Zenithal attraction, and beautiful sloooow meteors (was Re: (meteorobs) June Bootids - Call for observations)

[Lewis J. Gramer]

Bob Lunsford wrote:
> >Note that the apparent radiant is always shifted towards
> >the zenith by a few degrees. This is because of the slow
> >speed of the June Bootids.

And Richard Kramer then responded:
> This is a very interesting point. The reason for it alludes
> me. I wonder if someone would be kind enough to explain
> this subtlety.

Richard, the reason for this "zenithal attraction" is simply
the Earth's gravitational pull!

Basically, imagine a meteoroid coming in at a certain angle
relative to your local horizon. Now imagine what happens as
it nears the earth - just prior to its "fiery plunge" into
the atmosphere. Remember, gravity will gradually deflect the
meteor's path along a parabolic track toward Earth's center.

What's the result of this deflection, from the point of view
of an observer on the ground? The meteor's "radiant point" -
an imaginary spot on the celestial sphere that its apparent
path can be traced back to - will tend to shift toward your
local zenith! And of course, vector arithmetic (folks might
remember those fun diagrams from high school physics!) will
tell you that, the SLOWER the meteor is relative to gravity's
accelerating effect, the more NOTICEABLE this deflection is.

(All meteors are subject to this "Zenithal attraction", of
course: but meteors from most showers are fast enough that
it isn't too significant while observing. But as Rainer and
Bob Lunsford mention, the June Bootids - just like the Kappa
Cygnids later on this Summer - are a rare, slow exception!)


And by the way, even if you only get to see one or two JBOs
tonight, folks, remember - meteors *this* slow can really be
stunning sights! Assuming Bob's maximum of 10 degrees/second,
that means the average June Bootid well away from the radiant
might last a full half-second or more. That's enough time to
yell, "Look at that!" - and actually have someone get to turn
their head in time to see your meteor! :)


Clear skies,
Lew