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(meteorobs) Re: NM past South Delta Aquarid rates
"Kim S. Youmans" wrote:
>Not straying too far from the subject, I have a couple of other >lingering questions about velocities.
>A. Is is possible to see different but close velocities from the
>same shower? For instance, could I designate one leonid as a 5 and >another as a 4? Would this difference be subjective or due to >perspective?
Kim and all, the same shower can produce meteors of widely differing
APPARENT velocities. Take the upcoming Perseids for example. A Perseid
meteor seen 10 degrees from the radiant will have an angular velocity
from 3-5 degrees per second depending on the elevation of the meteor. A
Perseid meteor seen 40 degrees from the radiant will have an angular
velocity from 3-15 degrees per second, again depending on the elevation
of the meteor. If we use a constant elevation of 60 degrees then the
angular velocity would be 4 degrees per second (slow) for the Perseid
appearing 10 degrees from the radiant and 13 degrees per second (medium
speed) for the Perseid appearing 40 degrees distant. The upper limit for
Perseid meteors would be 30 degrees per second for those shower members
appearing 90 degrees in elevation and 90 degrees from the radiant
(essentially an "Earth Grazer").
So you see the angular velocity for the Perseids can range from 3 to 30
degrees per second making speed a poor indicator of shower association.
When trying to assign meteors during times of multiple radiants I would
suggest using the path, length, and finally the date to help in shower
association. I would ask: which radiant does it best line up with? Is it
too long for either radiant? And finally is the night of maximum
activity closer for radiant A or B.
During the last Leonid display I saw slow Leonids near the radiant and
swift ones much further out. This is all due to perspective. Using the
numeric scale, they would have fit all categories. An observer looking
at the radiant would have a slower average than somebody looking toward
Orion or the Big Dipper.
Is estimating velocities useless then? Not at all if you remember to
include the center of your field of view on your data sheet so that
those analyzing your data can determine your distance from the radiant
and your elevation above the horizon. Those observers looking closer to
a radiant and closer to the horizon will have lower estimates for
velocity than those looking higher and further from a radiant.
>B. When meteors have extremely short path lengths, I find it quite
> difficult to give many of them what I feel would be accurate velocity >ratings. Should I take Lew's advice and just "call 'em as I see 'em" >(as best I can)?
Lew's advice is sound. I may add that if you can see the actual
meteoroid itself, this would help in your estimate of speed. For short
meteors, if you easily see the actual meteoroid then I would call it a 1
or 2 on the numeric scale. If the meteoroid is just visible then call it
a 3. If it is just a streak of light with no meteoroid seen then I would
call it a 4 or 5. Actually this would probably be good advice for all
meteors, regardless of length, for those using the numeric scale of 0-5.
Enough Said!
Bob Lunsford
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