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(meteorobs) Radiant diameters
Imagine standing watching migrating butterflies. They have variations in
speed and directions of travel (ignoring that they don't fly in exactly
straight lines). During a migration, they tend to fly from the same
direction, but cover a large spread of directions. If you ran along at
the mean speed of the butterflies, they would appear to be moving at
random with respect to you, at very slow speeds in all directions.
Now get in a car and drive through the migrating butterflies at high speed.
Those that are unfortunate enough to hit the windshield will do so from a
narrow angle in front of the car. The butterflies have not changed their
flight, but their "radiant size" has changed dramatically from the
perspective of observers with different relative motion.
This is the same effect with meteoroids. The size of the radiant is
directly related to the geocentric velocity of the meteors (without the
gravitational attraction of the Earth). When this approaches
zero (meteors just catching up with the Earth from behind), these small
differences are all that is left. The meteoroids appear to move very slowly
coming from all directions. With relative velocities somewhat above zero,
the radiant becomes defined, but can still have a very large spread. Add
the Earth's gravitational attraction and diurnal aberration (effect of the
observer being on a continually rotating Earth) the radiant for very slow
meteors moves appreciably (tens of degrees) relative to its mean position,
during the course of each day as the radiant rises and sets.
A head-on collision adds the Earth's velocity to all the meteoroid
velocities and the small velocity differences between the meteoroids become
swamped by the overall larger combined velocity, hence a much smaller
radiant. The daily effects due to zenithal attraction and diurnal
aberration never amount to more than a 0.7 degree shift in the radiant of the
very fastest meteors from the mean radiant at that specific moment.
From the point of view of the geometry alone, I hope the above explanation
is clear. In reality different orbits can result is different dispersive
effects that can affect radiant size.
Observational error will also affect the radiant size, but would have a
lesser effect on the slow showers with intrinsically large radiants.
Cheers, Rob
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
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