In astronomy, the zenithal hourly rate (ZHR) of a meteor shower is the number of meteors a single observer would see in an hour of peak activity if it was at the zenith, assumed the conditions are excellent (stars visible up to magnitude 6,5). The rate that can effectively be seen is nearly always lower and decreases the closer the radiant is to the horizon.
The formula to calculate the ZHR is:
\( {\displaystyle ZHR={\cfrac {{\overline {HR}}\cdot F\cdot r^{6.5-lm}}{\sin(h_{R})}}} \)
where
\( \overline{HR} = \cfrac{N}{T_{eff}} \)
represents the hourly rate of the observer. N is the number of meteors observed, and Teff is the effective observation time of the observer.
Example: If the observer detected 12 meteors in 15 minutes, their hourly rate was 48. (12 divided by 0.25 hours).
\( F = \cfrac{1}{1-k} \)
This represents the field of view correction factor, where k is the percentage of the observer's field of view which is obstructed (by clouds, for example).
Example: If 20% of the observer's field of view were covered by clouds, k would be 0.2 and F would be 1.25. The observer should have seen 25% more meteors, therefore multiply by F = 1.25.
\( r^{6.5-lm}
This represents the limiting magnitude correction factor (Population index). For every change of 1 magnitude in the limiting magnitude of the observer, the number of meteors observed changes by a factor of r. Therefore, this must be taken into account.
Example: If r is 2, and the observer's limiting magnitude is 5.5, the hourly rate is multiplied by 2 (2 to the power 6.5–5.5), to know how many meteors they would have seen if their limiting magnitude was 6.5.
\( {\displaystyle \sin(h_{R})} \)
This represents the correction factor for the altitude of the radiant above the horizon (hR). The number of meteors seen by an observer changes as the sine of the radiant height.[dubious – discuss]
Example: If the radiant was at an average altitude of 30° during the observation period, the observer's hourly rate will need to be divided by 0.5 (sin 30°) to know how many meteors they would have seen if the radiant was at the zenith.
See also
List of meteor showers
References
External links
North American Meteor Network (NAMN)
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Meteor showers
By name
Andromedids Antihelion Source Aquariids
eta Aquariids kappa Aquariids North delta Aquariids North iota Aquariids South iota Aquariids Southern delta Aquariids tau Aquariids Aquilids
South June Aquilids Arietids
Daytime eps. Arietids Daytime May Arietids Aurigids
alpha Aurigids delta Aurigids Nov. iota Aurigids Bootids (June Bootids) Canis-Minorids Cancrids (delta Cancrids) Capricornids
alpha Capricornids Daytime Capri.-Sagitt. Carinids (eta Carinids) Cassiopeids (beta Cassiopeids) Centaurids
alpha Centaurids omicron Centaurids Cetids
omega Cetids pi Cetids Comae Berenicids Cygnids (kappa Cygnids) Doradids (gamma Doradids) Draconids Eridanids (eta Eridanids) Geminids (epsilon Geminids) Herculids (tau Herculids) Hydrids (sigma Hydrids) Leonids Leonis Minorids (Dec. Leonis Minorids) Lyrids
eta Lyrids June Lyrids Monocerotids
alpha Monocerotids Normids (gamma Normids) Orionids Pegasids Perseids
Daytime zeta Perseids September epsilon Perseids Phoenicids (July Phoenicids) Piscis Austrinids Puppids (pi Puppids) Puppid-Velids Quadrantids Scorpiids
alpha Scorpiids North omega Scorpiids South omega Scorpiids Serpentids (kappa Serpentids) Sextantids (Daytime Sextantids) Taurids
Daytime beta Taurids Northern Taurids Southern Taurids theta Centaurids Ursids Virginids
alpha Virginids gamma Virginids eta Virginids theta Virginids iota Virginids lambda Virginids mu Virginids pi Virginids psi Virginids March Virginids
By peak date
January 4 February 8 February 10 March 14 April 17 April 22–23 April 26 May 6 June 7 June 27 July 28–29 July 13 July 30 July 31 – August 1 August 13 September 1 October 8 October 10 October 21 October 24 November 9 November 12 November 18 November 21–22 December 5–6 December 13–14 December 15 December 22
Related topics
American Meteor Society Earth-grazing fireball International Meteor Organization Meteor procession North American Meteor Network Population index Radiant Zenithal Hourly Rate
Hellenica World - Scientific Library
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