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(meteorobs) Earth Grazers
Hello Bob, et al,
I must be in a number crunching mood tonight, and got to thinking about
those glorious "Earth grazing" meteors along with the rest of you. If we
hypothesize that our grazer is moving horizontally (or nearly so) directly
over it's local patch of Earth, and further stipulate that it's midpoint
has an altitude of about 100 km, we can use the curvature of the Earth (and
atmosphere) to gain some ideas as to the radiant altitudes and meteor
magnitudes necessary for those pre-radiant rise grazers to occur. Note
that I am assuming a fast enough shower not to be concerned with zenith
attraction, and that meteor altitudes and distances are measured from the
midpoint of the meteor's path.
Yet another chart:
radiant altitude (deg), meteor altitude (deg), meteor distance (km),
magnitude extinction (mag)
-10.1 deg, 0 deg, 1134 km, +7.9 mag
-9.1 deg, 1 deg, 1028 km, +7.5 mag
-8.3 deg, 2 deg, 934 km, +7.3 mag
-7.5 deg, 3 deg, 858 km, +7.0 mag
-6.8 deg, 4 deg, 773 km, +6.7 mag
-5.7 deg, 6deg, 649 km, +6.1 mag
-4.8 deg, 8 deg, 552 km, +5.6 mag
-4.2 deg, 10 deg, 477 km, +5.1 mag
-3.0 deg, 15 deg, 352 km, +4.1 mag
-2.3 deg, 20 deg, 277 km, +3.3 mag
-1.5 deg, 30 deg, 195.6 km, +2.2 mag
-1.0 deg, 40 deg, 154 km, +1.4 mag
-0.5 deg, 60 deg, 115 km, +0.5 mag
0.0 deg, 90 deg, 100 km, +0.0 mag
Based upon these estimations, the minimum radiant altitude at which grazers
begin to be possible is about -10 deg, but the meteors are so far away that
they must be quite bright to be noticed. A -4 mag would look like a +4 mag
at this distance. From -10 deg to -5 deg radiant altitude, the meteors
march steadily closer to the observer, with a -2 mag meteor looking like a
+3.3 when the radiant is at -5 deg. However, the really giant strides in
closing the meteor distances down to more common ranges (with little
magnitude extinction) occur during the last 5 degrees of radiant rise.
This should be the time in which the majority of long grazers are seen, and
I find it reassuring that Bob recommended this time frame based simply on
his own experience.
The Eta Aquarid shower was always one of my favorites, primarily because of
the lovely, trained, sky-crossing grazers it could produce prior to radiant
rise. Good luck to all in catching a few!
Take care,
Jim
James Richardson
Tallahassee, Florida
richardson@digitalexp.com
Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/
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