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Re: (meteorobs) RE: Horizon Perseid Meteors
On Mon, 23 Aug 1999, Kim S. Youmans wrote:
> Asaf's question on horizon meteors prompts me to ask a nagging
> question concerning the not-so-recent threads on angular velocities, a
> point which I must have missed in the discussion, namely, what happens
> to the apparent (perceived) velocities of fast (or any) meteors like the
> Perseids as they come down to the horizon. Do they appear to slow down,
> and if so, by how much as they approach the horizon?
Yes they appear slower nearer the horizon. The angular speed at 10
degrees altitude is about a fifth of the angular speed at 60 degrees
altitude. The difference roughly halves as the elevation doubles, so
at 20 degrees the angular velocity is 40%, and at 40 degrees altitude
the angular velocity is 75% of that at 60 degrees.
For the technically minded the angular speed is proportional to sin h,
where h is the elevation of the meteor's beginning point. There are
other factors too of course.
The various angular speeds versus meteor elevation and geocentric
velocity (i.e. intrinsic to the shower) is tabulated in the IMO Visual
Handbook and occasionally in WGN. It's probably at the IMO web site,
but I've not been able to connect to check.
Malcolm
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