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What’s a Zenithal Hourly Rate? Meteor Shower Terminology 101.

Doubtless you’ve heard astronomers and meteor shower observers kick around terms such as “bolide,” “sporadic” and “Zenithal Hourly Rate” when it comes to showers like this weekend’s Perseids. Like any field of endeavor, these terms and phrases and help to describe what we see (or expect to see) and aren’t just designed to make us unpopular at cocktail parties. Here’s a quick rundown on terms that should be in your meteor watcher’s lexicon; use em’ to impress (or annoy) your friends while you watch for this weekend’s Perseids;  

Meteor Stream: Meteors are part of a vast trail of cometary debris that orbits the Sun. Where a particular trail intersects the path of the Earth, a meteor shower occurs. As of this writing, the International Astronomical Union recognizes 64 established meteor showers, but only about six are intense enough to typically generate more than about a dozen meteors per hour at their peak.

Zenithal Hourly Rate: Typically abbreviated “ZHR,” the Zenithal Hourly Rate is an estimated ideal of how many meteors per hour you can expect to see from your location. This assumes that the radiant for the shower is directly overhead, your sky is pristine, your horizon is clear, & you have an omnipresent view in all directions. Of course, these factors are never optimal, and thus the actual observed number of meteors is almost always lower that the ZHR. In particular, a bright Moon can extinguish many fainter meteors.

Radiant: This is the imaginary point in the sky that meteors in a particular shower appear to be coming from. In the case of the Perseids, the radiant lies in the northern portion of the constellation Perseus (meteor showers receive their often unwieldy names from the constellations they appear to radiate from) and drifts towards the constellation Camelopardalis through mid-August. Thankfully, they’re not known as the Camelopardalids!

Total Effective observation time: This is often abbreviated at “TEFF” on meteor reports; this is the amount of effective time that an observer literally had “eyes on the sky” observing and counting meteors. Some observers and reporting systems use complex equations to calculate actual TEFF, but a good simple rule of thumb is “eyes on the sky, timer on,” and “eyes of the sky, timer off.”

Sporadics: If a meteor doesn’t trace back to the radiant of a particular active shower, it may well be a sporadic or random meteor. The typical rate for sporadics throughout the year never peaks much above five per hour. There are also six known sources of sporadic meteors; the helion (sunward), antihelion (anti-sunward), the northern & southern Apex (just above and below the ecliptic in the direction of the Earth’s motion) & the poorly understood northern & southern toroidal points high above and below the ecliptic.

Meteor Storm: No, this isn’t the name of a bad made-for-SyFy TV movie… very occasionally, the Earth rams into one of the great clumps of debris that litter known meteor streams. Though the rate at which a “shower” becomes a technical “storm” is a bit murky, the generally accepted informal level is an estimated ZHR of greater than 1,000. The November Leonids are known to have initiated some of the greatest meteor storms in history, as happened in 1933, 1966, & 1998 and may happen again in 2032 or 2033. Streams do evolve over time and may grow more intense or trickle out, as is the case of the once storm-producing but now defunct Andromedids.

Bolide: Though this term is somewhat unofficial, a bolide occurs when a bright meteor disintegrates in our atmosphere. A very rough standard for a bolide is a fireball of a visual magnitude of -14 or brighter and a super-bolide is maybe -17 or brighter; for some idea of scale, the Full Moon equals magnitude -13, and the measurement level of one lux is about -14. It’s rare to have a fireball of this intensity associated with a meteor shower, as even the largest meteors are mere dust grains.

Meteor Train: The streak that you’re seeing from a Perseid is not the dust grain itself, but a trail of incandescent gas as it burns up in the atmosphere. Particularly bright fireballs may leave a persistent smoke trail that stays visible for many minutes, and it’s worth keeping a pair of binoculars on hand to examine these.

Audible Meteors: Like “corkscrew” and “colored meteors,” this is a hotly disputed phenomenon. Some observers will swear that there’s an audible “hissing” or crackling component that accompanies some of the brightest meteors. Like reports of audible aurorae, many researchers have chalked this up to a purely psychological effect; the eye records a spectacular flash, and the brain reciprocates by assigning it an expected sound. Plus, most meteors occur about 50-60 miles up in the atmosphere where the air is too thin to transmit sound. A possible physical component was documented by Japanese observers in 1988, and it’s thought that electrophonic sound may be setting up a localized low frequency radio field using wires, poles or even wet blades of grass.

Radio Meteors: Did you know that you can “observe” meteors on your FM dial? The ionized gas from a meteor trail can cause “pings” similar to lightning storms; this can be heard by tuning an FM radio to an unoccupied frequency, and can also be picked on the lower channels of an old-school CRT television sets (that’s as in Cathode Ray Tube, used back in the olden days before plasmas and flat screens). A meteor trail may even briefly “focus” a distant radio station, causing it to fade in and out. Several showers such as the Daytime Arietids were discovered in this fashion. Meteorwatch.org also is excellent system to track and listen to radio meteors, and be sure to Tweet the ones you see this weekend or anytime to #Meteorwatch!

 

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