(meteorobs) Geminids vs. Sporadics?

[Lew Gramer]

Ray Jones asked:
> A few of you reported a significantly higher number of sporadics than
> Geminids in the last couple of nights' recordings. Is that common for
> this time of year, or is it possible they may have been Geminids that
> weren't absolutely classifiable?

Ray, this was a great question! In fact, on nights AWAY FROM the peak of a 
major shower (e.g., nights like last Saturday Dec 10/11, or even tonight Dec 
15/16) you often see more Sporadics (i.e., meteors not from any recognized 
IMO shower) in the wee hours than you see meteors from recognized showers.

Now normally, that's the only time we tend to observe - the wee hours, say 
from midnight on. That's because normally (most nights of the year), that is 
when it's most fun to watch, and also when you have the best chance to see 
statistically significant numbers of meteors per hour!

However, the Geminids are an unusual major shower, in that their radiant is up 
essentially all night for Northern observers! And of course, in the evening 
hours, there are usually very few Sporadics to be seen. So in the EVENING, 
within a night or two of the peak, you might expect the Geminids to dominate 
your counts. And yet, there ARE generally some Sporadics visible even in the 
pre-midnight hours; and in fact for Northern Hemisphere observers, Sporadics 
happen to be at an annual high right around this time of year...

Per Bob Lunsford's Weekly Meteor Outlook: "The Sporadic rates for the Northern 
Hemisphere are now at their high plateau for the year. Observers can expect 
around twelve random meteors per hour during the morning hours as seen from 
locations in the Northern hemisphere. During the dark evening hours perhaps 
three random meteors can be seen per hour north of the equator."

And keep in mind, Bob's comment above does not include the 1 or 2 meteors per 
hour from each of the minor showers he identifies as active in the evening 
hours: the Monocerotids, and what Bob calls the "Anthelion radiant", also 
known to observers by their IMO name of Chi Orionids (XOR)... So expect to 
see a few sporadics along with your Geminids, even in the evening hours. And 
of course, the darker your Limiting Magnitude, the more prominent Sporadics 
tend to become as compared with brighter showers like the GEMs!


Pete Bias also replied:
> I was thinking the same thing... Perhaps the problem is that some 
> (especially newer) observers tend to think that the radiant is a pin
> point... when it is generally much larger.

Pete's comment is true also, of course - although the radiants for many major 
showers like the Geminids may be extremely tight when they are determined by 
photographic or video data, the *effective radiant* to use when counting or 
plotting visually is usually much wider - from 5 degrees radius up to even as 
wide as 20 degs. for meteors seen very far from the radiant region.

However, my experience tends to be that newer observers do the opposite! That 
is they associate too many meteors with the current major shower, even if 
some of them may not match ALL THREE shower association criteria:

 1) traces back WITHIN "A FEW" (5-20) DEGREES of the radiant; AND

 2) APPROPRIATE SPEED for that shower's geocentric velocity; AND

 3) PATH NOT TOO LONG (length half the radiant distance or less).


Clear skies to all,
Lew Gramer