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Re: (meteorobs) plotting


Back on 07 Jan 2000, Robert Gardner wrote:
> I am interested in the meteor plotting.  Is there any problem with plot-
> ting due to the fact that you are mapping spherical images on to a flat
> surface? Do you have special star maps for this purpose. It would seem
> to me that you would ideally have a map made by projections from the
> very radiant you're trying to locate which of course doesn't make sense.


Robert, I apologize that your email got lost in the Holiday "blizzard"!

Actually, you are completely correct: the maps used for meteor plotting
are based on a different projection scheme than traditional maps of the
heavens (or the Earth). In place of the most common projection systems,
which project some variation of a cylinder or a conic section wrapped
around the celestial sphere's equator, meteor plotters use charts based
on the "gnomonic" projection system: this takes a plane tangent to the
celestial sphere at a single point - the center-point of each of the 12
IMO charts - and projects the chart area onto that plane.

The end result is that meteor paths, which in fact trace a segment of
one of the "Great Circles" around the celestial sphere, appear on the
plot charts as straight lines. With a cylindrical or conic projection,
we would be forced to plot meteors as curved arcs on a chart instead!

Note that there's no particular reason to favor the radiant point for
a shower (that I know of, anyway!), since most of us do not actually
face the radiant during a shower. For a major shower, we'll choose to
face 30-40o away from the radiant; or during times when several minor
showers are scattered around the sky, we may be forced to face even
further from one or more of these radiants than that.


How does one get ahold of a set of these weird "gnomonic" meteor plot
charts? That's where IMO or one of the regional societies such as the
American Meteor Society come in! By joining these organizations, or by
just contacting their coordinators, you can obtain complete chart sets
to copy from, as well as invaluable instruction on how to plot!

Check these Web sites for more info:

    http://www.imodot net
    http://www.amsmeteors.org

Clear skies and happy plotting!
Lew


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