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Re: (meteorobs) Meteor Velocities



> If I am correct, it is stated that many meteor showers
> have more or less their own characteristic speed,
> differing only slightly for each individual shower
> member.  Perseid and Leonid meteors are quoted as 
> being fast, and the Taurids as slow, for instance.
> 
> With that said, it is also realized that most meteors
> observed in the evening will be generally slow due to
> their "catching up" with earth, since the planet at
> that time is moving away from them.  As the night
> progresses, however, the meteors will become faster as
> the same side of earth rotates towards the front that
> is moving through space and plowing into meteoritic
> particles.

True, but this is just a statement about meteors in general
and not one about a specific meteor shower.  The important
factor is not that you see a meteor in the evening or morning,
but what the elongation of its radiant is from the direction the
Earth is moving towards.  This direction is very close to
90 degrees celestial longitude west of the position of the Sun.
In the evening, the point the Earth is moving _from_ is
in the sky so meteors must catch up with the Earth and overall
their velocity is smaller.  It's like very fast flying bugs
flying into your car's back window.  In the morning, we can see
meteors with radiants in the direction the Earth is travelling
towards and these are like bugs flying directly into the car
windshield.

A meteor shower will come from a specific direction and have a
pretty fixed elongation of the radiant, so the velocity is
pretty well fixed.  The meteor's velocity is dominated by the
intersection of the meteoroid's orbital velocity and the Earth's
orbital velocity.

The point that has caused the confusion is the observer being
on one side of the Earth or the other.  If you are in the car
looking out the back window, you don't see the bugs hit the 
windshield and vice versa, but this is just your perspective.
Thus if you look out the back window (and a little to the right),
you will see bugs hit the side of the car and if you look out
the windshield (and a little to the left) you will still see the
bugs hit the same side of the car.  How you choose to look at them
bugs flying from a specific direction and speed will always hit
the car at the same relative velocity.  The same with a meteor shower;
it hits the Earth's atmosphere at a velocity independent of what
part of the atmosphere it is hitting.

This said, there is a small component of a meteor's _apparent_
velocity caused by the observer's rotational velocity around the
Earth's axis.  This effect is small, and not relevant at the level
you are discussing.  This effect also moves the radiant position
slightly and is called diurnal aberration.

> Thus, wouldn't the meteors of a given shower that is
> active above the horizon from evening to dusk show any
> significant increase in speed?   Also, with this in
> mind, are the speeds given for a particular meteor
> shower an average, not only for shower members seen at
> a specific time, but for the whole night as well?     


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