[Prev][Next][Index][Thread]

(meteorobs) What do all the (meteor) numbers mean?? (LONG)


From time to time, meteor observers share their observing reports with us here 
on 'meteorobs'. These observing reports are of interest to us all for a number 
of reasons: First, they allow the posters to have a sense they are sharing what 
they do - apart from sending their reports off to unknown databases somewhere. 
Second, they give each of us a sense of the kinds of active observing which go 
on in many different parts of the world. Third, and maybe most important, these 
reports give us an idea of the type and quality of information we should all 
try to gather in our OWN meteor observing sessions...

In line with this, one of our recent subscribers asked me a very important 
question, and one which is probably on many subscribers' minds: namely, what DO 
all the numbers in those meteor reports mean??

The answer to this will take some time to relate - and I must **WARN** THE 
READER: this may seem fairly daunting at first! But believe it or not, the type 
of observing and data-recording which Marco did is replicated by many people 
around the world during the course of a year, and quickly comes to seem "second 
nature" to even us inexperienced amateurs...


Using one of Marco Langbroek's recent posts as an example, here is a short 
"Guide to Amateur Meteor Recording": this is NOT a complete manual on our hobby 
by any means! For that, I recommend the "NAMN Meteor Guide" if you're just 
getting started, or IMO's "Handbook for Visual Meteor Observers" (or their Web 
site http://www.imodot net ) if you have some experience.

OK, here goes! Comments will be preceded by "***"...

From http://www.tiacdot net/users/lewkaren/meteorobs/msg07051.html :

Date: February 17/18 1998
*** Don't forget the date you observed! Either "double-date" format
    (as above) or single date in UNIVERSAL TIME are always preferred.
Observer: LANMA
*** The observer's IMO five-letter ID. This is usually the first 3
    characters of your last name, followed by the first 2 of your
    first name. But if you don't already have an ID, contact IMO.
Location: Voorschoten, the Netherlands, 52d 07'N, 4d 28' E
*** Always determine the topographic coordinates of your observing
    site to at least one arcminute accuracy! This is important for
    later analysis. (Also, site elevation in meters can be useful.)
    For anyone with Web access, this is actually quite easy to do.

UT             Teff    Lm     DLE   VIR   Spor   Total
21:20-22:25    1.02    6.2     1     1     9      11
22:25-23:30    1.02    6.1     0     1     10     11

TOTAL          2.05    6.2     1     2     19     22

*** First, Marco breaks up his whole observing session into periods,
    each as close to a full hour as conveniently possible.

*** Then, he gives the start and end time for each period, again in
    *Universal Time* (GMT). This should be accurate to the minute!

*** Next, 'Teff' is the "EFFective observing Time": how many hours
    in each period Marco was actually looking at the sky. This is
    MINUS any time for breaks, or any time spent "looking down"
    while he recorded meteor data (usually 5-10 secs / meteor).

*** In column 3, Marco provides a *very* important number used to
    calibrate this night's observing: "visual limiting magnitude"
    or "LM". This is roughly speaking, the faintest star that is
    visible to the naked-eye directly overhead. But IMO provides
    a more statistical and (hopefully) accurate way of measuring
    LM, which requires the observer to count all visible stars in
    a defined sky area (a polygon with bright stars for vertices).
    There is a table IMO provides to convert these "star counts"
    into LMs: for example, if you can count 12 stars (inclusive)
    inside the triangle formed by Alpha, Beta, and Delta Cephei,
    then your "LM" at that moment is 5.5 (or better)!

*** Finally, in columns 4 and succeeding, Marco tells us HOW MANY
    meteors he saw during that period from EACH METEOR SHOWER: just
    counting meteors you see is essentially pointless for scientific
    purposes! Instead, you should learn to distinguish meteors which
    are associated with particular shower "radiants", from meteors
    which are simply "random" (these are called "Sporadics"). On a
    given night, there will usually be MORE THAN ONE SHOWER active,
    and so the experienced observer must actually distinguish each
    of these shower radiants from one another. During Marco's watch
    tonight, there were TWO active showers that IMO recognizes: one
    shower had meteors which appeared to "radiate" (or "trace back"
    to) a point in the sky near the star Delta Leonis. These meteors
    (and their shower) are thus called the "Delta Leonids", or "DLE"
    for short. In addition, there were meteors appearing to radiate
    from within Virgo - so he named them "Virginids" or "VIR". And
    finally, meteors from NEITHER shower were "Sporadic" or "Spor".

In the totals column, Marco toted up all the meteors from each shower, and also 
gave a time-averaged "LM" for the night. Based on just Marco's simple summary, 
some basic analysis can be done on what was going on in near-earth space that 
night! For one thing, most of the meteors were NOT from showers: this means 
that earth was passing through very sparse parts of the particle streams which 
form the DLEs and VIRs. Further, we know that the VIRs were relatively more 
dense than the DLEs in the region of space earth passed through tonight.

In a separate, private email later, Marco forwarded a more complete report on 
the data he gathered from this night's observing: this report had to give 
specific information about any cloud cover, any breaks he took, how much time 
he deducted for recording per meteor, and (VERY IMPORTANTLY) how bright each of 
the meteors he saw was tonight! (This is "meteor magnitude", which you estimate 
by comparing the meteor with nearby stars of similar brightness...)

This more detailed report served to confirm the data given in his summary, AND 
allowed still more detailed analysis to be done on his observations: in 
particular, the BRIGHTNESSES of the meteors he saw that night tells scientists 
how MASSIVE each of those meteors was, which in turn allows them to figure out 
what the mass DISTRIBUTION of each of the meteor showers (and the sporadics) 
was for that night. Again, for these numbers to make sense, ALL of the data 
Marco gathered was very important: for instance, without an estimate of Marco's 
LM for each hour-period, neither counts NOR brightnesses would make any sense.



Well, that's it... And if you're overwhelmed right now, you're probably not 
alone! Others probably have the same questions, so don't be shy about posting 
to 'meteorobs' to find out more. Or feel free to contact one of us "frequent 
posters" privately, if you'd rather. That's what we're here for!

Clear skies and many meteors to all!
Lew Gramer
owner-meteorobs@latrade.com