meteors - sky and telescope ebook

8/13/2019 Meteors - Sky and Telescope eBook

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Shooting Stars:

The history art and science of meteor-watching


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SKY & TELESCOPE 1

Discovering the Perseid MeteorsPrior to 1837, nobody realized the Perseids were an annual event.

Edward Claudius Herrickwas a bookworm.His father was a Yale graduate and founderof a girls' school. His mother was adescendant of one of Yale's founders. TheHerricks lived in New Haven, Connecticut,the home of Yale.

But young Edward did not go to Yale. Hedid not go to college. His parents felt that hischronic eyelid inflammation would keep himfrom succeeding in higher education. So in1827, at the age of 16, Edward became aclerk in a bookstore that served Yale studentsand faculty and was also the college'spublishing house.

Everyone in New Haven with intellectualinterests stopped by the bookstore, andHerrick reveled in conversations with

professors such as astronomer DenisonOlmsted and chemist Benjamin Silliman. Theyoung clerk worked hard, and by age 24 hehad become one of the bookstore's owners.Unfortunately, business stalled over the nextthree years and left Herrick broke.

On the evening of August 9, 1837, just ashis business was teetering toward collapse,Herrick observed an unusual number of

meteors in the night sky. From people whohad stayed up very late that night, he heardthat the meteors were even more numerousand brilliant after midnight.

American astronomers in 1837 were stillgripped by the excitement of the epic meteor

deluge that had taken place four yearsearlier. On the night of November 12-13,1833, more than a thousand shooting starsper minute had been seen radiating from theconstellation Leo.

Astronomers had been taken completelyby surprise; it was the first time that most ofthem had paid attention to meteors at all.They were especially startled by Olmsted'sdemonstration that the shower's meteorsmust have been flying together in parallel

from a distant region of space. Mostastronomers had believed that meteors weremere atmospheric phenomena, to be ignoredlike clouds and weather.

Now astronomers were searchinghistorical records and turning up accounts ofprevious mid-November meteor showers. Butto observe abundant meteors in August?That seemed odd.

In 1998, four brilliant Leonids streaked across Hydra, Canis Minor, and Orion in Italy. The radiant is in the Sickle of Leo at far left.

LorenzoLovato


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YaleUniversity

A Second Annual Meteor Shower?Herrick, alert andopportunistic, startedlooking for reports thathis August 9th displayhad been seen fromplaces other than NewHaven. He alsosearched historicalsources for evidencethat August meteorshad been seen inprevious years around

the same date. He found seven cases, from1029 in Egypt to 1833 in England.

Following Olmsted's example, Herrickwrote an article for the January 1838 issue of

Silliman's American Journal of Science andArts in which he proposed the existence of asecond annual meteor shower. He listed hisevidence and asked for information fromanyone else who had seen the display.

While his first article was beingpublished, Herrick turned up four moreaccounts from other years of meteorsplummeting through the skies on August 9thor 10th. He was convinced.

"There generally occurs on or about the9th of August in every year," he wrote, "aremarkably large number of shooting stars."

In a second article, Herrick drew furthercorrect conclusions:

The August meteors remain near theirpeak for about three days, and off-peakmeteors span perhaps two weeks.

Like those of November, the Augustmeteors have a "starting point," a spot inthe sky from which they seem to radiate.

Herrick could not yet fix its positionamong the stars; the meteors were notabundant enough to make their radiantobvious.

The August meteors are more numerousthan those of November almost everyyear, except on rare, overwhelming

occasions when the November meteorspour down in torrents

In addition to November and August,there was probably a third annual meteorshower around April 30th (now called theLyrids). Herrick found only three cases ofitin 1095, 1122, and 1803but mighty

storms of meteors had come in late Aprilof those years.

Herrick discarded notions that meteorswere meteorological, like lightning orrainbows, or debris falling back to Earth afterthe eruption of a volcano. "Shooting stars arewithout doubt cosmical or celestial bodies,"he wrote, "and not of atmospheric orterrestrial origin." What could that source be?

"It is not impossible," Herrick wrote, "that

these meteoric showers are derived fromnebulous or cometary bodies which, at statedtimes, the earth falls in." Here was anothercorrect insight, one that Olmsted hadbroached. The hypothesis that meteors havea cometary origin was confirmed 28 yearsafter Herrick's article, when the connection

Edward C. Herrick

(1811-1862)

This early color illustration depicts the Leonid meteors over

Niagara Falls during the 1833 shower over North America.

EdmundWeik/UniversityofVienna,Austria


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between meteor and comet orbits wasdemonstrated.

Pleased with his achievement, Herrickwrote up the new evidence for his annual

August shower, included his theories about ofmeteors, and gave the paper to Silliman forpublication in his journal. Less than twoweeks later Herrick received crushing news.He was not the shower's discoverer after all.

Parallel DiscoveriesAs so often happens in science, others wereworking along the same lines independently

just months apart. The 1833 Leonid stormhad galvanized interest in meteors, and thetime was ripe. Adolphe Quetelet, a Belgianstatistician and founder and director of theBrussels Observatory, had mentioned mid-

August meteors very tentatively six monthsearlier. His attention had been called tometeors by Franois Arago of France, whodominated European science at the time withhis skill in discerning important scientificproblems and suggesting experiments tosolve them. What, asked Arago in the wakeof the 1833 display, constituted a shower ofmeteors, and what was the rate of theordinary, everynight drizzle?

By the following year, Quetelet hadaccidentally found records in his observatoryof exceptional meteor displays on August10th of 1834 and 1835 to accompany theincrease he had seen in 1836. He called forscientists at the March 4, 1837, session ofthe Royal Academy of Brussel to watch thesky on August 10, 1837.

Herrick stumbled onto another annualmeteor shower that occurred aroundDecember 7th, the Andromedids or Bielids

(so named much later, in 1872, for theirassociation with Biela's Comet). This wasfun, discovering meteor showers. Herrickcalled for worldwide observations all nightand year round. He also offered somepractical advice. "Shooting stars must alwaysbe watched in the open air: observationsthrough a window can not be trusted."

When the following August (1839) camearound, Herrick and three friendsconcentrated on determining the radiant oftheir favorite shower. They concluded that the

August meteors appear to come fromPerseusand were right.

But, yet again, Herrick's discovery wasnot the first. This time he was five years toolate.

Locke and the PerseidsJohn Locke, aphysician andhigh schoolheadmaster inCincinnati,Ohio, was thefirst person to

discover thatthe Augustmeteor showercomes from aradiant inPerseus. On August 11, 1834, the CincinnatiDaily Gazette published a letter to the editorfrom John Locke, headmaster of a girls'school. (Locke was about to begin, at age 43,a highly productive career as a physicist,geologist, and scientific instrument maker.)

Locke had seen a meteor shower on theevening of August 9th and, impressed withOlmsted's writings on the radiant of theLeonid storm less than a year earlier,watched the display carefully and detectedthat it too had a radiant. It was in Perseus(true), near the star Algol (about 17too farsouth).

Locke's letter in a small newspaper onthe Western frontier went completelyunnoticed. But he read and contributed to the

scientific journals and was angry whenHerrick and Quetelet gained acclaim fordiscovering both the August meteors andtheir radiant. He wrote to Silliman (for whomhe had once worked as a lab assistant)claiming credit and snubbing the later

Archives&RareBooksDept.

UniversityofCincinnati

John Locke (1632-1704)


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discoverers. Silliman passed the letter toHerrick, who immediately wrote up a noticefor the American Journal of Science and Artsacknowledging Locke's observations. So nowthere were three independent discoverers ofthe Perseid meteor shower.

Well, not three after all, it turned out.Thousands.

The Tears of Saint LawrenceThe earliest discoverers of the Perseids wereanonymous, and their feat lay buried in anEnglish farmer's almanac. Both Quetelet andHerrick chanced upon it. Bravely, Herrickacknowledged, "The annual occurrence of ameteoric display about the 10th of Augustappears to have been recognized for a verygreat length of time." Thomas Furley Forster

of London had recorded it in 1827 in hisPocket Encyclopaedia of NaturalPhenomena. "According to Mr. T. Forster,"Herrick reported in October 1839, citingQuetelet, "a superstition has 'for ages' existedamong the Catholics of some parts ofEngland and Germany that the burning tearsof St. Lawrence are seen in the sky on thenight of the 10th of August; this day being theanniversary of his martyrdom.

Saint Lawrence was tortured and killed inRome on August 10, 258, during the reign ofthe anti-Christian emperor Valerian. "Thepeasants of Franconia and Saxonyhave believed for ages past that St.Lawrence weeps tears of firewhich fall from the sky everyyear on his feet (the 10thof August)," Herrickwrote, quoting aBrusselsnewspaper. "This

ancient popularGerman tradition orsuperstition has beenfound within these [past]few years to be a factwhich engages the attentionof astronomers."

Herrick never seemed bitter about beingrepeatedly upstaged. He continued to tendhis August meteors with great faithfulnessand to report their activity in Silliman's journalall the remaining years of his life.

In 1838, soon after his first scientificarticles appeared in print, Herrick lost hisbookstore. But Yale was so impressed by hisscholarship that it awarded him an honoraryMaster of Arts degree. Five years later, Yalebuilt a new library and made Herrick collegelibrarian. It was a pleasant irony for a manwhose eye trouble had kept him from collegeand who had complained about New Haven'spoor libraries. Herrick spent the next 15 yearsvigorously developing the Yale librarycollections. He never married. He never tooka vacation. Later he assumed the duty of

writing and publishing Yale's obituaries ofgraduates and faculty. Herrick was soorganized and efficient that he wrote his owndeath notice a few days before he died in1862 at the age of 51.

By: Mark Littmann

Bottom: Russel Sipe (1985)

Mid-left: J. F. Funderburg (1993

Upper right: Fred Bruenjes (200

Perseids of the past.


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DecipheringMeteor-eseAn introduction to all words meteor-related.

At one time or another, almost everyonehas glimpsed a swift little streak of lightdashing across the night sky. Thesesudden celestial visitors are meteors,commonly called falling or shooting stars.Meteors are pieces of space debris thatplow into the Earth's atmosphere. Becausethey arrive at very high speedsanywherefrom 11 to 74 kilometers (7 to 46 miles) persecondthey vaporize by air friction in awhite-hot streak. Most meteor parents(meteoroids ) range in size from sand

grains to pebbles. Occasionally, a largerobject will survive its descent and fall toEarththen it's called a meteorite.

A meteor that appears brighter than anyof the stars and planets is called a fireball.The sudden appearance and fast motion ofa bright meteor produces an illusion ofcloseness that can fool even well-trainedprofessionals. Airline pilots have swerved toavoid meteors that were actually 160kilometers (100 miles) away.

Most meteors are seen 80 to 120kilometers (50 to 75 miles) above theground. Occasionally, someone will claim tosee a fireball land just beyond a tree or ahilltop, but in fact a typical fireball firstappears at a height of about 125 kilometers(80 miles) and loses its brightness while stillat least 20 kilometers (12 miles) above theground.

Much more abundant are smaller,

everyday meteors. While most look white,some appear blue, green, yellow, orange,or red. One that explodes at the end of itsvisible flight is called a bolide.

Meteor ShowersAt certain times of the year we see

more meteors than usual. This happens

when Earth passes near a comet's orbit andsweeps through debris that the comet has

shed. Such events are called meteorshowers. For the major annual meteorshowers, seeing one meteor every fewminutes is typical, though there are oftenbursts and lulls.

Shower meteors can appear anywherein the sky, but their direction of motion isaway from the constellation whose namethe shower bears. This apparent point oforigin is known as the radiant. Someobservers feel that the best place to watch

is between a shower's radiant and thezenith (the point directly overhead). Ingeneral, you'll do best by watching thedarkest part of your sky, wherever you maybe.

All you need to observe these celestialdisplays are a dark sky, a way to staycomfortable, and a little patience. Lightpollution or moonlight will drastically reducethe number of meteors you see, so planaccordingly. Give your eyes at least 15

minutes to adjust to the dark. Make yourselfcomfortable with a reclining lawn chair,sleeping bag, snacks, music, the companyof other stargazers, or whatever will helpyou remain interested enough to keep youreyes turned toward the sky.By: Paul Deans

This meteor from the 1998 Leonid shower in Kansas is as bright as

the full moon.


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The Basics of Meteor ObservingEverything you need to know to get started.

Meteor watchingis one of the easiest formsof astronomy. Anyone can go out in theearly morning hours, lie back in a loungechair, and wait for the occasional shootingstar. Plan to start watch around midnight.By then the radiant of most showers will behigh enough above the horizon. The hour ortwo before dawn should be best of all.

Bring a reclining lawn chair to a darksite with an open view of the sky. No trees

or buildings should intrude into your viewexcept maybe at the very edges.Depending on the time of year, you maywant to bring a sleeping bag for protectionagainst the cold, dew, and mosquitoes.You'll also need a watch and a dim, red-filtered flashlight to read it by. You canmake notes with a clipboard and pencil, butmuch better is a tape recorder with amicrophone switch. This way you candictate notes in the dark without taking your

eyes off the sky.

Give your eyes at least 15 minutes toadjust to the dark. Settle in, look up, andrelax. When you're ready to begin watchingsteadily, note the time to the nearestminute.

The simplest project is just to count thenumber of "shower" (S) and "non-shower"(NS) meteors that you see. Showermeteors will seem to come from the radiantof the particular shower you are observing.The name of the shower will tell you thegeneral location of the radiant. Forexample, the Perseid meteor shower'sradiant is in the constellation Perseus.

Trace the path of a meteor backwards

across the sky. If the line comes near theradiant, then you have observed a showermeteor. If the line goes elsewhere, then youhave observed a non-shower meteor.

Watch the sky at least 50up, and picka direction away from the radiant. Keepyour field of vision filled with sky. If objectsdo intrude, they should block no more than20 percent of your view.

ind the Skys Limiting MagnitudeWhile gazing and waiting, you'll have plentyof time to find the limiting magnitude in thepart of the sky you're watching. One way isto check the visibility of stars in and aroundthe Little Dipper (if you live at a northerlylatitude). Use the chart to find the visualmagnitude of the faintest star you can seewith the naked eye. Check again at least

Noting your sky's

limiting magnitude is

essential for makingmeaningful meteor

counts. Check the

visibility of stars in

and around the Little

Dipper (if you live at a

northerly latitude) and

find the visual

magnitude of the

faintest one you can

see with the naked

eye.

Sky&Tele

scope


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once every hour to track subtle changes insky conditions, always noting the time.Even a small change in sky clarity has a bigeffect on the number of meteors you see.

Most observers like to take a break

once an hour to get up, move around, andhave a cup of coffee. Note the beginningand end times of each break. If you'rewriting, also record how much time youspend looking down at your clipboard torecord a meteor if this amounts to morethan a few percent of the total. Count howmany seconds your note-taking requiresper meteor; you may be surprised at howmuch time it adds up to.

Even if you observe without a break,

separate your records with a timeannotation at least once an hour. A watchthat beeps on the hour will help remind you.

Also note the part of the sky where youspend most of the time looking.

For simple meteor observing andcounting, that's about it. If you want to go tothe next level check out our AdvancedMeteor Observing article on our website.

By: Alan M. MacRobert

Dark Nights for the PerseidsPlan to meteor-watch on the nights of August 11-12 and 12-13.

The Perseid meteor shower peaks this yearwhen theres almost no Moon, making thelate-night sky nice and dark for shooting-starspectators and counters. The thick waxingcrescent Moon sets in mid-evening. SomePerseids do appear during the evening, butthe shower is always better from about 11 ormidnight until the first light of dawn. This iswhen the showers radiant point, in northernPerseus, climbs high in the northeasternsky. Or to put it another way, this is when

your side of Earth turns to face theoncoming meteors more directly.

Earth should go through the thickest part

of the stream for many hours centeredaround 19h Universal Time August 12th,which is 3 p.m. on that date Eastern DaylightTime. That means the shower splits thedifference between the early-morning hoursof August 12th and 13th for North Americaand Europe. So the number you count oneach of those mornings, even in ideal

A bright Perseid streaked across southern Cetus around

moonrise in the Alamut Valley of Iran last August.


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conditions, maybe somewhat off theInternational Meteor Organizationspredicted peak of 100 visible per houreven if you have no light pollution and theradiant is near the zenith. But surprises canalways happen.

Two much weaker showers are alsoactive at this time of year, the Delta

Aquariids and Kappa Cygnids. And there arealways a few sporadics too. Keep all theseseparate from Perseids in your notes.

Find a spot with an open sky view andno bright lights. Lie back in a reclining lawnchair, bundle up in a sleeping bag againstmosquitoes and the late-night cold, andwatch the stars. Expect an average of

roughly a Perseid a minute under fineconditions, or fewer in light pollution.

If youd like to try making a reportablemeteor count for the IMO, follow themethods at imo.net/visual/major. You canwatch other observers counts accumulatealmost in real time at imo.net.

The Perseids are the ionization trailsmade by little debris bits from Comet109P/Swift-Tuttle streaking into Earths

upper atmosphere at 37 miles (60 km) persecond. The Perseids were especiallydramatic in the 1990s around the time ofSwift-Tuttles mostrecent return, but theyvesince reverted to normal. The comet isntdue back until around 2122.

By: Alan M. MacRobert

Hundreds of meteor observers counted Perseids last year

using the standardized methods of the International MeteorOrganization for intercomparison. They tallied 27,537

Perseids during 3,981 time intervals. As the resulting activity

curve here shows, the shower remains active for several

days before and after its peak. The zenithal hourly rate is the

corrected number that someone would have seen if the

showers radiant were near the zenith in an unobstructed sky

with 6.5-magnitude stars visible. Vertical bars show the

statistical uncertainty for each combined data point.

TheInternationalM

eteorOrganization

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