David Levy’s Guide to Observing Meteor Showers
Cambridge University Press
9780521696913 - DAVID LEVY’S GUIDE TO OBSERVING METEOR SHOWERS - by David H. Levy
July 4, 1956
Twin Lake Camp
Catch a falling star and put it in your pocket
Never let it fade away.
Catch a falling star and put it in your pocket
Save it for a rainy day.1
As a spindly asthmatic eight-year-old Canadian boy, I was not familiar with the Fourth of July, 1956, as a national holiday. Four days earlier, I had boarded a train in Montreal with my two brothers Richard and Gerry and my sister Joyce. It was the first time in my life I would be away from Mom and Dad, and I was not happy about it. I was homesick before the train pulled to a stop by the small station at Whitehall in New York state. We boarded buses – one for Joyce who would attend nearby Camp Awanee, and another for the rest of us. Richard was looking forward to his elite experience on Senior Hill; Gerry was wondering about life in Freshman House, and I was headed for Bunk B on Junior Row. With these thoughts in our minds, our buses crossed the border into Vermont, near the historic Fort Ticonderoga where American soldiers fought the British during their war of independence. We reached Vermont Route 30 and headed souththrough beautiful undulating mountain scenery. Finally we passed a restaurant on the right called The White House, descended a hill, slowed down, then turned left. We had reached Twin Lake. (See Figure 1.1.)
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|Figure 1.1 A group of children at Twin Lake Camp in the Summer of 1956. The thin boy second from right is the author, then eight years old. He saw his first meteor that summer.|
I didn’t enjoy that first night. My first request of the Camp Director, my one lifeline to big city life, was to be allowed to watch the Perry Como show each week. “If I let you do that, David, then I’d have to let 160 other boys watch their favorite shows from my living room!” he explained. Yielding to his logic, I returned to Bunk B where I tossed and turned. I walked onto the porch and joined the counselors, who pointed out the mountain peaks and how they appeared to interact with the cloudy sky. I was surprised by how dark it was. It was my first night away from the bright lights of Montreal, and my first night away from television shows like Perry Como. But they showed me how the country had a new kind of magic.
Bunk B had five children, and we all quickly became good friends. Our first major celebration at camp took place four days into the season, on July 4. The evening began with an event in the social hall. Then we headed down to an open field to enjoy the camp’s fireworks display. When it ended, we little guys were sent back to our bunks. The way back to Bunk B was a slow uphill climb toward the north. As we walked casually up the gentle slope, my gaze turned skyward. Toward the right, a bright star highlighted the east. I was amazed at how many stars there were. I stared for a few seconds. Then a “falling star” – a meteor – streaked across a small portion of sky. It wasn’t particularly bright – perhaps as bright as Polaris, the north star, and it did not leave a long trail. It moved fairly slowly and headed toward that bright star that was almost certainly Vega. It lasted only about a second.
To germinate and sprout
That’s the story – except that that one-second event planted a seed in my mind. It didn’t germinate for about 15 months. I do remember walking home from school on an early October afternoon in 1957, and one of my classmates told me that there was a new moon in the sky called Sputnik. I looked up into the cirrusy afternoon sky and wondered if I could see this remarkable thing there and then. I was grabbed by the wonder of the new moon, but was too young to share the concern my parents felt over what it represented that evening at dinner.
In the spring of 1960, the seed finally sprouted. On June 11, I started writing a book called A look at the stars. (I doubt I finished it.) On June 21, while cycling to graduation and a picnic at Roslyn School, I fell off my bicycle and broke my arm. My cousin, Roy Kaufman, gave me a book called Our Sun and the Worlds Around It. It was filled with pictures and good information; I used that book as a basis for writing The Wonders of the Earth, which was about our Solar System. By the end of the summer of 1960, the night sky had become an all-consuming passion which culminated on the evening of September 1, 1960, when my parents and I set up my new 3.5-inch reflector telescope and turned it toward the brightest thing in the sky. As I focused the telescope, the image sharpened to a bright disk with two dark bands crossing it, and four little stars nearby. From a meteor to Jupiter, I was now a lover of the night sky.
Dating the meteor
So many memories of youth get lost over the years, but not the meteor incident. I recall precisely where at the camp I was when I saw it. I also remember that it followed a Fourth of July celebration. But I spent three summers at Twin Lake; which year was it? I assumed it was my final year there, 1958, when at age 10 the incident would have meant the most to me. I did remember a very dark night, something that would be easy to confirm using a planetarium program that displays the sky, with proper position and phase of the Moon, on any night. When it showed the sky from central Vermont on July 4, 1958, a bright waning gibbous Moon rose less than an hour after dark. By the time the social evening and fireworks were over, I would have been observing a sky not much darker than a Montreal sky. The previous summer was not much better: On July 4, 1957, an 8-day old Moon next to Spica and slightly-past first quarter brightened the southwestern sky.
On the evening of July 4, 1956, the waning crescent Moon was within three days of new, and not in the sky.
But was it clear? What about what the weather was gonna do, would the sky be clear? What was the weather on July 4 of each of the three summers I was at camp? For answers I turned to two sources: the National Weather Service, and John Martin, senior systems analyst at Newsbank, a company headquartered in Chester, Vermont. Both the Weather Service and John, through his access to newspaper archives, confirmed that Independence Day 1958 was cloudy with rain, as part of a long-lasting cold system that, I recall, seriously aggravated my asthma that summer. The 1957 evening had variable clouds with thundershowers scattered throughout the region. The National Weather Service, who provided me with weather data for those three nights, reported that on that evening the sky was clear and dry throughout Vermont. July 4, 1956, was the night I saw my first meteor.
A seed was planted that night, but it took some time to germinate and then to sprout. I had no idea at the time that what I had witnessed was the end of a 4.5-billion year life story. I’d later learn that my falling star was actually a speck of dust from the Omicron Draconid stream, which might have begun its life as part of Comet Metcalf, but those details will come later in Chapter , after we have some more background about meteors. For now, what counts is that after wandering across the Solar System, past the Sun, planets, moons, comets, and asteroids, this tiny particle separated from the comet and began a life of its own. Exploring space by itself now, the particle moved through the Solar System for hundreds of thousands of years. In the summer of 1955 the Earth was a bright star in the particle’s sky, by the spring of 1956 the Earth was getting bigger by the day. Throughout the day on July 4, the Earth loomed larger as nightfall approached the eastern United States. At last the particle slammed into Earth’s atmosphere at a speed of 14.7 miles/s (23.7 km/s).2 It heated the surrounding air until it was incandescent. Then it was gone.
I am writing this chapter on July 4, 2006, precisely 50 years after that meteor. A lot has happened in those 50 years. By 1960 I was completely hooked on astronomy. All the comets I have found, all the eclipses I have seen, all the young people who have looked through my telescopes – all those things can be traced back to that one simple event, a meteor falling from a clear, dark sky while I stood in a field with some friends at camp.
What is a meteor?
Never tell me that not one star of all
That slip from heaven at night and softly fall
Has been picked up with stones to build a wall.
Some laborer found one faded and stone cold …1
A meteor is actually not an object but an event that occurs when a meteoroid, or a speck of dust, makes contact with Earth’s atmosphere and heats the surrounding air to incandescence. But if the physics behind this sand-grain-sized speck of dust is complicated – its orbit around the Sun, and how it can disappear in an instant after wandering through the Solar System for billions of years – then the idea of a meteor brightening the sky for a second or two is something an inquiring child can grasp. And for that child, the first step towards understanding is to appreciate that after each meteor falls there are still just as many visible stars as there were before. Every star is a sun, in some ways like our own. These distant suns will not fall to Earth. A meteor takes place close to us, perhaps 40 miles above us in the Earth’s own upper atmosphere.
So, once again: a meteor is an event that takes place when a tiny particle, called a meteoroid, enters the Earth’s upper atmosphere, heating the air around it to incandescence. It is that incandescent glow that we see as a meteor. A meteoroid is the particle that produces a meteor. It starts out as a particle of dust in a comet or, less often, in an asteroid. The particle continues circling the Sun in roughly the same orbit as the comet from which it came, but over time it gets farther from that comet, whose orbit becomes littered with these particles. See Figure 2.1.
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|Figure 2.1 Comet McNaught at the moment of perihelion, its closest approach to the Sun. This photograph was taken at noon, with the comet visible in broad daylight near the Sun.|
The meteor event
A meteoroid’s final plunge begins somewhere from a height of 75 to 50 miles, and can vanish at altitudes between 50 and 40 miles. Its velocity depends on whether it is catching up from behind or hitting the planet head on. If the meteoroid is directly behind Earth, it will strike at about 7.5 miles per second; if the collision is directly head-on, its velocity can reach 45 miles per second. Usually the velocity will be somewhere in between those extremes. Some meteors show a trail, a streak lasting a second or two. Rarely a bright meteor will leave a train of faint light that can last for several minutes; I once saw a Taurid so bright that dogs were wakened and started barking, and its train lasted for 20 minutes.
Micrometeoroids and Brownlee particles
There are particles even smaller than meteoroids. These particles, called micrometeoroids or Brownlee particles (after Donald Brownlee who first described them) are tiny cometary particles from the zodiacal light cloud, some of which are from ancient comets, and actually arrive on Earth. As small as these particles are, micrometeorites are actually easy to spot on a dark night. In northern hemisphere winter, they appear best as a tepee-shaped cone of light, called the zodiacal light, which climbs sharply up from the western horizon. In the southern hemisphere the zodiacal light climbs high in the sky on June, July, and August mornings. Experienced observers under very dark conditions can follow the zodiacal light all around the sky. As the tepee shaped zodiacal light fades, it continues as a thin, faint zodiacal band that follows the ecliptic across the sky. At the point in the sky opposite the Sun, it widens out into the Gegenschein, or counterglow.
In 1970 the first balloon experiment to collect dust particles in the stratosphere retrieved artificially produced and volcanic dust as well as some microscopic samples of feathery cometary dust. These Brownlee particles are extraterrestrial in origin. Specially equipped planes flying some 20 kilometers into the stratosphere routinely collect such particles today. The Brownlee particles eventually drop to the Earth’s surface at the approximate rate of a single particle on each square meter of the Earth’s surface every day.2
In the early years of the Solar System, the zodiacal dust was far more extensive than it is now. Over a long period of time, uncountable billions of such grains entered the atmospheres of Earth, Mercury, Venus, Mars, and struck the airless Moon. They were not large enough to be consumed as the meteors we see in the sky at night; instead they were so tiny that they came to a stop in the upper atmosphere and then wafted gently down to the planetary surfaces. Today, lesser numbers of these primordial grains reach the surface. Occasionally a micrometeorite may form the nucleus of a raindrop.
Fireballs and bolides
If a meteor reaches or exceeds the brightness of Venus, it is called a fireball. Sometimes a fireball explodes as it falls; if you see a bright meteor break up in a shower of sparks, then you have seen a bolide. A fireball brighter than the quarter Moon might actually survive its plunge through the atmosphere to land on Earth’s surface. Listen for rumbling or detonation sounds after you witness such a thrilling event. In September 1968 I saw such a meteor fall towards the northeast. It lasted about half a minute, suffered several explosions, and finally disappeared over the northeastern horizon. Several minutes later I heard a low rumbling noise that lasted almost half a minute. It is possible that this object survived its fall to Earth, and that the meteorite still awaits discovery in the forests of Prince Edward Island. But to have any chance of recovering a meteorite, accurate observations of the altitude and azimuth of the beginning and end points of the fireball are necessary from several sites. From these measurements, one can try to calculate where the meteorite landed.
Meteors are more numerous on certain nights
Although meteors can be seen on any night of the year, at certain times of the year the Earth rushes through a stream of particles that are strung along the orbit of a comet. Kicking off the new year are the Quadrantids, which are active for only a few hours on the night of January 2/3. The Lyrids peak at the end of April. We call this shower the Lyrids because its meteors appear to radiate from the constellation of Lyra. The Eta Aquarids, from Halley’s comet, are active around the fourth of May.
The next good shower is the Delta Aquarids, which reaches its maximum on July 29 with meteors one could trace back to a point in the sky near the star Delta in the constellation of Aquarius. By itself, the Delta Aquarids is only a moderate shower, but its maximum marks the onset of one of the northern hemisphere’s great showers, the Perseids. They peak two weeks later on August 12. The next major shower, the Orionids, follows on October 20, and, like the Eta Aquarids of May, the Orionids derive from Halley’s comet. The Taurids, which can display some dramatic bright meteors, are visible during the first part of November. The Leonids occur on November 17, and the Geminids reach their strong maximum on December 13.
Why do meteors of a particular shower appear to radiate from a single point in the sky? The effect is due to perspective, and is similar to looking down a railroad track. The tracks appear to converge at a specific point, but they obviously can’t, since the tracks must be parallel or the trains will all fall off. Similarly, the meteors from a particular shower appear to be coming from a specific place in the sky. But they actually hit Earth from the same direction – they are on parallel paths.
A meteor shower radiant is actually a small area, the size of which indicates the age of the stream. A small radiant size indicates a youthful, hardly dispersed stream. A few meteor showers are complexes that show several radiants a few degrees apart in the sky. Because the Earth moves relative to the meteoroid orbits, the apparent radiant shifts from night to night. Not all meteors belong to showers; sporadic meteors are thought to be the remains of ancient streams that have had their orbits dispersed beyond recognition.
More meteors are seen in the hours before dawn than after sunset. This effect occurs because in the morning hours meteoroids are colliding more nearly head-on, rather than catching up with Earth as they do in the evening hours. The closer to the zenith the radiant is, the higher the meteor rate usually will be.
Amateur meteor observing
In the early 1920s Charles P. Olivier founded the American Meteor Society (AMS), a group of dedicated meteor observers whose observations would build up a central archival file of meteor work done over many years. Dr. Olivier endorsed a mode of operation that concentrated on the efforts of single observers and not with group reports. In the 1950s, when planning for the International Geophysical Year (IGY), large numbers of meteor observations were accumulated. This international program of cooperation between countries included professional and amateur astronomers alike. In no country was this program taken more seriously than Canada, where Dr. Peter Millman of the National Research Council carefully prepared an observing form that would be a model of simplicity and could be understood by almost anyone. For each meteor, only two pieces of data were needed: magnitude and shower membership. The time column was left unlined, so that observers could record time to the hour, minute, or second. Here was an “observer-friendly” program that, if successful, could attract hundreds of Canadian amateur astronomers. These forms are used in this book.
From the time the program began at the dawn of the IGY in 1957 to its official close almost fifteen years later, groups of observers across the country organized major observing projects for most of the major annual showers. The observing became so intense that for some astronomy societies in Canada, meteor observing was the major observational activity. From years of observing by the AMS, IGY, and other groups, amateurs have achieved a monumental file of archival data that shows the changing strength of meteor showers over three-fourths of a century. The data required are easily obtained and provide both the novice and the experienced observer with an opportunity to contribute to the exciting field of meteor research.
Some historical notes
Goe, and catche a falling starre,
Get with child a mandrake roote,
Tell me, where all past yeares are,
Or who cleft the Devil’s foot.
Teach me to heare Mermaides singing,
Or to keep off envies stinging,
Serves to advance an honest minde.1
I would more easily believe that two Yankee professors would lie, than that stones would fall from heaven.2
Catch a falling star, and put it in your pocket,
Save it for a rainy day.3
Go and catch a falling star. This four-century-old piece of advice was modernized in the late 1950s: go outdoors at night, look at the stars, and catch a meteor. As unbelievable as Thomas Jefferson claimed it was, stones do fall from heaven. What you catch are not real stones but the excitement of that flash of light that brightens the sky and makes you wonder. When you see a meteor, you share an experience with many, many people throughout history. Meteors are often mentioned in the works of Shakespeare, John Donne, and other writers from many times.
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