storm predictions, note that there will still be
regular Leonid activity occurring. The time of the maximum for the regular
activity is listed by the IMO in their 2002 Meteor Shower Calendar as November
17th at 20h UT, ie. for observers on Eastern Standard Time (subtracting 5 hours)
means 15h EST, which (subtracting 12) means 3 p.m. in the afternoon on November
17th. According to the IMO, "the November 17 timing favours places from
Asia and Russia eastwards to the Far East and Australia."
The available storm predictions that most observers are aware of, as at the
time of writing in late October, are the following. The storm components will
occur a day and a bit later than the regular activity peak.
Note: When the duration of the trail peak is given, it is referring to 'full
width half maximum' (FWHM), the period of time over which the rates are
predicted to be at least half the maximum number of hourly meteors predicted.
Hence observers should make sure to start watching ahead of time, and keep
watching after the predicted time!
The team of Esko Lyytinen, Tom Van Flandern and Markku Nissenen predict (in
- Nov. 19 04.03 UT (11.03 pm EST on Nov. 18) 7 rev trail 3500/hr
19 06.36 UT (01.36 am EST on Nov. 19) 5 rev trail 160/hr
- Nov. 19 10.40 UT (05.40
am EST on Nov. 19) 4 rev trail 2600/hr
This team also mentions that the "7 rev trail meteors are brighter, so
they are more easily seen in the sky brightened by the Moon" and that there
may be a "possible weak 7 rev trail sub-peak" slightly before the main
7 rev trail peak. The 7 rev trail peak is predicted to last about 106 minutes,
and the 4 rev trail peak about 122 minutes. More details are available at
Dr. Peter Jenniskens predicts (in ZHR rates):
- Nov. 19 03.48 UT (10.48 pm EST on Nov. 18) 7 rev trail 5900/hr
- Nov. 19 04.50 UT (11.50 pm EST on Nov. 18) 6 rev trail 51/hr
- Nov. 19 05.59 UT (00.59 am EST on Nov. 19) 5 rev trail 28/hr
- Nov. 19 10.23 UT (05.23 am EST on Nov. 19) 4 rev trail 5400/hr
According to Jenniskens, the 7-rev trail peak will last about .64 hours
(about 38 minutes), the 6-rev trail peak about 4.1 hours, the 5-rev trail peak
about 4.8 hours, and the 4-rev trail peak about .60 hours (about 36 minutes).
More details are available at http://leonid.arc.nasa.gov/1998.html and
predictions covering many cities around the world are posted at http://science.nasa.gov/headlines/y2002/09oct_leonidsforecast.htm?
The team of Jeremie Vaubaillon and Francois Colas predict (in ZHR rates):
- Nov. 19 04.04 UT (11.04 pm EST on Nov. 18) 7 rev trail
3400/hr with a range
- Nov. 19 10.47 UT (05.47 am EST on Nov. 19) 4 rev trail
3000/hr with a range
According to Vaubaillon and Colas, the 7-rev peak will last about 2 hours,
and the 4-rev peak about 3 hours, with a possible secondary peak. A basic
description of how this was computed is found on their (updated) website at
The team of Robert McNaught and David Asher predict (in ZHR rates), according
to the article published in WGN 30-5 of October 2002:
- Nov. 19 03.56 UT (10.56 pm EST on Nov. 18) 7 rev trail
1000/hr with a range
- Nov. 19 10.34 UT (05.34 am EST on Nov. 19) 4 rev trail
6000/hr with a range
According to McNaught and Asher, the 7-rev trail peak will last about 130
minutes (with a possible range of 105-150 minutes), and the 4-rev about 71
minutes. In their article, they mention that "the 4-rev dust trail... has
had no close encounters with the Earth since it was formed... The situation is
very different for the 7-rev trail... Numerous disruptions are caused by the
Earth's passage close to the trail." They also comment on the brightness of
the meteors that will be seen: "Despite the probably higher ZHR of the
4-rev trail in 2002, the lower <difference in semi-major axis between the
ejected particle and the comet at the time of ejection> of the 7-rev trail...
will result in a higher proportion of bright meteors. This will have a marked
bearing on observed meteors for lower limiting magnitudes as would be expected
in full moonlight."
McNaught and Asher, in their conclusion to the article in WGN, comment:
"Some uncertainty in the peak ZHR exists for both these trails that
could increase the predictions by up to a factor of three. For the 7-rev trail
over European longitudes the uncertainty results from the high ZHR from the same
trail in 2001. Overall, it does not appear warranted to assume the observed
activity of the non-linear encounter in 2001 should automatically imply higher
than nominal rates in 2002, but without very extensive calculations we cannot
deny this possibility. The 4-rev trail over N. American longitudes falls in
roughly the same ZHR parameter space as the 1833 and 1966 Leonid storms. Given
that both these storms seem rather underpredicted by our ZHR model, and bearing
in mind that these are the only two linear encounters that are so badly
predicted, it seems reasonable that the 4-rev encounter in 2002 could be double
the nominal ZHR prediction."
So, how many meteors will we really see? It will be up to observers all
around the globe to gather observations to find out! According to the
researchers, in spite of all their efforts, we could still be in for some
surprises. The International Meteor Organization, in their 2002 Meteor Shower
Calendar, emphasizes that "other unexpected peaks are not excluded, so all
observers should be alert right over the probable maximum dates, from November
As Jeremie Vaubaillon phrased it so well on the MeteorObs email list on
"I think the important thing is to consider that, on next Nov. 19th,
there will be a very strong Leonid shower, and perhaps the last you can see in
your whole life (so strong I mean). So just remember to observe!"
Links to Leonids Web Sites
Peak Online Estimator: If you want to find out the estimated peak times for where you live, go to this site, and check out your location. At the bottom of the screen is a flux calculator. Pick the city closest to you and launch the calculator. It should give you a pretty good idea what the times will be for your area.