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(meteorobs) AMS FAQ on Meteors and Meteor Showers
Hello folks,
With the high number of beginers currently on the list, all asking many
excellent questions about how to get started in meteor observing, I
thought it might be helpful to post the American Meteor Society (AMS) FAQ
(Frequently Asked Questions) Sheet on "Meteors and Meteor Showers." This
FAQ, along with other reference material and links to other resources is
also located on the AMS web site at:
http://www.serve.com/meteors/
Below is the FAQ:
The American Meteor Society, Ltd.
Frequently Asked Questions (FAQ) About Meteors and Meteor Showers
Version 1.2
Question List:
1. What is the difference between a meteor, a meteorite, and a meteoroid?
2. How high up do meteors occur?
3. How big are most meteoroids? How fast do they travel?
4. Where does a meteor's light and color come from? What is a meteor
train?
5. How many meteors can I expect to see if I go out to observe them when
no meteor shower is occurring?
6. What is a meteor shower? Does a shower occur "all at once" or over a
period of time?
7. How can I find out when a meteor shower is occurring, where and how to
look, and what to expect?
8. Does the published meteor rate for a shower really represent what I
should expect to see?
9. I thought I saw a lot of meteors coming from a certain part of the sky
last night, but I can't find any shower listed in my books. What was
going on?
10. What is a meteor storm, and how often do they occur?
11. Where can I get more information about the Leonid storm which is
expected to occur between 1997 and 2000?
12. Is there a chance of a meteor from a meteor shower or storm reaching
the ground as a meteorite, and is it dangerous to observe meteor
storms?
13. Where can I find information on historical meteor observations?
Below are some relatively concise answers to the above questions. If you
need further clarification or have further questions, please feel free to
contact us via electronic mail.
1. What is the difference between a meteor, a meteorite, and a meteoroid?
Meteoroids are the smallest members of the solar system, ranging in size
from large fragments of asteroids or comets, to extremely small
micrometeoroids. Whenever a meteoroid plows into the Earth's atmosphere, it
will create a brief flash of moving light in the sky, called a meteor.
Meteors were once thought to be a purely atmospheric phenomena, and the
study of these and other atmospheric effects, especially weather, spawned
the science of meteorology. It was not until the mid-1800's that the
extra-terrestrial nature of meteors was widely recognized. If remnants of
the parent meteoroid survive the trip through the atmosphere to reach the
ground, then these remnants are called meteorites.
2. How high up do meteors occur?
Most meteors occur in the region of the atmosphere called the thermosphere.
This "meteoric region" lies between about 80 km and 120 km (50 to 75 miles)
in altitude. This is a general guideline only, since very fast meteors may
first become visible above this height, and slow, bright meteors may
penetrate below this band.
3. How big are most meteoroids? How fast do they travel?
The majority of visible meteors are caused by particles ranging in size
from about that of a small pebble down to a grain of sand, and generally
weigh less than 1-2 grams. Those of asteroid origin can be composed of
dense stony or metallic material (the minority) while those of cometary
origin (the majority) have low densities and are composed of a "fluffy"
conglomerate of material, frequently called a "dustball." The brilliant
flash of light from a meteor is not caused so much by the meteoroid's mass,
but by its high level of kinetic energy as it collides with the atmosphere.
Meteors enter the atmosphere at speeds ranging from 11 km/sec (25,000 mph),
to 72 km/sec (160,000 mph!). When the meteoroid collides with air
molecules, its high level of kinetic energy rapidly ionizes and excites a
long, thin column of atmospheric atoms along the meteoroid's path, creating
a flash of light visible from the ground below. This column, or meteor
trail, is usually less than 1 meter in diameter, but will be tens of
kilometers long.
The wide range in meteoroid speeds is caused partly by the fact that the
Earth itself is traveling at about 30 km/sec (67,000 mph) as it revolves
around the sun. On the evening side, or trailing edge of the Earth,
meteoroids must catch up to the earth's atmosphere to cause a meteor, and
tend to be slow. On the morning side, or leading edge of the earth,
meteoroids can collide head-on with the atmosphere and tend to be fast.
4. Where does a meteor's light and color come from? What is a meteor train?
The majority of light from a meteor radiates from a compact cloud of
gaseous atoms and molecules immediately surrounding the meteoroid or
closely trailing it. This cloud consists of a mixture of atoms and
molecules ablated from the meteoroid itself as well as from the surrounding
air. These excited particles will emit light at wavelengths characteristic
for each element/compound. The most common emission lines from meteors
originate from iron (Fe), oxygen (O), magnesium (Mg), sodium (Na), nitrogen
(N), and calcium (Ca). Less frequently seen are the emission lines of
hydrogen (H), Silicon (Si), Manganese (Mn), and Chromium (Cr).
While most meteors produce a wide blend of these emissions, giving the
meteor an overall white color, specifically colored meteors are often
reported by meteor observers. Usually, such colors are rather weak in
appearance; however, vivid colors are occasionally reported, especially
with fireballs. Reported colors range across the spectrum, from reds,
yellows, greens, and blues, to gold, orange, and (infrequently violet. The
velocity of the meteor also plays an important role, since a higher level
of kinetic energy will excite the atoms/molecules to a higher degree. Slow
meteors are often reported as red or orange, while fast meteors frequently
have a blue color. Due to the nearly identical composition and velocity of
meteors belonging to a particular shower, several showers are known for
their characteristically colored meteors.
Often, a brief glow will remain after the passage of the meteor. If this
glow persists for less than 0.5 seconds, it is called a wake. This residual
glow is caused by the same atoms which produced the original light from the
meteor, only at lower excitation energies.
If the glow from the meteor trail persists for a longer period, this is
called a meteor train. Trains are most often seen from fast, bright
meteors, in the altitude band from about 100 to 120 km (62 - 75 miles).
This type of train usually lasts about 1-2 seconds, and is primarily
generated by the green emissions of the neutral nitrogen atom. On very rare
occasions, a train may persist for several minutes, and will be observed to
change shape as the trail is blown by upper atmosphere winds. Such
persistent meteor trains provided scientists with their first data on winds
in this region.
5. How many meteors can I expect to see if I go out to observe for them
when no meteor shower is occurring?
The number of random, or "sporadic" meteors that can be seen in the night
sky is quite variable, depending upon such factors as the time of night,
time of year, light pollution, and cloud conditions.
Perhaps the most important factors necessary in order to observe meteors
are to have a clear, unobstructed view, out in the open, and under as dark
sky conditions as possible.
Over the course of a night, it will be noticed that more sporadic meteors
can be seen in the hours before sunrise than in the hours after sunset.
This is due to the motion of the Earth as it revolves around the sun, with
the leading edge (morning side) of the Earth encountering more meteoroids
than the trailing edge (evening side). In general, 2 to 3 times as many
meteors can be seen in the hour or so just before morning twilight, than
can be seen in the early evening. Additionally, the numbers of random, or
sporadic meteors will also vary from season to season, due to the tilt of
the Earth on its axis and other factors. As a general rule, about 2 to 3
times as many sporadic meteors can be seen in the early fall (September) as
can be seen in the early spring (March). Together, these two effects can
generate a fluctuation in the hourly rate of sporadic meteors by a factor
of 4 to 9 times, over the course of the year.
Under good conditions, only about 2-4 sporadic meteors can be seen per hour
in the early evening in March, with this rate increasing to about 4-8
sporadic meteors per hour by morning twilight. These rates will then slowly
increase throughout the spring and summer. By the month of September, the
evening sporadic rate will be up to about 4-8 meteors per hour, increasing
up to about 8-16 sporadic meteors per hour by morning twilight. Throughout
the remainder of the fall and winter, these rates will slowly drop off,
returning to the March levels again. Note that these rates are rough
guidelines only, with random statistical fluctuations, observing
conditions, and personal perception all playing a role in the actual number
of meteors seen.
6. What is a meteor shower? Does a shower occur "all at once" or over a
period of time?
Most meteor showers have their origins with comets. Each time a comet
swings by the sun, it produces copious amounts of meteoroid sized particles
which will eventually spread out along the entire orbit of the comet to
form a meteoroid "stream." If the Earth's orbit and the comet's orbit
intersect at some point, then the Earth will pass through this stream for a
few days at roughly the same time each year, encountering a meteor shower.
The only major shower clearly shown to be non-cometary is the Geminid
shower, which shares an orbit with the asteroid (3200 Phaethon), one that
comes unusually close to the sun as well as passing through the earth's
orbit. Most shower meteoroids appear to be "fluffy", but the Geminids are
much more durable as might be expected from asteroid fragments.
Because meteor shower particles are all traveling in parallel paths, at the
same velocity, they will all appear to radiate from a single point in the
sky to an observer below. This radiant point is caused by the effect of
perspective, similar to railroad tracks converging at a single vanishing
point on the horizon when viewed from the middle of the tracks. Meteor
showers are usually named for the constellation in which their radiant lies
at the time of shower maximum. Thus, the Perseid meteor shower (peaking
about August 12) will appear to radiate from the constellation of Perseus,
while the Leonid meteor shower (peaking about November 17) will appear to
radiate from the constellation Leo.
Meteor shower rates are highly variable, with the number of shower meteors
seen following a curve of activity which usually lasts several days.
Beginning at some level below the sporadic meteor background rate, the
number of shower meteors seen will increase exponentially as the Earth
approaches the densest portion of the stream. The rate will then peak at
some maximum level, followed by a decreasing exponential decay back below
the normal sporadic level as the Earth leaves the stream. The duration of
peak activity can vary widely between showers. Some meteor showers (such as
the Quadrantids) have very sharp maximums, displaying their best rates for
only a few hours each year. Other major showers (such as the Taurids) have
a broader maximum, which can span across a few nights.
Meteor streams also vary greatly in strength between each other, depending
upon such factors as the stream age, parent body composition, stream
particle density and distribution, and how close the earth approaches to
the stream core. Of the 10 major meteor showers, the low-rate showers (such
as the Taurids and April Lyrids) will produce only about 10-15 meteors per
hour at their peak under good conditions, while the high-rate showers (such
as the Perseids or Geminids) can produce up to 40-90 meteors per hour at
their peaks. It is important to note that even the high rate showers will
still produce only about 1 to 2 meteors each minute, with faster or slower
periods occurring over time.
Along with the major meteor showers, there are also a number of minor
meteor showers which, while greater in number than the major streams, are
difficult to detect above the background sporadic meteor rate. These
showers will generally yield only about 1-5 meteors per hour at their
maximums, with only a sprinkling of meteors produced on non-maximum nights.
It usually requires many hours of observing experience in order to
correctly recognize and classify minor shower meteors.
7. How can I find out when a meteor shower is occurring, where and how to
look, and what to expect?
There are a variety of sources for information on meteor showers, ranging
from encyclopedia articles, to amateur astronomy books, to periodicals such
as Astronomy and Sky & Telescope. In addition, the Internet is a rapidly
growing source for information on astronomical topics. A few meteor shower
observing guidelines are included below:
In order to successfully observe a meteor shower, some familiarity with the
night sky is usually required, including the use of star charts to locate
constellations and locations on the celestial sphere using the Right
Ascension / Declination coordinate system. Plan your observing session as
close to the time of shower maximum as possible. Meteor showers are usually
quite disappointing under city and suburban conditions, so a dark
observation site, far from city lights is preferred. Similarly, meteor
showers which occur near the time of gibbous or full moon usually do not
perform well. Many meteor shower radiants do not rise before midnight,
making most meteor showers best between midnight and morning twilight.
Once at the observation site, ample time should be allotted for your eyes
to adjust to dark conditions, as this can take over an hour for full dark
adaptation. No magnification devices will be necessary. The use of all
lights should be minimized, with only dim, red pen-lights or flash-lights
used sparingly.
Most meteor observers observe from a reclining position, either in a lawn
chair or sleeping bag, with their gaze directed about 45 to 75 degrees
above the horizon, in the general direction of the shower radiant. The best
portion of the sky to watch is usually an area of sky about 25 to 45
degrees away from the radiant point for the shower.
Due to the effect of perspective, shower meteors which appear very close to
the radiant will be quite short in length, while those which appear some
distance from the radiant can be quite long. Members of the same shower,
while varying greatly in brightness, will share common characteristics,
such as speed, color range, and potential for leaving behind a train (a
glowing wake of air left behind after the meteor has passed).
It will also be noticed that the number of shower meteors seen will improve
as the radiant gets higher in the sky. This is because meteors seen near
the horizon are much farther away than those seen directly overhead, making
them dimmer and harder to notice. Also, the light from a meteor near the
horizon must pass through much more atmosphere to reach the observer than
for a meteor overhead, further attenuating the light from meteors at low
elevation angles.
Perhaps the key work to remember in meteor observing is patience. Most
meteor showers will not produce a spectacular display, but will instead
produce a steady, reliable show -- sometimes with a few surprises. Meteor
watching is like watching a graceful, natural fireworks display, and you
never know when or how bright the next "shot" will be.
8. Does the published meteor rate for a shower really represent what I
should expect to see?
Many publications which list meteor shower rates will often give a
corrected value, called the Zenith hourly Rate (ZHR) which standardizes the
shower rate to optimum observing conditions. The shower rates listed are
usually corrected for fully dark skies, and the meteor radiant point has
been artificially located at the zenith, directly overhead. The actual rate
of meteors seen by most observers, however, will be lower than this
corrected value.
Below is a table showing actual expected values for the major meteor
showers, along with their corrected ZHR's. Other publications may show
somewhat different rates. These rates have been oriented to central U.S.
latitudes. The quoted values are "smoothed" and do not represent those
rarer times when abnormally high or low rates occur. We have selected the
better years, assuming that a sharp observable peak occurs in your
longitude. Four different rates are given for each shower, under the
following conditions:
(1) city sky or rural sky with full moon,
(2) suburb sky or rural sky with quarter moon,
(3) rural sky and moonless,
(4) calculated Zenith hourly Rate, ZHR.
Date Shower Name (1) (2) (3) (4)
---------------------------------------------------------
Jan 3-4 Quadrantids 5 10 25 120
Apr 21-22 April Lyrids 4 7 15 15
May 4-5 Eta Aquarids 4 6 10 40
Jul 28-29 Delta Aquarids 4 7 15 20
Aug 12-13 Perseids 10 20 40 120
Oct 21-22 Orionids 5 10 25 25
Nov 3-13 Taurids 4 6 10 10
Nov 16-17 Leonids 5 10 15 15
Dec 13-14 Geminids 18 35 85 85
Dec 21-22 Ursids 3 5 10 20
Five of the showers pass close enough to overhead that their ZHR's can be
taken as more or less equal to the rural sky rate. Quadrantids more often
than not will give a display in the low 20's ; you have to be fortunately
placed to do better. We have used a comfortable average for the showers
with unstable rates (Eta Aquarids, Taurids, Ursids) . Sharp-peak showers
are Quadrantids and Perseids. The Leonids don't fit any of these categories
for the next few years; these numbers cover their slow years (1975-1994,
the latest such period just completed).
9. I thought I saw a lot of meteors coming from a certain part of the sky
last night, but I can't find any shower listed in my books. What was going
on?
There are several possibilities here. First, It is possible that you caught
the peak of a minor shower, not listed in most texts. Consulting a more
extensive shower list may reveal a match. Second, random sporadic meteor
activity will occasionally increase above the average level, giving rise to
the suspicion that a shower may be in progress. Third, meteor observers
have, for many years, suspected the existence of small clusters or
"outbursts" of meteor activity not formally associated with a recognized
shower. The reason for these pockets of activity range from statistical
fluctuations in the sporadic meteor distribution to isolated remnants of
old extinct meteor streams. This "clustering" effect is not yet well
understood.
10. What is a meteor storm, and how often do they occur?
In meteor science, the month of November is best known for the meteor
storms which have occasionally given us one of the most spectacular
displays the night sky has to offer. On a single night, meteors sometimes
fell so thick it would appear as though the entire sky was falling, or gave
the appearance of rapid forward motion of the Earth through the stars. The
great Leonid meteor storm of 1833 did more to spawn the study of meteors
than any other single event, along with great excitement by the general
public. Meteor storms are not limited to only November, and in a historical
parallel, the famous October Draconid (or Giacobinid) storm of 1946 also
did much to spawn the study of meteors by radio methods.
Meteor storms are generally caused by young meteor streams, in which the
majority of the streams' mass is still concentrated along that portion of
the orbit occupied by the parent comet. Meteor storms occur when the Earth
crosses the orbit of the meteor stream, at the same time that the main mass
of the young meteor stream is crossing the orbit of the Earth. For streams
with a low potential for orbital perturbation, this event may occur on a
periodic basis, generally at around the same time that the parent comet
becomes visible in the inner solar system. Streams which tend to undergo
frequent orbital perturbations may only cause infrequent and rare storms,
some never occurring again. To make the possibility even more remote, these
streams also tend to be very narrow, with the Earth taking only a few hours
to cross the concentrated portion of the streams' path. Being on the right
side of the globe, under good weather, on the right night is very important
toward seeing these events.
Two meteor streams are associated with the November storms, the Andromedid
(or Bielid) stream, and the Leonid stream. The Andromedid stream is one
that is subject to frequent orbital perturbations, and as such, only rarely
crosses the Earth's' orbit in a manner favorable for producing a meteor
storm. The last storm produced from this stream was on November 27, 1885,
with 13,000 meteors per hour visible at the peak. By contrast, the last
appearance of a shower from this stream was in 1940, with only 30 meteors
per hour at the peak.
The Leonid stream is much more favorable for producing storms, and
generally tends to produce one every 33 years or so, although it has
sometimes been disappointing. After feeble displays in 1899 and 1933, The
last appearance, on November 17, 1966, provided the highest known rate of
any meteor stream ever recorded. An approximate rate of 40 meteors per
second (144,000 meteors per hour), was seen for about 1 hour as viewed from
the western portion of North America, and the Pacific. Unfortunately, the
east coast and Midwest were enveloped in clouds that night, disappointing a
lot of amateurs and professionals alike. The next appearance of this storm
should be between the years 1997 and 2000, on November 17th, plus or minus
a day. Reserve those nights to get out under dark skies. If nothing happens
you haven't lost anything except sleep, but if something does, you will
never forget it.
The Leonid meteors represent the fastest known shower meteors, barreling in
at 72 km/sec. They are well known for their bright magnitudes, and their
ability to produce extremely long duration trains, some lasting up to
several minutes. On the other end of the spectrum, the October Draconids
(Giacobinids), which last produced a brief outburst in 1984, have extremely
slow meteors, coming in at less than 11 km/sec.
11. Where can I get more information about the Leonid storm which is
expected to occur between 1997 and 2000?
As the time for the potential Leonid storm approaches, information about
this occurrence should become widely available in a large range of
publications and periodicals. In the United States, the best authorities to
look for are Dr. Donald Yeomans, of NASA-JPL, and Dr. Peter Jenniskens, of
NASA-Ames. Professional and amateur meteor organizations, such as the
American Meteor Society (AMS) and the International Meteor. Organization
(IMO) will be tracking this meteor shower closely, and can provide
information and updates, particularly through Internet channels.
12. Is there a chance of a meteor from a meteor shower or storm reaching
the ground as a meteorite, and Is it dangerous to observe meteor storms?
The meteoroids which make up a meteor shower or storm are very fragile in
nature, and are composed of a somewhat "fluffy" composite of material from
which all volatile material has escaped, due to many trips near the sun.
This material readily vaporizes in the upper atmosphere, and is given the
descriptive name of "friable" material. While quite spectacular to watch, a
meteor storm presents no real danger to the viewer, who is protected by
miles of atmosphere.
13. Where can I find information on historical meteor observations?
Obtaining good historical information in the area of meteor science can
often prove difficult, due to the limited publication and circulation of
professional texts in this field. It is highly recommended that researchers
obtain access to a university or large city library which caters to
astronomical and planetary science research. The below listed books are
highly recommended by us, and their bibliographies can point the researcher
in other desired directions:
(1) Olivier, C. P., (1925). Meteors. Baltimore: The Williams &
Wilkins Company, (276 pp).
(2) Porter, J. G., (1952). Comets and Meteor Streams. London:
Chapman & Hall, Ltd., (123 pp).
(3) Lovell, A. C. B., (1954). Meteor Astronomy. Oxford, New York:
University Press, (463 pp).
(4) McKinley, D. W. R., (1961). Meteor Science and Engineering.
New York: McGraw-Hill Book Co., (309 pp).
FAQ compiled by:
James Richardson, AMS Operations Manager / Radiometeor Project Coordinator
James Bedient, AMS Electronic Information Coordinator
FAQ References:
Adams, M. T., (1980). "Observing Falling Stars," Mercury
(March-April, 1980).
Cook, A. F., (1973). "A Working List of Meteor Streams,"
Evolution and Physical Properties of Meteoroids. NASA , United
States Govt. Publication.
Hey, M. H., & Rea, D. G., (1986), "Solar System/ Meteors,"
Encyclopedia Britannica (Vol 27, pg. 587).
Jenniskens, P., (1994). ""Meteor Stream Activity I: The annual
streams," Astronomy and Astrophysics, (1994: 990-1013).
Jenniskens, P., (1994). "Meteor Stream Activity II: Meteor
Outbursts," Astronomy and Astrophysics, (1994).
McKinley, D. W. R., (1961). Meteor Science and Engineering. New
York: McGraw-Hill Book Co.
McLeod, N. (1997), AMS staff correspondence.
Meisel, D. D., (1990). "Meteor," McGraw-Hill Encyclopedia / EST
7th Ed.
Olivier, C. P., (1965). "Catalogue of Hourly Meteor Rates,"
Smithsonian Contributions to Astrophysics Vol. 8 Number 6,1965.
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© 1997 American Meteor Society, Ltd.
References: