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Celestial showers: catch these displays of "falling stars" throughout the year.

Many summers ago, when I was just a stargazing novice, I joined an astronomy club in my hometown of Ottawa, Ontario. My fellow club members urged me to take up meteor observing. They said that watching meteors would be a fun activity requiring no expensive gear, and I could continue learning the constellations as I went along. They were right on all counts.

My introduction to the world of meteors came on the night of August 12th, when the Perseid meteor shower (which I'd never even heard of before) produced hundreds of falling stars. Dozens of them were gleaming light-sabers that cut the starry sky in two. Sometimes a Perseid fireball brighter than Venus would explode with a flash that instantly illuminated the night. What magic!

I soon learned that several other meteor showers recur on an annual basis. Decades later they remain old favorites that I try to watch every year. I'll explain how you, too, can enjoy these meteor feasts--but first a little scientific background is in order.

The Facts on Falling Stars

Obviously, stars don't fall to Earth. But bits of space debris called meteoroids do. Every day millions of these interplanetary particles, most of them smaller than a kernel of corn, enter Earth's atmosphere at high speed (typically between 10 and 40 miles per second) and vaporize. As the meteoroid incinerates, the surrounding air becomes superheated and glows for a second or two. The resulting burst of light we see in the sky is a meteor, popularly called a falling star or shooting star. Bright meteors frequently leave a long, glowing train (trail) that takes a few additional seconds to fade away.

Scientists estimate that at least 100 tons of space dust vaporize in our atmosphere every day. Yet watching the sky most nights, you'll notice only a few meteors per hour. This background, or sporadic, activity may not seem like much, but the occasional meteor slicing across the sky always garners a shout from appreciative stargazers. The shouts become more frequent when a meteor shower occurs. During a shower, dozens, or even hundreds, of meteors rain down in a single night. The rate of meteor activity typically increases for several nights leading up to the main event, then decreases afterward.

A shower's meteors can appear anywhere in the sky, but if you trace their streaks backward you'll find that they appear to come from a small area, called the radiant. The main showers are named after the constellations that host their radiants. For instance, the Perseid shower in August radiates from a point in northern Perseus; the Leonids in November issue from northwestern Leo; and the Geminids in December emanate from northeastern Gemini. The existence of specific radiants is a clue that each shower has a single, identifiable source in space. In most cases, that source is a comet.

When a comet swings close to the Sun, its icy surface heats up and releases clouds of gas and dust. Over time this material spreads out along the comet's orbit. Whenever the Earth plows across that path, it sweeps up a portion of the dusty debris. The flakes of comet dust follow parallel paths into our atmosphere, but their incandescent streaks appear to radiate from a single point in the sky--the radiant--for the same reason that snowflakes seemingly radiate from a single point on the highway when you drive through a snowstorm. (This effect also occurs in a rainstorm, but it's not as dramatic.)

Showers Great and Small

Not all meteor showers are created equal. The Perseids and Geminids, each of which can deliver roughly 100 meteors per hour, are the two most powerful. The Quadrantid shower in January is also strong, but very brief. Several second-tier events, such as the Lyrids in April and the Orionids in October, yield significantly lower rates, typically about 20 per hour. The remaining annual showers usually aren't much more active than the background sporadic rate. In the table on page 77, the maximum rate listed for each meteor shower indicates what an attentive observer might witness under a clear country sky. But beware: the devil is in the details!

A shower's impact depends on good weather, yes, but also on three main astronomical factors. Chief among them is the whereabouts of the Moon. Moonlight can greatly reduce the strength of a meteor display--even the powerhouse Perseid and Geminid showers--and we'll see this effect at work in 2016. On the big Perseid night of August 11-12, a waxing gibbous Moon won't set until around 1 a.m. (local daylight time), leaving only a few dark hours for meteor spotting. The Geminids of December 12-13 will be worse off, as the glaring full Moon will be up all night. Lunar phases are included in the table on page 77.

The second key factor concerns exactly when the Earth crosses the mid-point of a meteor stream. The concentration of particles is usually much greater along the stream's central portion, and that dense middle is often not very wide. In the case of the Perseids, for example, the Earth plows across the middle in a matter of hours. It's during that short period that the meteor activity peaks. However, you get the full dose only if the shower peaks when it's nighttime at your longitude.

And if that special time occurs after local midnight (with no Moon), so much the better.

My "after midnight" comment leads to the third main factor, which I'll call radiant height. If a shower's radiant is barely above the treetops, a lot of the action will be lost below the horizon. A few radiants are located in southern constellations that ride low in our North American skies. Fortunately for us, though, most radiants are farther north and climb high. All other things being equal, the frequency of meteors in a shower is greatest when the radiant is highest. On each date in our table, the shower radiant reaches its height after midnight.

Meteor Watching is Cool

Meteor watching is a relaxing pastime that can be enjoyed quietly on your own or with a group of enthusiastic friends. Either way, you'll see more meteors if you lie back on a garden lounge chair and take in the show more intently. For best results, look halfway up the sky, away from any nearby lights or large obstructions such as houses or trees. Meteors will be visible in every direction, but if you want to see longer trails look well away from the shower's radiant point.

Is it worth observing when there are no main showers? Sure, especially in the second half of the year when meteor traffic is generally strongest. I recall one moonless autumn night when several minor showers combined with "sporadics" to produce after-midnight rates of 30-plus per hour. Just bear in mind that an autumn meteor watch can be a "cool" experience --particularly if you live in Canada or the northern United States. Dress for the season (I bundle up in a sleeping bag). A thermos bottle of coffee or hot chocolate will help keep you going as the hour grows late and the temperature drops.

Remember: on any given night of observing you never know what will happen. The motto of the meteor maven is "keep your eyes peeled." If you take that advice to heart, you're bound to catch some falling stars this year. Good luck!

Sky & Telescope contributing editor Ken Hewitt-White has been observing meteor showers on and off since 1967.

Major Meteor Showers in 2016

                   Morning          Radiant          Peak rate, ideal
Name               of peak         location            conditions *

Quadrantids        Jan. 4       Northern Bootes           60-100
Lyrids             Apr. 23           Lyra                 10-20
Eta Aquariids       May 5    Water Jar of Aquarius        20-60
Delta Aquariids    July 27         Aquarius                 20
Perseids           Aug. 12     Northern Perseus           60-80
Orionids           Oct. 21      Northern Orion            10-20
Leonids            Nov. 17       Sickle of Leo            10-20
Geminids           Dec. 14     Castor in Gemini            100

                     Parent comet           Moon
Name                 (or asteroid)          phase

Quadrantids         2003 [EH.sub.1]    waning crescent
Lyrids             1861 G1/Thatcher         full
Eta Aquariids          1P/Halley             new
Delta Aquariids      96P/Machholz       last quarter
Perseids           109P/Swift-Tuttle   waxing gibbous
Orionids               1P/Halley       waning gibbous
Leonids            55P/Tempel-Tuttle   waning gibbous
Geminids             3200 Phaethon          full

* The likely peak zenithal hourly rate (ZHR): the number a
single observer would see per hour If the radiant were at
the zenith and the sky were dark enough for magnitude-6.5
stars to be seen.
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Title Annotation:METEOR SHOWERS OF 2016
Author:Hewitt-White, Ken
Publication:SkyWatch
Date:Jan 1, 2016
Words:1399
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