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(meteorobs) ZHR and HR confusion
Michael Boschat wrote:
>
> >How do the Japanese get such high ZHR's !! while others had less
> >numbers, like 40-100 for a ZHR?
I think there is perhaps a confusion here between ZHR and HR. The 40-100
Mike is referring to is the Hourly Rate observed, i.e. the real number of
meteors observed. This is not equal to the Zenith Hourly Rate, which is
reached by correcting the Hourly Rates observed for deviating limiting
magnitude and radiant altitude. A zenith hourly rate (ZHR) strictly
speaking is what an
'standard' observer would observe if he had the radiant in the zenith and
a limiting mgnitude of +6.5. If his Limiting magnitude is lower than +6.5
and/or the radiant altitude lower than 90 degrees, his hourly rate (HR, =the
number of meteors he observes) will be lower.
The Japanese observed with low radiant altitude and low Limiting
magnitude due to moonlight, so their HR is much lower than the actual ZHR
computed from it.
This is a constant source of confusion even among the professional
astronomers that are not meteor astronomers: note for example that HR's
are quoted as if it are ZHR's in the October 9 IAU circular, and that the
'ZHR' of 7000 quoted in many articles for the 1866 Leonids is actually a
HR (it is derived from a value of 120 meteors/minute observed from
Greenwich: but not corrected for the fact that the radiant altitude was
only 23 degrees! The resulting ZHR is almost 20 000 even if we assume a deep
Lm of +6.8-+6.9!)
- Marco Langbroek
Dutch Meteor Society
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