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Re: (meteorobs) Leonids in the news

On Wed, 11 October 2000, "Terry Johnson" wrote:
> 
> I live in Tennessee (CST), and it looks to me like the times are going to
> fall on the nights of the 16/17 and 17/18 for me.  Will the drop-off of the
> rate be drastic enough to discourage me from observing on the night of the
> 18/19th?  I was hoping make a public announcement of the shower and get
> about 50 people together for the show.

It's probable that the ZHR will be very low (<<20?) by the night of the 18/19. At least, that's how it has been in past years. Moonlight will lower the observed rates (see below) to well below the public interest level. Of course, you could get some telescopes out for viewing the Moon, Jupiter and Saturn. Public Leonid watches in my neck of the woods are invariably busts because: 1) The weather is likely to be lousy; and 2) You can't get people to stay up that late. The radiant rises around 11pm, so you're likely to see zero before that and not much for the next hour. In 1999, numerous watchers retired at midnight with the Moon in the sky, the radiant low, and grand totals of two to five Leonids. Those of us who stayed up until morning twilight got to see a couple of awesome fireballs as well as good activity (my best hour was 73 Leonids and 28! sporadics in very dark skies, 11/18; 12-13h UT). Unfortunately, the Moon won't be setting this year, but you'll at least have a highe!
r radiant elevation for the  predicted outbursts on the 17th and the 18th.
 
> Also, with a last-quarter moon in the midst of the action, what is the
> expected visible rate if the ZHR is >100?

That depends on just how much your limiting magnitude is affected by the Moon. The ZHR is standardized to LM=6.5. During the Perseids, I have seen LM=6.0 or better with a quarter Moon. Assuming an r-index of 2.3 for the Leonids, if the limiting magnitude is 5.5, you can divide the ZHR by 2.3 to find the effect of limiting magnitude. If it is 6.0, you divide by 2.3^0.5=1.5. YOU ALSO HAVE TO CORRECT FOR THE RADIANT ELEVATION, which will knock rates down even further. For example, if the ZHR is 200, you are observing in LM=5.5, and the radiant elevation is 20 degrees, you can expect an observed rate of:

200 divided by

2.9 (correction for radiant elevation; 1/sin(20))

=68.4 divided by

2.3 (correction to LM=6.5; r^1)

=~30

Not exactly spectacular stuff, even though the ZHR is high. Of course, who's to say the ZHR won't be a lot higher (or lower)? In the West, all of the predicted outbursts occur when the radiant is very low in the sky. Things are a little better for you, farther east in the US.

--
Wes Stone
Portland, Oregon

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