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Re: (meteorobs) ZHR and HR confusion
At 18:31 1998-10-12 +0000, you wrote:
>
> 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
Tsja: soms is het goed weer wat voorlichting te verzorgen.
References: