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(meteorobs) Leonid radiant rise

Looking back over emails from last November I noted this reference (below)
to the time of Leonid radiant rise at 111km elevation.  This is a mistaken
idea.  From ground level, when the zenithally corrected radiant (without
refraction correction) is on the horizon, then a Leonid OVERHEAD will
be travelling horizontally and be at it's closest to the Earth (it's
perigee on a hyperbolic path).  If you do the calculation for 111km, you
would require any Leonids that pass thru that point to have actually passed
thru the atmosphere down to the Earth's surface before rising up to
that position!

When the radiant is on your horizon shower meteors will be slightly
more common in the direction of the radiant as that part of the atmosphere
is above regions with the radiant above the horizon.  Looking very low
towards the direction of the rising radiant can give Leonids some tens
of minutes earlier (depending on your latitude).  Looking in the
other direction, any Leonids will have passed their closest approach to
the Earth and be on the rise back into space.

It seemed the case last year that Earth grazers were at a great
height.  Their angular velocity was quite low compared to Leonids
with the radiant higher in the sky.  I assume someone can quantify this
from observational data.

Cheers, Rob


Date: Wed, 28 Nov 2001 16:43:08 -0700
From: Larry Wood <chukwood@telusplanetdot net>
Reply-To: meteorobs@atmob.org
To: meteorobs <meteorobs@atmob.org>
Subject: Re: (meteorobs) Leonid report - narrative - Nov 17/18, Mt. Lemmon,
     AZ (ATAJU)

I checked using Guide 7 and here in Edmonton (Elev 672 m) on the 18th the
radiant center rose at  05:27 UT.
If at 111 km elevation the radiant center would have risen at 03:37 UT.

Larry Wood

> Mike,
> It is also possible for the radiant to be below the horizon for an
> observer on the ground, but the 100 to 120 km height of Leonid's start of
> light to be above the horizon. Jim  Richardson wrote an article about it a
> few years ago. I believe the conclusion was that the radiant can be almost
> 10 degrees below the horizon, but I'll have to check on that...my math
> isn't currently in use enough to figure it out.

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