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Re: (meteorobs) Re: Angular velocities
The angular velocity of meteors has certainly become involved. From GZay we
heard,
> For what purpose
>other than shower association is visual meteor speed estimates used for? I
>can't think of any.
>.... All you need to be is accurate enough to provide for adequate
>shower association. You don't need to measure anything or estimate any
>durations or do any kind of math in your head....just record the speed
>impression you get from Slow to Very Fast and have some rough sense of the
>meteor's distance from radiant and know what it means.
I'll drink to that .... as long as it's fresh cow's milk, being a
teetotaler. As you become familiar with each major shower, you learn what
meteors of various true velocities look like. Then you will know similar
velocities when sporadic meteors appear. It does become second-nature with
experience.
The AMS data forms requested distance and duration, so that is what I
learned. Distance was by far the easier of the two, particularly since I
had a good working knowledge of the constellations before starting in
meteors. Durations took about 3 years to learn well. Everybody begins with
estimates much too long. My first Perseids were all noted as 1 and 2
seconds! Over the next 3 years my durations gradually got shorter, then
stabilized.
A good method for learning the length of a second is using a watch with a
second hand. I had a watch with seconds subdivided into 5 parts so that was
a big help in learning fractions of a second as well. For years I would
study the watch and mentally image meteors as the second hand moved,
constantly refreshing my sense of time. The most common meteor durations
are 0.3s and 0.4s.
With these two quantities recorded, then if I wanted to know the velocity in
degrees per second, it is just a matter of doing the arithmetic at leisure.
In fact, I have never desired to do it. As far as recording this data goes,
I discontinued these items after the summer of 1976 as no new information
was coming from it, and I wanted to streamline field work. Around 40000
meteors had been recorded by that time anyway.
Perceptions of distance and duration do vary between observers. I see
meteors as very short, averaging probably under 5 degrees. That comes
partially from being in dark skies, but when observing with Bill Gates (not
from Microsoft) who has 4 times my perception, he felt like I was seeing
only a portion of each meteor path. That is remarkable considering that I
see most meteors centrally or nearly so. Evidently there is no guarantee
that an observer will see a meteor even if it occurs at central vision.
Olivier was struck by the shortness of my plots from the beginning. Then
for durations, as an example, I found out Bob is dramatically different.
When he remarked that he had never seen a meteor last longer than 4 seconds,
I was stunned. That is far from the longest I have seen, up to 10 seconds ;
and something lasting 4 seconds is not at all unusual to me. Thinking about
Bob's long Leonid lasting 4 seconds, a similar Leonid for me lasted 8
seconds. Bob's sense of time must run a lot slower than mine. I would have
thought that training with a watch would not permit such a large difference,
but now that's out the window.
Bob said,
>1/6th of a degree? You must have micrometer eyes!
Yes, I have very acute central vision which aids in seeing faint stars and
other objects plus resolving short distances. I could have detected the
motion in a meteor half that length. But acute vision is not well related
to meteor perception, at which I am just normal.
>This very short meteor
>WAS traveling slowly if your estimates are correct. It's the duration
>that was quick making it appear as a swift meteor.
Exactly right, that was my intention. For meteor speeds my goal these days
is estimating true velocity within 3 categories : slow, medium, fast. I am
not concerned with just the angular velocity. I take the distance and
duration under consideration together to come up with a speed class. It
becomes second-nature with experience. This information weeds out an
occasional meteor from a known radiant that doesn't have the proper speed.
It also weeds out meteors from new radiants that don't fit the majority
profile ; if most of the meteors from a developing radiant are fast, I would
drop any slow meteor from the same radiant, for example.
Norman
Norman W. McLeod III
Asst Visual Program Coordinator
American Meteor Society
Fort Myers, Florida
nmcleod@peganet.com
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