(meteorobs) Meteoroid streams and the Moon

[David Entwistle]

In message <PhW+VXBYKmMJFwdD at radiometeor.plus.com>, David Entwistle 
<David at radiometeor.plus.com> writes
>Following up on a question from 2006 09 22...
>
>On 14th December 2008 the almost full Moon will pass within a few
>degrees of the active Geminid radiant. Does this mean that the Geminid
>meteoroids that reach Earth, as visible meteors, have passed through the
>vicinity of the Moon on their way here?  Or, is the alignment of the
>Moon and radiant just an optical effect and the meteoroids don't get
>anywhere near the Moon, on their way to Earth?
>
>I favour the second answer, but am not sure...


Thanks to all who have made comments on this - I'm still struggling to 
visualize the relative motions of the Earth and the meteoroids, so all 
comments have been helpful. Special thanks to Pete for pointing me to 
the article on the subject and quantifying any (un)likely effect.

Although we're not expecting to see any significant effect, I now favour 
the view, as indicated by Rainer, that the Geminid meteoroids that reach 
Earth, as visible meteors, will have passed through the vicinity of the 
Moon on their way here, when the Moon is close to the radiant on 14th 
December. Given the travel time, the significant interval will actually 
be three hours after the Moons closest approach to the radiant. I have 
to say that I'm still not 100% certain of this though.

I've sketched what I see as the significant circumstances of the Moon 
passing through Gemini while the Geminids are active.

<http://www.radiometeor.plus.com/meteors/showers/GEM/radiant.html>

My rough and ready calculation for the Geminids, in the plane of the 
ecliptic, has:

Earth's approximate velocity:
(149.6 x 10e6 x 2 x Pi)/(356.25 x 24 x 60 x 60) = 30 km/s
Moving towards the Apex of Earth's way at 172 degrees ecliptic longitude
(corrected to Leo/Virgo boundary).

Geminid meteoroid's approximate apparent geocentric velocity (the 
resultant):
35 km/s (that's Vinf, if that's right to use)
112 degrees ecliptic longitude

Roughly plotting the vectors on paper, I think that if we could somehow 
stop and 'park' the Earth in its orbit, then the Geminids would appear 
to radiate at 34 km/s from Taurus at 61 degrees ecliptic longitude, near
the Hyades. This is the area where the Moon will be on 2008 12 11 15:00.

Those meteoroids passing by the Moon will take 384,400 / (35 x 60 x 60)
= 3 hours to reach Earth.

Many thanks,

-- 
David Entwistle