Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making

    Most changes in the sky are subtle, very slow and gradual, and highly predictable.  The daily rising and setting of
the sun, the phases of the moon, the slow dance of the five visible planets against the “fixed” background stars, and
the gradual seasonal shift of the constellations are all so staunch and reliable that you can, literally, set your clocks
and calendars by them, as cultures have done for thousands of years.

    But meteors are different.

    In an instant a bright pinpoint of light draws out a streak against the sky, hanging there for a second then rapidly
fading.  On any given evening meteors can appear anywhere in the sky, at any moment, traveling in any direction.  
Some flash so faintly and rapidly you only catch them in the corner of your eye; others are much brighter and slower,
and cast shadows.  Once in a lifetime you might see a meteor that rivals the full moon in brightness, painful to dark-
adapted eyes.  And very, very rarely a meteor appears so bright that it is plainly visible in daylight and approaches the
sun in brilliance.

    On all clear dark nights you can see meteors.  Most nights they are infrequent and appear at random—the so-
called sporadic meteors.  But about twenty times a year meteors are much more frequent, and for periods of several
days can reach numbers many times the sporadic background level.  During these meteor showers one can see tens,
hundreds, or even thousands of meteors an hour appearing to streak from the same point in the sky.  This weekend
we hope to witness the annual shower that streams from the head of the ancient constellation of the Lion—the Leonid

Diverse cultural views

    People have watched meteors since ancient times.  Their unpredictability and varied appearance have inspired a
diversity of explanations, each drawing on the worldview and knowledge of the different cultures of the world, past and
present.  Distinct from the more regular movements of the planets and background stars, the flash of a meteor was
often given special meaning and significance.  Many peoples have considered meteors to be portents of coming good
fortune, or omens of impending doom.  The ancient Chinese considered meteors to be evidence of turmoil in the
heavenly realm of the ancestors; and turmoil in the heavens will inevitably bring unrest on Earth.  Since it was the job
of the Emperor to mediate between heaven and Earth, for centuries court astrologers kept detailed records of
exceptional meteor falls, often describing nights when “stars fell like rain.”  Modern astronomers have mined these
imperial records going back to the 7th Century BC for the evidence of the shifting dates and intensities of meteor
showers and the comets that cause them.  The Babylonians, Greeks, Romans, Mayans, and other early civilizations
similarly leave us accounts of exceptional meteor falls, and the meanings they attributed to them.

    Some cultures associate the fall of a meteor with the flight of a human soul.  The Germans thought each star was
tied to a person by an invisible thread; when the thread breaks, our star falls and we pass into death.  The Bororo
Indians of Brazil believe that a streaking meteor marks the theft of a soul by a powerful sorcerer.  Medieval Christians
said a prayer with each meteor, asking God to accept a new soul into Heaven.

    The Timbira, a kin-based Native American society in Brazil, consider meteors to be stars from one sky-clan flying to
marry stars from the clan on the opposite side of the sky, like a Timbira man running to the village of his future bride.

    Some cultural explanations are more prosaic—to the Pilaga of Argentina meteors are merely glimmering star poop.

    Many cultures, though, proposed explanations similar to the one astronomer’s believe today: meteors are caused
by the fall of celestial objects to Earth.  The Yukpa Indians I lived with in the early 1980s believed that meteors were
shiraptu—lesser stars—fallen to Earth, and would travel long distances to find them.  Many peoples around the world
have collected meteorites—the scorched remnants of the objects that make meteors—and revered them as cultural
relics.  The Black Stone at the Kaaba in Mecca is thought to be such a meteorite.

    Greeks and Romans wrote of stones falling from the sky.  In Medieval Europe falls of “thunderstones” were
occasionally mentioned by chroniclers and illustrated in drawings and woodcut prints.  However, early scientific
astronomers generally rejected these accounts as unreliable and superstitious, preferring Aristotle’s explanation:
meteors are merely wisps of flammable air set alight by friction with the rotating celestial spheres.

The Leonids and the rise of scientific meteoritics

    The intellectual tide began to turn in the late 1700s when well-documented stone falls in Italy, England, Russia,
France, and India attracted serious scientific attention.  A crucial event occurred in November of 1833, when an
intense storm of Leonid meteors fell over Europe.  Observers noted that the meteors seemed to stream from the
direction of the star Gamma Leonis, in the neck of the constellation Leo.  This was the earliest recognition of a shower
radiant, and confirmed that meteors (or at least the objects that cause them) come from interplanetary space.  In 1867
a comet discovered independently by William Temple and Horace Tuttle (55P/Tempel-Tuttle) was found to lie in the
same orbit as the Leonid meteoroids, confirming a link between meteors and celestial objects.  Thus began the
science of meteoritics: the study of meteors, meteorites, and their parent bodies.

Where do meteors come from?

    We need to introduce a little terminology.  Meteoroids are particles in solar orbit, ranging in size from tiny dust
grains to massive menacing mountain-sized chunks of rock, metal, and ice.  Meteors are the streaks of light that
appear in the sky when a meteoroid traveling at cosmic velocity slams into Earth’s upper atmosphere.  Small
meteoroids (like Leonid meteoroids) are reduced to vapor and do not survive the transit through the atmosphere.  
However, fragments of bigger ones can fall to Earth as meteorites.  Very large meteoroids, those considerably bigger
than a house, can survive the transit through the atmosphere with much of their mass and cosmic velocity intact, and
impact the surface with the explosive energy of many nuclear bombs.  The results are impact craters such as Meteor
Crater, Arizona, the impact structure that floors Chesapeake Bay, and the dinosaur-killing multi-ringed Chicxulub basin
on the northern tip of the Yucatan Peninsula.

    Nearly all meteoroids come from either asteroids or comets.  Asteroids are rocky and metallic bodies considered
the reworked remnants of planetesimals that formed in the inner parts of the early solar system about 4.5 billion years
ago.  Most of the early asteroids collided and coalesced to form the inner planets Mercury, Venus, Earth, and Mars,
but some remain today mostly in an asteroid belt beyond Mars’ orbit shepherded by the intense gravitational field of
the giant planet Jupiter.  Even today occasionally two asteroids collide, producing a cloud of meteorite particles
ranging from the size of dust grains to mini-asteroids miles across.  Some of these meteoroids are sent into Earth-
crossing orbits and eventually enter our atmosphere.  Meteors from asteroids tend to be sporadic and appear to come
from all directions.  Many are faint, but some form bright fireballs and explode when they reach the lower atmosphere
in a shower of sparks (and sometimes fist-sized chunks of rock and iron).  Most of the stony, iron, and stony-iron
meteorites displayed in museums are fragments of asteroids; the rest are fragments ejected during impacts on planets
like Mars and our moon.

    Other meteoroids—including all shower meteoroids—come from comets, interplanetary bodies of dust and organic
matter cemented together by ice.  Comet nuclei are thought to have assembled from planetesimals that formed in the
colder outer reaches of the early solar system.  Many of these collided to form the cores of the giant outer planets
Jupiter, Saturn, Uranus, and Neptune.  As these planets grew and their gravitational fields became more intense, the
remaining comets were slung out of their original orbits.  Some were sent closer to the sun, where many impacted on
the inner planets delivering large amounts of water and organic material.  It may not be a coincidence that the
elements carbon, hydrogen, oxygen, and nitrogen (the so-called CHON elements) are abundant both in comets and in
the bodies of living organisms on Earth!  Other comets were flung out of the realm of the planets, forming two great
cometary reservoirs—the thin disk of the Kuiper belt just beyond the orbit of Neptune, and the more rarefied spherical
expanse of the Oort cloud that stretches out in all directions to about a light-year from the sun, about a quarter of the
distance to the nearest star.  It is estimated that a trillion comet nuclei travel in elliptical orbits through this vast volume
of space.  Many comet nuclei must have been ejected altogether from the solar system, and today wander between
the stars.

    When perturbed by the gravitational attraction of the outer planets, passing stars, or massive galactic gas clouds,
comets can be sent into orbits that bring them periodically into the inner solar system.  As they approach the sun their
ices begin to sublimate, ejecting dust and vapor forming a wide hazy coma around the comet nucleus.  Pressure from
sunlight spreads the dust into streams along the comet’s orbit; after several revolutions these streams spread out
along the entire elliptical orbit of the comet.  When the earth passes through these cometary dust streams we
experience a shower of meteoroids, all traveling in parallel orbital trajectories, creating a meteor shower.  Cometary
meteors tend to be fainter and faster-moving than many asteroidal meteors, and appear to radiate from a common
point in the sky.  Few (if any) of the small and fluffy cometary meteoroids survive their entry into the atmosphere and
reach the ground intact as meteorites.  So far as we know, you won’t find a piece of a comet in a museum or for
auction on Ebay.

The Leonids

    The particles that give us the annual Leonid meteor shower were ejected from the comet 55P/Tempel-Tuttle.  The
comet was spotted in 1366 by Chinese astrologers, although Leonid meteors were first recorded on October 14, 902
AD over Sicily and Egypt.  Following the 1866 apparition mentioned in a previous section, it was recognized that comet
Tempel-Tuttle lies in an elongated elliptical orbit passing close to the sun every 33.23 years of so.  At aphelion, the
farthest point from the sun, its orbit extends beyond that of Uranus, making it a Halley-type comet.   (Short-period
Jupiter-family comets have orbital periods less than 20 years; long-period comets take over 200 years to orbit the sun,
and some have orbital periods measured in millions of years.  Halley-type comets have periods in between these
extremes, often in orbital resonance with Jupiter.)  

    The nucleus of Tempel-Tuttle is potato shaped, about 3.5 km (~2 miles) long, and is thought to be composed of
about two parts dust to one part ice.  During its last apparition in 1998 it rotated once each 15 hours.  Like other
comets, the nucleus of Tempel-Tuttle is quite dark, with an albedo similar to charcoal or fresh asphalt, and covered in
a fluffy crust of dust.  As it passes through the inner solar system Temple-Tuttle releases about 13 billion kg of dust
into solar orbit.  Comets like Temple-Tuttle are expected to make an average of about 400 orbits before they fall into
the sun, smash into a planet, or are ejected from the solar system by a close encounter with Jupiter or Saturn.

    Its retrograde (“against the flow”) orbit, obliquity to the common orbital plane of the planets, and 5:14 orbital
resonance with Jupiter suggests that it originally came from the distant Oort cloud, and was nudged into its present
orbit by gravitational interactions with the giant planet.  

    In the modern era the Earth encounters the in-coming leg of Tempel-Tuttle’s dust trail for several days each year
around the date of November 17.  The retrograde motion of the cometary particles added to the Earth’s prograde
orbital velocity means that Leonid meteoroids strike our atmosphere with the astounding velocity of 72 km/s (that’s 45
miles per second, or 162,000 miles per hour).  That is about as fast as anything can move and still be bound to our
solar system; most cometary and asteroidal meteoroids approach us at much lower velocities.  This is why Leonid
meteors flash across the sky so quickly, and one of the reasons Leonids are often brighter than other shower meteors.

    Why do Leonid meteors glow?  Though we often speak of a meteor “burning up” in the atmosphere, the light
comes neither from chemical combustion as in a candle flame nor through intense frictional heating of the meteoroid
itself.  During entry a shower meteoroid is far too small to see, and it is enveloped in a cloud of vapor that blocks light
from the object itself.  Rather, it is the interaction of this meteoroid vapor with the thin upper atmosphere that
produces the visible fireworks.  At an altitude of about 120 km (75 miles) impacts with air molecules sputters and
vaporizes molecules from the surface of the meteoroid.  The resulting cloud of atoms and molecures travels with the
meteoroid at 72 km/s relative to the surrounding rarefied air.  When these molecules slam into relatively stationary air
molecules, electrons in both become highly excited from the release of kinetic energy.  These electrons emit photons—
light particles—as they return to the ground state.  The light we see comes from these photons.  Some of this light is
emitted in invisible hard ultraviolet wavelengths, causing surrounding trace molecules in air to fluoresce creating a
bright halo of visible light around the incoming meteoroid.  Though the meteoroid itself is tiny, often smaller than the
average grain of sand, the bright light we see at the head of a meteor can come from an area a hundred yards across.

    By the time a Leonid descends to about 90 km (56 miles) most of the particle is vaporized and the meteor ceases
to produce new light.  However, the excited atoms and ions left in its wake can continue to glow for a second or two,
causing a distinct after-image (it isn’t all in your eye; if you sweep your vision just as a meteor flashes, this afterglow
moves with the sky, not with your eye).  Brighter fireballs can leave a glowing train that can last several minutes.  Akin
to the chemical phosphorescence of a firefly, the persistent glow comes from chemical reactions in atmospheric
oxygen catalyzed by iron and sodium molecules vaporized off the meteoroid.

    As they encounter the atmosphere shower meteoroids move in nearly straight parallel trajectories.  Their apparent
direction is determined by both their own orbit in space around the sun and the motion of the Earth.  Shower meteors
appear to radiate from this direction, with meteors closer to the radiant appearing to move across the sky more slowly,
leaving shorter trails.  Meteors far from the radiant appear faster and longer.  However, this is an optical illusion
caused by parallax; in reality, shower meteors travel at much the same velocity and in the same direction.

Observing the Leonids

    In 2006 the Earth will be passing through 55P/Tempel-Tuttle’s dust train from November 14 through 21.  During
this period at least some Leonids will be visible most nights after the head of the constellation Leo clears the horizon,
around 11:30 pm.  Optimal viewing during most nights is likely to be around 2-3 am when Leo is high in the eastern
sky.  It should be easy to spot the Leonids: compared to sporadics, they are very fast, mostly faint, and their trails
trace back to the reverse-question-mark of stars forming Leo’s head.  On most nights expect to see several Leonids
each hour, matched by several sporadics coming from different directions.

    While the number of meteors is expected to peak on November 17, they won’t be visible from North America.  
However, a brief outburst is predicted between 11:45 pm and 1:33 am EST on the night of November 18/19.  The
Earth will be passing through the dust trail laid down by Tempel-Tuttle in 1932.  While the best show will be in Europe
and North Africa, we can expect some “atmosphere-grazing” Leonids climbing up the sky out of the east moving
towards the zenith.

    What are the chances of another Leonid meteor storm this year?  Storms occur when the Earth passes through a
dense knot of cometary debris.  For a brief time observers can see meteors falling at a rate thousand or more per
hour.  Storms are most likely in the several years following the passage of a comet; unfortunately, the last apparition
of Tempel-Tuttle was in 1998 so it is unlikely that the Leonids will reach storm levels this year.   (In fact, some meteor
forecasters don’t expect another Leonid storm until the year 2098!)

    But you never know.  Meteor forecasting is not yet an exact science.  Dust streams can be diverted and
concentrated by their orbital passage past other planets; there may be unforeseen knots of Leonids meteoroids
hurling towards us at this very moment ready to give us a great show over the coming evenings.

Beyond the Leonids

    You can see some meteors on any clear dark night.  Sporadic meteors are most frequent after midnight, as the
Earth’s rotation “catches up” with meteoroids traveling in our orbital direction.  The very brightest meteors—fireballs—
are often sporadics.  Pick an evening when the moon is new or sets early in the evening, and observe far from the
pollution of city lights.  Your naked eyes are your best instrument: your chances of seeing a meteor in the narrow field
of binoculars or a telescope are very poor.

    Over 40 annual meteor showers are well-documented, and there are probably many more.  Many of these are
quite faint and spread thinly over many days.  But here’s a list of the better showers that you might consider observing:

Geminids.  Active between December 7 and 17, peaking near December 13.  Geminids appear to stream out of the
twin “heads” of the constellation Gemini, and are one of the few showers that are best before midnight.  The Geminids
are one of the best showers currently visible, with many bright medium-speed meteors.

Quadrantids.  Active between December 28 and January 6 with a sharp peak around January 3.  Quadrantids appear
to stream from a point midway between the constellations Draco and Bootes.  

Lyrids.  Active between April 16 and 25, peaking around April 21.  The radiant lies between Hercules and Lyra.  Lyrids
tend to be bright and rather fast.

Perseids.  Active from July 17 to August 24, peaking around August 12.  Perseids stream from the constellation
Perseus, near the easily recognized “lounge chair” of Cassiopeia.  Perseids are often bright and fast.  Warm summer
weather makes the Perseids one of the favorite annual showers.

Copyright 2009 Ross Sackett
From Thunderstones to
Comet Dust:
Leonids 2006

Talk given at the Weathersfield Inn, Springfield, VT  
November 2006