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(meteorobs) Re: Best Time to See the Leonids
David,
You have put a lot of thought and effort into this problem.
Unfortunately from your location it is impossible to see ANY Leonid
activity before 10 PM due to the fact that the Leonid radiant will be
below the horizon and the Leonid meteors cannot contact the atmosphere
above your location. As 10 PM passes the Leonid meteors will just begin
to graze the atmosphere above you creating awe-inspiring long Leonids
that often trace a path over one half the length of the sky. As time
passes the Leonids will begin to enter the atmosphere above you at a
steeper angle creating shorter lengths.
Since 10 PM is 3 hours after the predicted time of maximum activity I
would venture to guess that your visual rates would be steady throughout
the night and morning. As the radiant rises higher into the sky the
activity should increase but the decreasing intensity of the shower will
counteract this providing a steady display.
If I were at your location I would begin watching at 10 PM to see these
impressive "Earth Grazers". If by chance the maximum is late then the
activity you will see in the late evening could be better than that seen
later in the morning.
I hope this helps!
Robert Lunsford
American Meteor Society
International Meteor Organization
David Cross wrote:
>
> People (not the sort who are going to observe for hours on a freezing cold
> night) ask me when the "best" time to watch for the Leonids is. I'd like to
> know, myself. The short answer for the UK in November is "anytime you're
> lucky enough to have a clear sky, and as dark a location as you can get
> to". But it got me thinking, particularly about the balance between the
> time of the expected peak and the "you see more meteors after midnight"
> principle. I've outlined my thinking below, and I'd appreciate comments on
> factors I might have missed or got wrong or on faults in my logic. More to
> the point, perhaps, does this accord with observations?
>
> I've seen various times quoted for the predicted maximum, but a consensus
> range would be between 1800 and 2300 UT on November 17. Since UT is our
> local time in winter in the UK, this puts the expected maximum well before
> midnight.
>
> It seems to me that the following factors affect the number of observed meteors
> 1 The expected time of maximum ZHR (the published peak).
> 2 The height of the radiant above the horizon - the higher the better.
> 3 The local time. More meteors will be seen after local midnight, since the
> observer will be on the side of the earth going "forwards" in its orbit.
> 4 The phase of the moon and the time of moonrise and moonset.
> 5 The limiting magnitude.
>
> The moon: Since the moon is about new for this year's Leonids, it will set
> around sunset, so I'll ignore it.
>
> The limiting magnitude: This is unlikely to be predictably time-dependent,
> so I'll ignore that.
>
> Height of the radiant: I'm not clear how far meteors can go from the
> radiant, but if they can go up to 90 degrees away, then you would see all
> meteors from a radiant at the zenith, but only half the meteors from a
> radiant at the horizon (because half would go down below the horizon, where
> you wouldn't see them, and half would go above the horizon). So this makes
> a difference of a factor of two. If the meteors can be seen more than 90
> degrees from the radiant, then the number seen becomes *less* sensitive to
> the height of the radiant.
>
> Local time: I don't know exactly how the number of meteorites varies over
> the 24 hours but the maximum will be at 6 am when the observer is on the
> "frontmost" part of the planet in its orbit, and the minimum will be at 6
> pm. Based on radiodetection of sporadics Philip Gebhardt (Sky & Telescope,
> Dec 97) quotes a ratio of maximum to minimum of 4 to 1 (for mid-latitudes).
> So the difference between after midnight and before can't be more than
> fourfold.
>
> Taking the last two factors together, assuming the radiant is above the
> horizon, then the most difference that the time of night can make is an
> eightfold difference. Provided that the ZHR around the peak is more than
> eight times the ZHR outside the peak, then you are better off watching at
> the expected peak than waiting until after midnight. Does this all make
> sense?
>
> Clear skies
>
> David
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