Getting to know meteor terms
Everything you need to know about meteor terminology
Introducing Zenithal Hourly Rate (ZHR)
Zenithal Hourly Rate (ZHR) is used to normalise meteor shower rates for comparative purposes and isn’t intended to represent expected visual rates.
The term assumes a shower’s radiant is directly overhead (at the zenith), a perfectly clear, dark sky with a limiting magnitude of +6.5 and no visual obstructions. Few, if any of these conditions will be met in reality, so the actual number of meteors seen is often significantly lower than the quoted ZHR.
Visual hourly rate (Nv)
An observer’s visual hourly rate is the number of shower meteors recorded per hour. If the observing period is less than an hour, the count should be multiplied by 1 over the hour fraction (T). For example, seven shower meteors over 15 minutes (T = 0.25 hours) gives a visual hourly rate of 7 ÷ 0.25 = 28 meteors per hour.
Field of view correction (F)
In an ideal world, you’d be looking at a totally clear sky. In reality, foreground objects or passing clouds get in the way. F corrects for this, calculated as
1 ÷ 1–k, k representing the fraction of sky lost
(0 = clear, 1 = obscured). If a one-third loss of sky is noted, F works out as 1 ÷ 1–0.33 = 1.5.
Population index (r)
A shower’s population index (r) indicates the average dimness or brightness of its trails. A value below 2.5 indicates more bright meteors than average, while values below 3.0 indicate the shower has a larger proportion of fainter trails.
Values for r can be obtained from various sources such as the International Meteor Organisation (www.imo.net).
Limiting magnitude (nelm)
Naked-eye limiting magnitude (nelm) is a way to assess how clear your sky is by monitoring the faintest stars visible using nothing more than your eyes. For typical ZHR calculations, a limiting magnitude of +6.5 is assumed. Moonlight can greatly affect the nelm value. Several magnitudes are lost when a bright Moon is present.
Radiant altitude (Hr)
The meteor radiant’s height makes a big difference to the number of meteors seen. A low radiant altitude will produce trails that will either occur below the horizon or be seriously dimmed by the thicker layer of atmosphere near to the horizon. The value is measured in degrees.
Rate variation over time
Earth entering the outer edge of a comet’s debris stream marks the start of shower activity. The shower’s ZHR will be low at this point, increasing to maximum as Earth moves ever deeper into the stream. The peak is related to Earth’s orbital position
defined by the Sun’s ecliptic longitude (λ ). The
rise to maximum may result in a narrow, sharp peak lasting only a few hours. Alternatively, if the densest part of the stream is wide, the peak may last for days.
In addition to this natural rate of variation, on any
given night the number of meteors seen fluctuates
according to radiant altitude and whether you’re observing before or after midnight local standard time (UT for the UK). Before midnight, meteoroids play catch up to enter the atmosphere. After midnight, Earth turns so trails are the result of meteoroids colliding head on with the atmosphere. This raises the collision energy, resulting in brighter
and, consequently, visually more numerous trails.
Naked-eye limiting magnitude (nelm) is a way to assess how clear your sky is by monitoring the faintest stars visible to just your eyes