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Estimating Velocities - NAMN Scale vs Deg/sec
Below is the jest of the NAMN method of velocity estimations...
perhaps if I changed the title from Estimating Velocities to
that of Shower Determinations from Apparent Motion would be more accurate?
The only real relationship is that the information from both methods fills
the
same slot on a meteor report, but still only one or the other is necessary.
If tabled items don't align up, print a copy for clairification.
George
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ESTIMATING VELOCITIES - The NAMN Scale vs Degrees/second
George Zay
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Estimating meteor velocities for the purpose of shower association
is usually determined by estimating the degrees traveled per sec-
cond. Another method used by the North American Meteor Network
(NAMN), is the NAMN scale. Here it is explained and compared to
the degree/sec method normally used thru-out IMO.
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1. Introduction
When I first began meteor observing, I perceived making velocity
estimates in degrees/second as an awkward procedure. As a solu-
tion, I incorporated the 0-5 velocity scale found on the FIDAC
Fireball Report forms into a user friendly method. The major and
perhaps sole purpose for velocity estimates is to help determine
shower associations. When Mark Davis and I formed NAMN, the pri-
mary purpose is to get new people interested in meteor observing
and teach them how to record good meteor data. The method to
record meteor data basically follows what is taught by IMO. From
the outset, we recognized the importance of making data gathering
for the novice as simple as possible without jeopardizing it's
quality. When I considered teaching velocity estimates, it was
only natural to teach the easiest scale method first. The ini-
tial intention was when a new observer gains experience, the
degree/second method would be presented. Recently, the topic on
velocity estimates made by degree/sec vs the NAMN scale was dis-
cussed over the internet at length. In the process of explaining
the NAMN scale, I've convinced myself that it was not only use-
able, but superior to the degree/sec method that's predominant-
ly used.
2. THE NAMN SCALE METHOD
Using the NAMN scale method is rather simple. For each meteor,
the velocity recorded will simply be a number of 0 thru 5 repre-
senting it's apparent motion.
The Scale and their Representatives are as follows:
Scale # Meteor Appearance Meteor Shower Base Velocity Range
0 = Stationary = No velocity Range
1 = Very Slow = 20km/s and less
2 = Slow = 20km/s to 30 km/s
3 = Medium = 30km/s to 40 km/s
4 = Fast = 40km/s to 50 km/s
5 = Very Fast = Over 50 km/s
Definition:
Stationary = A meteor head on...no movement.
Very Slow = Visible for 10 seconds or more if traveled across
most of the sky.
Slow = Visible 5 to 10 seconds or more if traveled across
most of the sky.
Medium = Visible for 2 to 5 seconds as a sky crosser.
Fast = Rather fast with the meteor head recognizable.
Very Fast = Just a streak with no meteor head discernible.
To use the scale, you simply list the number that represents
how the motion of the meteor appeared. Ignore any attempts at
trying to equate any km/s velocities. The scheme of the NAMN
scale is to allow a judgement on the odds of any meteor being a
shower member. This is based on the fastest expected possible
appearance a shower member can have. With velocities appearing
to be slower as it originates near the radiant or with a lower
radiant elevation, a meteor can appear not related if you try
to equate the shower's velocity directly to a scale number.
Thus, a 3 number range is used in determining shower membership
in conjunction with meteor length and radiant alignment. If a
meteor falls on any of the 3 scale numbers with the shower base
scale number in the middle, it's a shower member. Or if the
shower base scale number is 1 or 5, and the assigned scale number
is a number within 2 of the base scale number, it's also a shower
member.
3 THE NAMN SCALE IN USE
With the following examples, it is assumed that proper meteor
length and radiant alignment is also present for simplicity of
discussion.
Example 1. An Alpha Aurigid candidate. With a shower velocity
of 66 km/s, it's meteor shower base velocity corresponds to a
meteor appearance of Very Fast and a scale number 5. Whether
plotting or simple counting, when the meteor candidate is first
recorded you simply write down the number to indicate it's veloci-
ty appearance. The Scale number will not necessarily be a 5, but
it's fastest possible expected appearance would be a 5. If plot-
ting shower determinations can be made later. When counting, the
judgement must be immediately made. In this case, with a velocity
base scale number of 5, if the meteor was given a scale number of
3,4,or 5, it will be counted as an Alpha Aurigid. But if it was
a 1 or 2, it will be listed as something else or a sporadic.
Example 2. An Alpha Capricornid. It's listed shower velocity
is 25km/s...Slow. It's assigned a 2 for it's fastest possible
appearance. If I assigned a scale number of 1,2, or 3 to the met-
eor it would be counted as an Alpha Capricornid. But if the given
scale number was a 4 or 5..dot it immediately becomes something other
than an Alpha Capricornid. In any place in the sky, it would be
very difficult to conceive seeing a 25 km/s meteor appearing fast
or just leaving a streak. The odds of it being an A. Cap is very
slim then.
Example 3. A Geminid candidate. It's listed velocity is 35
km/s. This would give a scale number of 3 for it's fastest possi-
ble appearance. In this case, it is possible that it would appear
as a 2 or 4 dependent on where the meteor is in relation to the
radiant and the radiant's location in relation to the horizon. If
I assigned the meteor a scale number of 2,3,or 4, it would be des-
ignated a Geminid. But if the scale number assigned was a 1 or 5,
it becomes something else.
4. NAMN SCALE vs DEGREES/SECOND
The major argument against the NAMN scale method is that it's not
as accurate in determining velocities as the Deg/sec method. This
is true, but the end result is not to find a value of measurement,
But rather allow an individual to make
judgement calls for shower association. We aren't looking for a
value that is accurate...just a means to quickly determine shower
membership. If you wanted to get accurate enough to go beyond
shower determinations you need a camera system of some sort with
rotating shutters or some other electronic means that goes be-
yond the human senses. All either method is good for is making
a determination of what shower a meteor belongs to. If this could
be done satisfactorily, why bother to get velocities to the nitty
grittieth power if that isn't good enough for anything else? Why
go thru all the mental gymnastics? When determining a meteor's
degrees/sec, you need to estimate durations in 1/5th or 1/10th of
a second. Is it worth the effort having to think in tenths or
fifths of a second while your mine is needed to note magnitudes,
train presence, times etc. I note all these too, but if I can elim-
inate having to think about one more factor and still accomplish
the same resultant goal if I did, why put yourself thru it? An
experienced observer should be able to at least look at any meteor
and judge whether it appeared Very Fast, Medium, or Very Slow and
other in-between motions. After viewing a few major showers, a
new observer can make these kind of judgements as well with a lot
less frustration. There is no need making various data gathering
factors more complicated than what they can be ultimately used
for. When you estimate in deg/sec you are making two estimates...
time and distance. Whereas with the NAMN scale you are only esti-
mating an apparent look of movement. It would seem to me that the
more separate estimates made for a factor, the more inaccurate
it's final value will be. I know some very experienced observers
are quite good at estimating deg/sec and that's great! But why
shoot gnats with an elephant gun when a pea shooter is just as
deadly? In this case, making deg/sec estimates is overkill for
something that can be accomplished in a more simpler manner. Now
if there were a genuine need for visual deg/sec data beyond shower
determinations, then I can see the advantage. But what are they?
I haven't come across anything that would dictate deg/sec over the
NAMN scale. At best they are equal, and I personally feel that
the scale is superior for it's intended purpose mainly on account
of simplicity.
There should be no difficulty for anyone when they receive a re-
port that uses the NAMN scale, because it's purpose has already
been used prior to anyone else receiving it. It's basically like
the work sheet you use on the back of IMO's Summary sheet to de-
termine LM's for any given period. It's all done and not entered
on the front except for the conclusion. The only difference, each
meteor is judged on the front of a meteor data sheet. I highly
doubt anyone ever looks too much at either the deg/sec or NAMN
scale in the velocity column for anything other than an obvious
error...they are mainly concerned whether it was a certain shower
member or a sporadic.
5. CONCLUSION
I'm not advocating the discontinued use of deg/sec if an indivi-
dual enjoys the mental exercises. I know once an individual is
accustomed to a method he can become very efficient in utilizing
it. But it's important to not make observing a chore.
Wouldn't doing more steps than necessary constitute a chore? Al-
beit a rather small chore multiplied with the number of meteors
seen. It's important to have good meteor data. The simpler the
efforts needed, the more readily a new observer will enjoy the
process and continue. It's quite impressive to hear an experi-
enced observer rattle off some mumbled calculations just to deter-
mine what shower a meteor belonged to...but it's not necessary...
unless estimated deg/sec is needed for some other computations.
Also consider this, As an experienced observer I have no problems
following the listed guidelines in making shower determin-
nations. The other factors for shower determinations (path length
and radiant alignment) re-enforces my judgements enough to tell me
that if I'm making any errors with the NAMN method, there aren't
that many... The only real pre-conceived "inaccuracies" is those
that are manufactured by bias and pre-conceived notions as to how
the NAMN method really works. To make a judgement, one should try
it and compare the results. If it's inaccurate...how come I
can make it work?