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;   (more…)

AstroEvents- Hunting things that “Flash” in the January Sky

Credit: David Dickinsen

2012 is here, and the world shows no sign of ending as the heavens spin on their appointed rounds high overhead. But the diligent observer may be rewarded with several unique an spurious sights, both natural and manmade…

1st up is everyone’s favorite meteor shower named after an obsolete constellation; the Quadrantids peak the morning of January 4th in what is the first large meteor shower of the year. The peak is very swift, only lasting about 12 hours or so and is centered this year on 2:00 AM EST/7:00 AM UTC. This favors the U.S. East Coast in 2012, as the 79% waxing gibbous Moon will set around 2AM local the morning of the 4th for observers in mid-northern latitudes. The radiant of the shower lies at a declination of 52° degrees north at the junction of the modern constellations of Draco, Bootes and Hercules, and thus activity may be visible pre-midnite local, although the setting of the Moon and the rising of the radiant will raise sighting prospects considerably. Expect swift-moving meteors headed outward from the radiant above the handle of the Big Dipper to appear anywhere in the sky. The Quadrantids have been known since the early 1800’s, but there has been much conjecture as to the source parent body. Astronomer Fred Whipple noted in 1963 that the stream bears some resemblance to the Delta Aquarids, and that the orbital path has undergone alterations by the planet Jupiter in the last few thousand years. In 2003, SETI researcher Peter Jenniskens proposed that the source may be then recently discovered asteroid 2003 EH1, which has been tentatively linked to Comet C/1490 Y1, which approached Earth at a distance of 0.52 Astronomical Units on January 12th 1491. Be sure to keep an eye out for Quadrantids on these chilly January mornings, as we commemorate Quadrans Muralis, a constellation that is no longer! (more…)

The Geminid Meteor Shower Rounds Off 2011

2011 has been quite a year, both terrestrial and otherwise. This week sees the last of the big scheduled astronomical happenings of the year in the form of the Geminid meteor shower.

This shower is one of the yearly standbys along with the Perseids that are always sure to produce. The Geminids have a long peak centered on the morning of December 14th when an idealized Zenithal Hourly Rate (ZHR) of up to 120 meteors per hour may be seen.

Problems will arise, however, from an 82% illuminated waning gibbous Moon in the adjacent constellation of Cancer. Rising roughly around 10PM local on the night of the peak, this makes for the worst possible Moon phase as it’ll be high and bright in the early AM hours, just as the meteor shower is getting into high gear. But as always, I wouldn’t let that stop you from looking! (more…)

What Are the Draconids? A Brief History

2011 Perseid+Jupiter Credit: David Dickinsen

Sure, you’ve heard of the August Perseids that come blazing across the sky and are invariably trumpeted by the news as the “Meteor Shower of the Millennium!” on a yearly basis. Perhaps you’ve even heard of the Leonids, normally a feeble November shower prone to legendary outbursts roughly every 33 years. But have you ever heard of the October Draconids?

I’ll admit I hadn’t until the October 1998 issue of Sky & Telescope arrived on my doorstep. Or should I say tent-flap? You see, the last half of 1998 found me deployed with my U.S. Air Force squadron to Al Jaber, Kuwait. In those days, email (and spam) were still a hip “new thing,” giant washing-machine-like CRT monitors adorned many office desks, and we were involved with a cat-and-mouse game between Saddam and the U.N. Inspectors that eventually became the shooting war of Operation Desert Fox. A military base in the desert was also a fine place to do some causal astronomy. The Leonids put on a fine show that year approaching storm levels of 1,000 per hour from our longitude. I remember mentioning the Leonid meteors to one of our F-16 pilots, and they later briefed not to mistake the fireball flashes for Iraqi AAA (an important distinction!) (more…)

The 2011 Perseids: A Survival Kit

Perseid Credit: science1.nasa.gov

A classic summer astronomical standby may be in trouble this year, but that shouldn’t stop you from looking. That’s right, we’re talking about the Perseids, that “old faithful” of meteor showers which sees northern hemisphere residents getting bundled up to camp out under the summer stars every mid-August.

Some of our earliest astronomical memories come from watching this very shower under the dark northern Maine skies of our 1970’s childhood… yes, the Perseids are public crowd-pleaser and even an occasional Yahoo-trending fave that can even knock the likes of Brittany & friends down to the number 2 slot for a 12 hour period… so, what’s this news of the Perseids being “troubled?” They haven’t been threatened by irate pop stars, have they?

Unfortunately, this year’s menace is a more inviolate force; the Full Moon. Yes, this year’s Perseid meteor shower peaks the morning of August 13th, mere hours before the Moon reaches Full status on the same day at 14:57EDT/18:57UTC. Instead of the usual respectable zenithal hourly rate (ZHR) of around 100 meteors per hour, observers can only expect to see a paltry few of the very brightest fireballs…

But all is not lost. With a little planning, a few factors can work in YOUR favor this year. Keep in mind that the Perseids have a very broad peak, and the radiant near the head of the constellation Perseus lies above the horizon all night for observers north of 32° latitude. Plus, this month’s Full Moon (known as a Sturgeon Moon) has a southerly declination of about -11° degrees; your best bet for catching Perseids this year may lie with watching several mornings prior, in the window of time between the setting of the waxing gibbous Moon and the beginning of local twilight… and yes, you CAN still watch a meteor shower during a Full Moon and see an occasional Perseid.

Generally speaking, the farther north in latitude you are located, the more likely you’ll be able to take advantage of these twin factors. Finding as dark a site as possible and putting something physically between yourself and the bright Moon is the key. A building or a hill makes an excellent “Moon-block.” Perseids are swift movers with an atmospheric entry velocity of about 61 km/sec.

The meteors are tiny grain-like debris shed by the comet Swift-Tuttle, and the shower occurs as the Earth intersects the path of the comet every mid-August. The Perseids are also famously known as the “Tears of Saint Lawrence” who was martyred on August 10th, and are probably the most well known of all meteor showers because of their annual dependability and their timely occurrence with the northern hemisphere summer vacation and holiday season.

Historic ZHR rates have always hovered around 60, but the past several years have seen an enhanced ZHR of +100. This shower is also infamous for sparking “Meteor Shower of the Millennium!” headlines annually, Although I have yet to see the same occur this year… a shower that MAY be a contender for said headline is the Giacobinids (or do you say Draconid?) meteors in early October 2011, an obscure shower that may dazzle this year… more on this to come in October.

Wonder what that the above graphic has to do with anything astronomical? You are looking at what is known as a nomogram, a graphical device that is a neat way to show interrelationships between factors. (Hey, how do you spend your weekend?) The graphic above was adapted from a nomogram featured in the November 2002 issue of Sky & Telescope; it shows the interrelationship between the two biggest factors that affect the zenithal hourly rate; namely, the radiant’s elevation above the horizon, and the limiting magnitude of your sky. (For a way “math-y-ier” dissertation on nomograms and the ZHR, click here).

This device will be your indispensible friend and secret weapon as you plot your meteor observing adventures for this or any shower. The ZHR is a theoretical limit; it assumes that you have absolutely perfect skies with a radiant directly overhead and an unwavering 360°degree view. Unrealistic, right? Well, it’s something to strive for. As you can see after playing around with the graphic a bit, the biggest killer of how many meteors you’ll see is how dark a sky you are under.

Even when the Perseid radiant is half way (45° degrees) in altitude towards the zenith, a limiting magnitude of +4 (as is typical around a Full Moon) means you’ll see roughly 10% of the zenithal hourly rate, i.e. 6-10 meteors per hour instead of 60-100. Yes, light pollution, both natural and manmade, is a real meteor shower buzz-kill. And keep in mind, other factors can conspire to lower that rate even further, such as obstructions on the horizon and the fact that a sole observer can only cover a limited swath of the sky visually. Still, the sight of a well placed fireball can be unforgettable and just plain pure magic to witness.

Do get out there on days leading up to this week’s Perseid meteors, and be sure to follow and report the magic via #meteorwatch on Twitter; they’re worth setting an early alarm for!

David Dickinsen



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