Perseids on August 12-13

7 Aug 2007

      Kim, from an old post of mine on this topic. - Kurt 

alpha =  altitude of the radiant above the local horizon at a local time * pi() / 180 
hr_alpha = zhr * sin(alpha) 

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Conversion of ZHR to hourly rate off-zenith 

The zenith hourly rate is the rate observed when the shower is at local observer's zenith.  The International Meteor Organization gives the following information regarding when the meteor shower's radiant is not on the observer's local zenith: 

"Table 1 gives the number of meteors an observer might expect to see at different radiant elevations if the ZHR amounted to the 
impressive value of 100. Now you can imagine what you will see with a radiant elevation of 10° if the ZHR is not as impressive as 
100. Therefore, when planning your watch you should take care that the radiant will have a reasonable elevation for the whole watch. 
For several reasons the lower limit is about 20°. 

Table 1: Numbers of meteors visible per hour for a ZHR=100 shower and different radiant elevations h (limiting magnitude of 6.5m) 
h in °  |  90  |  70  |  50  |  40  |  30  |  20  |  10 
n        | 100 |  94  |  77  |  64  |  50  |  34  |  17  " 

Source: IMO.  http://www.imo.net/visual/major01.html 

IMO Table 1 appears to follow a simple sin function - 
alpha =  altitude of the radiant above the local horizon at a local time * pi() / 180 
hr_alpha = zhr * sin(alpha) 

where hr_alpha is the predicated hourly rate at the selected altitude of the radiant above the local observer's horizon.

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Kurt Fisher

Kim

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