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Re: (meteorobs) Meteor Velocities
Thanks to Rob and Bob for a wonderful clarification on this question. But just
so we all know we're on the same page, I wanted to clear up one final thing:
Meteors really have TWO speeds... Namely, one is how fast they APPEAR to be
moving in the sky, from the perspective of a particular observer's location.
This "speed" cannot be based on actual linear distance: after all, there's no
way to measure linear distances in your sky! So instead, it must be measured
in terms of the ANGLE a meteor subtends in the observer's sky: and so this is
the mysterious "degrees per second" people sometimes refer to. And as both Rob
and Bob noted, this value is dependent on WHERE you see a given meteor. Also,
note that some very experienced observers do not bother to estimate a precise
angular speed in degrees/s for meteors, instead choosing to use a much simpler
"Slow-Medium-Fast", or "1-5" (slow/medium-slow/medium/medium-fast/fast) scale.
Now on the other hand, the actual physical particle (the "meteoroid") that is
CAUSING a meteor, *does* have a real linear velocity relative to the earth...
This velocity therefore, is usually measured in *kilometers* per second (or in
"km/s" for short), and is what is usually quoted in Annual Meteor Calendars.
Naturally enough, the relationship between "degrees/sec" (the angular speed,
which is specific to a given location) and "km/sec" (the linear velocity, an
absolute) is dependent on some fairly complex geometry, which I wouldn't even
hope to explain here. But you can find fine references on this very subject,
both in print and on the Web, via the following site:
http://www.imodot net
(Now as it happens, a particle's velocity relative to the earth depends in
complex ways on its orbital geometry relative to that of earth, and on its
mutual gravitational attraction with the earth. But as we say here in the
States, "that is a whole other kettle of fish"... :>)
Take care all!
Lew Gramer
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