(meteorobs) Zay's Numeric Speed Scale
The following describes the speed scale scheme I use with various examples
for shower association. In use, I have no problem with it at all. It comes
To record the speed of a meteor, I developed a numeric scale method that is
rather simple. For each meteor, the speed recorded will simply be a number of
0 through 5 representing itís apparent relative motion.
The scale and their representatives are as follows:
Scale # Meteor Appearance
0 = Stationary
1 = Very Slow
2 = Slow
3 = Medium
4 = Fast
5 = Very Fast
Stationary = A meteor head on with no apparent movement.
Very Slow = Visible for 30 seconds or more if traveled across 120
degrees of the sky. Speed is almost comparable to that of a burning satellite
Slow = Visible for 15 to 30 seconds if traveled across 120
degrees of the sky. The traditional slow moving meteor that is usually
recorded in the early evenings.
Medium = Visible for about 5 to 15 seconds if traveled across 120
degrees of the sky.
Fast = Fast enough for the meteor head to be barely
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 km/s
velocities. The scheme of the numeric scale is to allow for a judgment based
on the relative odds of any meteor being a shower member. This is based on
the fastest expected appearance a shower member may have. With velocities
appearing to be slower as a meteor originates near the radiant or with a
lower radiant elevation, a meteor can appear not related if you try to equate
the showerís velocity directly to a scale number. Thus, a three number speed
range is used in determining shower membership in conjunction with meteor
length and radiant alignment. For meteors outside an approximate 10 degrees
from a radiant, if the meteor falls on any of the 3 scale numbers with the
Shower Base Scale number in the middle, it becomes a shower member. If the
Shower Base Scale number is 1 or 5, and the assigned scale number is within
two digits from the base scale numbers, itís also a shower member.
To determine the Shower Base Scale number for a shower, you simply match
numbers from the 0-5 numeric scale to that of the showerís entry velocity.
Remember, the numeric scale number and the shower entry velocity are not
equivalent. This is simply a means to establish a range of numbers to
initially work with.
Base Scale # Shower Entry Velocity
0 = No velocity range
1 = 20 km/s and less
2 = 20 km/s to 30 km/s
3 = 30 km/s to 40 km/s
4 = 40 km/s to 50 km/s
5 = Over 50 km/s
Special Exceptions: There are three notable exceptions concerning meteor
speeds that one has to be aware of. These exceptions will have results
leading to apparent speeds slower than expected. They are:
a) If a meteor appeared within about 10 degrees of a radiant, due to
foreshortening it can have apparent slow meteors of scales 0, 1 and 2, even
if from a shower that is listed as having itís members being very fast. This
is if the meteor was no longer than 5 degrees and coming directly from a
known radiant. Here shower membership determination is more weighted when
referring to the meteorís path length rather than apparent speed. Also, Very
Fast appearing meteors in this circle should not be considered as belonging
to the radiant in question. Be sure not to confuse short durationís for that
of Fast motion.
b) Another exception are meteors that appear from a radiant that is near the
horizon, whether that being several degrees below or about 20 degrees above.
Under these circumstances, meteors may appear relatively slower and often
long as well.
c) The last exception are meteors that appear relatively low on the horizon
from a radiant that is relatively high in the sky. Here in this region,
meteors can appear deceptively slower than same shower members higher above
the horizon due to path alignment effects and distance.
These three exceptions do not complicate the numeric scale once they are
understood and recognized.
With the following examples, it is assumed that proper meteor length and
radiant alignment are also present for simplicity of discussion. Examples 1,
2 and 3 are outside 10 degrees of a radiant, while the radiant is high in the
sky. Also the meteors appeared somewhat in a mid-sky location.
Example 1. An Alpha Aurigid candidate. With a shower velocity of 66 km/s,
itís Shower Base Scale number 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
itís relative motion...that is, how it appeared to you. The scale number will
not necessarily be a 5, but itís fastest expected appearance should be a 5.
If Plotting, shower determinations can be made later. When Counting, the
judgment must be immediately made. In this case, with a Shower Base Scale
number of 5, if you gave the meteor a scale number of 3, 4 or 5 when you saw
it, 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 candidate. Itís listed shower entry velocity
is 25 km/s, which appears Slow. It will have a 2 for itís Shower Base Scale
number. If I assigned a scale number of 1, 2 or 3 to the meteor it would be
counted as an Alpha Capricornid. If I gave it a scale number of 4 or 5, 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 alpha
Capricornid is very slim then and thus eliminated.
Example 3. A Geminid candidate. Itís listed entry velocity is 35 km/s. This
would give it a Shower Base Scale number of 3. Outside of the exceptions, it
normally would appear as a 2, 3 or 4. The closer the meteor comes to the
conditional exceptions, the slower they will appear. The grouping of the
scale numbers 2, 3, and 4 allows for any possible uncertainties of motion
judgment since the Geminids are nearing the range of the Fast scale. If I
gave the meteor a scale number of 2, 3 or 4, it would be designated a
Geminid. If the given scale number was a 1 or 5, it becomes something else.
Example 4. A Perseid candidate within 10 degrees of the radiant. With a
shower velocity of 59 km/s, itís meteors would appear to be Very Fast if
viewed at a distance of about 45 degrees from a high placed radiant. Being
close to the radiant, the meteorís apparent trajectory is foreshortened. This
causes the meteor to exist longer with an apparent shorter travel distance
and thus appear to be moving slower than if it was farther from the radiant.
In this case the meteor would have the appearance of being Slow with a scale
of 2 and conceivably a 1. Here, velocity is not traditionally as important
for shower determination as meteor path length and radiant alignment are. A
non-fireball meteor within 10 degrees of a radiant should be no longer than a
few degrees, definitely less than 5 degrees and more than likely about 2. If
our Perseid candidate was only 3 degrees long I would count it as a Perseid.
If it was about 8 degrees long, I would count it as a non-Perseid. If the
meteor was about 3 degrees long inside the 10 degrees surrounding the radiant
and it appeared to be Very Fast, it also will be judged as a non-Perseid.
Its fast velocity should not be apparent this close to a radiant.
Example 5. A Perseid candidate appearing 10 degrees above the horizon with
the radiant near the zenith. Again path alignment shows itself and the meteor
will appear Slow. It could have the apparent motion scale number of 1 or 2.
Again in this case, alignment and path distance become important.
Example 6. An Eta Aquarid candidate with itís radiant near the horizon. Entry
velocity for Eta Aquarids is 66 km/s, Very Fast giving a Shower Base Scale
number of 5. This is one of the exceptions. From the numeric scale grouping
scheme, itís likely to see meteors with scale numbers of 3, 4 or 5. With the
radiant this low, most of these meteors will appear near 3. Some people may
try to give them a scale number of 2, but I do not get the sensation that
they are going this slow. This is one of the exceptional conditions and itís
possible to record a 2. In this case, the other factors such as path length
and alignment will have to be considered as well.
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