(meteorobs) Geminid comparisons

[Bruce McCurdy]

Norman wrote:

> Onward to the Quadrantids.  Good news for American zones on this one -- it
> is our turn for a good year.  Two cycles back, in 1997, I saw 61/hr Quads
> but with a thick crescent moon.  Of all the rotten luck, 2004 isn't
> moonless either.  Just glad it's past last quarter.

    Of course you mean 2005. And it really doesn't have anything to do with
luck. Although the best repetition of moon cycles is the well-know Metonic
cycle of 19 years, this cycle can be subdivided into periods of 8 and 11
years in which Moon phases only change a day or so, later and earlier
respectively.

    So those meteor showers with narrow peaks which are longitude dependent
and therefore follow a four-year leap cycle, will have a similar phase every
second cycle, which slowly changes over longer periods.

    Because there are an odd number of lunations (99) in 8 years plus ~ 1
day, after four years there are 99/2 = 49.5 lunations, so one can expect
almost an opposite phase at the alternate cycles. Check out the following
dates of moon phases relative to the Quadrantids:

    First quarter            Third quarter

1993 Jan 1                1997 Jan 2
2001 Jan 2                2005 Jan 3
2009 Jan 4                2013 Jan 5
2017 Jan 5                2021 Jan 6
                    ...etc...

    Because we are talking differentials of a leap year, the Quadrantid peak
occurs right around the same time on the same date each of these years,
namely ~12h UT on Jan. 3. It follows that each good North American peak for
the foreseeable future will have a quarter Moon, but in half those cases the
first quarter moon will have set well before the peak; in the other cases,
as upcoming, the third quarter Moon will be a big ole pain in the butt,
getting slightly bigger each eight year interval.

    Other considerations are that Earth arrives at the same point in its
orbit about three quarters of an hour earlier each leap cycle, which will,
also gradually, change which cycles are favourable for which of Norman's
longitudinal quadrants.

    Presumably also, and this is where I have little expertise, the
meteoroid streams themselves are evolving so that their 'point' of
intersection with Earth's orbit is gradually shifting over time. Or the
'best' peak might actually shift from one filament to another, as I believe
may have happened with the Perseids fairly recently. (RASC Observer's
Handbook showed the predicted Perseids peak moving from 10 h UT on 2001 Aug.
12 to 22h on 2002 Aug. 12, a shift of some six hours against the tropical
year. The solar longitude in the relevant table was shown to change from
139.8° to 140.0°; suggesting either a refined observation or recognition of
a new 'best' filament.)

    I would welcome the more expert (although not *too* technical, please)
comments of others re: the evolution of individual meteoroid streams and
their changing relationship with Earth expressed as a factor of time (or if
you prefer, solar longitude). I would like to develop a better mental
picture of the changing dynamics of the streams, especially the rate of
change.

    regards, Bruce