Download Meteor Shower Observations

Name: ________________________________
Date:___________________
Meteor Shower Observations
“Now slides the silent meteor on, and leaves
A shining furrow, as thy thoughts in me.”
Tennyson: The Princess VII
INSTRUCTIONS:
Read the laboratory exercise carefully before attempting to do the laboratory. Be sure to
complete the pre-observation questions hours before attempting to do the laboratory
exercise. When you do your observations, remember that it is often a much lower
temperature in the pre-dawn hours of the day than it is at sunset. Be sure to plan to dress
accordingly. You will need your red filtered flashlight.
BACKGROUND INFORMATION:
Since ancient times, sky watchers have seen flashes of light in the sky that looked
like extremely fast moving stars. These flashes of light in layman terms are often called
shooting stars or falling stars. Carefully observing the stars, though, astronomers found
that no stars were missing. Furthermore, widely separated observers would often see the
shooting star at slightly different locations in the sky, indicating that the phenomenon was
located high in the atmosphere, rather than among the distant stars. Astronomers, thus,
believed that the shooting star was an atmospheric event, similar to lightning. The term
meteor was applied to the sudden flash of light. It wasn’t until the Nineteenth Century
that astronomers realized that the meteors were in fact extraterrestrial in origin. They
were caused by something hitting the atmosphere moving at very high speed many miles
overhead.
The Solar System is composed not just of the Sun, planets, and moons. There are
a large number of small objects orbiting the Sun. These objects range in size from
microscopic particles of dust to larger bodies hundreds of miles across. The larger
objects are called asteroids if they are rocky or iron rich in nature and comets if they are
composed of significant amounts of ice and frozen gasses. Most comets are believed to
be a mixture of ices and rocky material. Smaller objects are called meteoroids. There is
not an exact size distinction between a meteoroid and an asteroid, so many of the larger
meteoroids are often referred to as small asteroids. Very small meteoroids are sometimes
called micrometeoroids. Most of the meteoroids orbiting the Sun are very small,
typically the size of sand grains or rice grains. Orbital motions around the Sun are very
fast, with the Earth moving at 29.8km/s around the Sun. Many meteoroids can move
faster than this. Thus, when they encounter the Earth, they can be moving very fast. The
highest speeds come from meteoroids moving quickly towards the Earth and hitting it
head on as the Earth orbits the Sun. As a meteoroid strikes Earth’s atmosphere, friction
with the atmosphere heats up both the meteoroid and the atmosphere through which it is
passing. Small meteoroids often do not survive the heating intact, and they burn up. As
they pass through the atmosphere, however, the meteoroid is called a meteor. Passing
through the atmosphere, the meteoroid ionizes and heats the air through which it is
passing. This leaves a tube of hot ionized air that briefly glows behind the meteor. It is
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this trail of light that observers from the ground usually see as a shooting star. In addition
to be used to refer to the object passing through the atmosphere, the term meteor is also
sometimes used to refer to the flash of light generated in the atmosphere by the passage
of the meteoroid. Should the meteoroid be large enough when it strikes the atmosphere,
or be going slow enough to avoid completely burning up, then it can survive the passage
through the atmosphere to strike the ground. A meteoroid that strikes the ground is called
a meteorite.
Not all meteoroids are the same size. When a somewhat larger one hits the
atmosphere, it is often much brighter than the typical meteor. Such a very bright meteor
is often called a fireball or a bolide. The term bolide is the more technical term.
Occasionally, a bolide can result when a meteoroid is large enough to survive the trip to
the ground and become a meteorite. Sometimes bolides are associated with a popping or
crackling sound as it passes by at supersonic velocities. Most of the meteors that survive
long enough to pass low enough into the atmosphere to be heard from the ground yield
meteorite falls. With bolides, and sometimes with smaller meteors, a train can remain
after the passage of the meteor. This train appears as glowing trail through the sky
marking the path of the meteor. The train is due to air ionized by the passage of the
meteor. Usually the train lasts just seconds after the passage of a meteor, but sometimes
it can last several minutes. Such a long lasting train is called a persistent train. The
likelihood of forming a train depends upon the speed of the original meteoroid, angle of
impact with the atmosphere, and size and composition of the meteoroid.
Most meteors are simply random bits of material orbiting the Sun that run into the
Earth. These are called sporadic meteors. As the Earth goes around the Sun, more
meteors hit the front of the Earth in it orbit, just as more bugs hit the front of a car driving
down the highway. The front of the Earth is defined as the side of the Earth that is facing
the direction that the Earth is moving around the Sun. An observer is on the front side of
the Earth from local midnight until local noon. From a dark, clear location, about five or
six such sporadic meteors per hour can be seen on any given night between local
midnight and dawn, with the most meteors generally just before dawn. Prior to midnight,
fewer sporadic meteors are seen. In general, slightly more sporadic meteors tend to be
seen during the summer than during winter months.
Beginning in 1833, astronomers noticed that sometimes there are periods on
greater than average meteor activity. Such periods of time are called meteor showers,
and are caused when Earth passes through a swarm of meteoroids. The meteoroid
swarms are generally believed to be remnants of material ejected from comets. During a
meteor shower, meteors strike Earth’s atmosphere from the same direction. Due to our
perspective on Earth, the meteors appear to radiate away from one point in space. This
point is called the radiant of the meteor shower. Not all meteoroid swarms are evenly
distributed. Most, in fact are rather clumpy, meaning that sometimes there are more
meteors in a shower than other times. If the Earth crosses the orbit of the meteoroid
swarm every year, then the shower is called an annual meteor shower.
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Meteor showers were not recognized until about 1833 when a meteor shower was
observed with meteors radiating away from the constellation Leo in mid November. By
1837, astronomers had also observed that there was a meteor shower in mid August with
a radiant in the constellation Perseus. Over the years, astronomers have observed
numerous meteor showers. To designate which meteor shower is which, the meteor
shower is generally designated by the location of its radiant. For example, the meteor
shower with a radiant in Leo is called the Leonid meteor shower. The individual meteors
are called Leonids. The meteor shower with a radiant in Perseus is called the Perseid
meteor shower. If more than one meteor shower had a radiant in a constellation, then the
meteor shower is often named for the nearest star. For example, there are two meteor
showers with radiants in Aquarius: the Eta Aquarid shower (radiant near η Aqr), and the
Delta Aquarid shower (radiant near δ Aqr). One meteor shower, the Quadrantid meteor
shower that occurs in January, is named for a constellation that is no longer recognized
by astronomers.
Just as sporadic meteors tend to be more numerous in the predawn hours, so do
shower meteors. A few meteoroid swarms, however, catch Earth from behind. Most
notably the Arietid Meteor shower catches Earth from behind in June of each year. This
results in evening meteors rather day predawn meteors. This is the exception, though.
Not all meteoroid streams are alike. Some are dense and some are fairly thinly
distributed. Thus, not all meteor showers display the same number of meteors. The most
common way to describe the activity of a meteor shower would be to describe the
number of meteors that an observer would see under a dark clear sky if the radiant were
directly overhead. This is called the zenithal hourly rate (ZHR). Typical major meteor
showers have at ZHR of about 30 to 40 meteors per hours. Some are more impressive
with a ZHR of nearly 100 meteors per hour. Note that the ZHR is a calculated value, and
does not really represent the actual number of meteors that an observer would expect to
see, unless the radiant were near the zenith and the shower were being observed from a
dark sky. Thus, for a typical observer, with some light pollution, and the radiant near the
horizon, a meteor shower with a ZHR of 80 might only produce a visible meteor every
couple of minutes. Minor meteor showers have a ZHR of only 5 to 10. This is roughly
the same number of meteors from the shower during optimal viewing conditions that one
would expect from sporadic meteors! On extremely rare occasions, the Earth passes
through a particularly dense meteoroid stream, and the ZHR of the resulting meteor
shower can climb to values in excess of 1000 meteors per hour (about one every few
seconds). In this case, the meteor shower is generally called a meteor storm. Only a
handful of meteor storms have been recorded in history.
Meteoroid streams come in a variety of sizes and distributions of particles. Some
meteoroid streams are very broad, and it takes the Earth several days to cross the stream.
In such cases, observers may see meteor activity from the shower for several nights in a
row. In other cases, the meteoroid streams are very narrow. In such a case, the meteor
shower may show peak activity for just one night, or even for as short a period of time as
just one hour. Some meteoroid streams have meteoroids that have dispersed throughout
the stream’s orbit. In this case, there will be a consistent meteor shower every year as the
Earth crosses the meteoroid stream orbit. In other cases, the meteoroids are bunched
together into swarms. There may be a minor meteor shower each year as the Earth
crosses the stream’s orbit, but a major shower happens only when Earth is crossing the
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orbit at the same time as the swarm happens to be in that part of its orbit. The Leonid
meteor shower is an example of a meteor shower that is normally a very minor shower,
but exhibits major activity about every 33 years or so when Earth passes through denser
portions of the meteoroid stream.
Different meteoroids have slightly different compositions and strike the Earth’s
atmosphere at different speeds. Not all produce trains. Furthermore, not all meteors are
simply white, but some meteors exhibit color. A few are orange or yellow. Others
appear red, green, or blue. A great many do, in fact appear as simply white. The colors
are due in part to the composition of the meteor, but also due to the speed at which they
enter the atmosphere. In addition to color, some meteoroids, particularly the slightly
larger ones, can break apart as they pass through the atmosphere. This is seen as a
meteor sometimes breaking into two or more pieces that then diverge in their paths across
the sky. Sometimes, bits and pieces of a meteoroid break off as it flies through the sky,
giving an impression of a shower of sparks coming off of the meteor. This is particularly
common in bolides.
OBSERVING METEORS:
Observing meteors is one of the easiest things in astronomy. You just look up at
the sky! Any night of the year, at any time of the night, there will be occasional sporadic
meteors. If you happen to be looking in the right part of the sky at the right time, and if
the sky is dark enough, then you will see the meteor. Your chances of seeing meteors
improves if you happen to be looking up after local midnight. If your goal is to observe
meteors, it is generally not very productive to try to observe prior to local midnight.
Also, as indicated earlier, there are slightly more meteors in the summer months than in
the winter months. Winter nights are longer, though, giving you more time to look.
Winter nights are also often clearer than summer nights, especially after a hard cold front
drives the temperature below freezing, helping to drive moisture out of the lower
atmosphere. Clearer, darker nights tend to allow you to see dimmer meteors, the greater
number of which offsets the fewer total meteors seen in the summer. Obviously, if you
happen to go out on the night of a meteor shower, then your odds of seeing a meteor
greatly improve! During the meteor shower, you will see not only the meteors of the
shower, but the usual complement of sporadic meteors, too.
If you wish to look at meteors from a particular meteor shower, they you should
look on a star chart, star wheel, or appropriate computer program to see when the radiant
of the meteor shower rises. You are unlikely to see many meteors when the radiant is
below the horizon. You should then situate yourself where you can observe the sky.
Ideally, you wish to look at a portion of the sky 45 to 60 degrees away from the radiant.
If you look right at the radiant, any meteors that you see will be moving straight at you.
They will appear as brief flashes like stars. These are called point meteors, and are
generally much less impressive than the ones that shoot across the sky, sometimes
leaving trains.
Meteor astronomy differs a bit from many other forms of astronomy in that you
do not need a telescope in order to observe meteors. While telescopes or binoculars
would allow you to see dimmer meteors, such instruments restrict your view of the sky to
very small areas, thus you miss most of the brighter meteors. Most recognized meteor
showers result from swarms of meteoroids that produce fairly bright meteors, so you
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don’t gain much by being able to see dimmer ones. Thus, the best way to observe
meteors is with the naked eye. Additionally, since the normal human reaction is to have
one’s attention drawn to motion, just looking into a region of the sky allows you to see
meteors not just where you are concentrating, but in nearby areas, too. As long as your
peripheral vision encompasses a portion of the sky, your attention will be drawn to the
meteor. Thus, the best way to look is to look upwards at a steep enough angle that your
entire field of vision is on the sky. This is best accomplished by laying either on the
ground or on a blanket or lawn chair inclined at 45 degrees or more to the vertical.
Don’t worry about looking directly towards the radiant. The best views are generally
some distance in the sky from the radiant. The best thing is to look overhead.
Though most recognized meteor showers tend to have fairly bright meteors, this
statement means fairly bright compared to stars, not streetlights. The more light
pollution, the fewer meteors that you will be able to see. The brighter the moon light, the
fewer meteors that you will see, too. If possible, observe when the moon is not up. Also,
if possible, find a location with as little light pollution as possible. Even from fairly light
polluted areas, though, you may be able to see several meteors per hour during a meteor
shower, so don’t completely give up if you can not find a dark location, especially if a
major meteor shower is predicted.
Finally, since meteor observing is best done in the predawn hours, lots of sleep
the night before, or at least something to drink such as coffee, cola, or hot chocolate to
keep the observer awake is often useful. It is also very useful to have a friend or family
member observing with you. Conversation (as long as you don’t look away from the sky)
helps to keep you awake. Also, the predawn hours are the coldest part of the day. Be
prepared to dress as warmly as possible. It can be MUCH colder just before dawn than it
is in the afternoon, especially in the winter. Since meteor observing does not involve
much activity, then you tend to lose heat very readily. In the winter, you should dress
like you are going on an Arctic expedition. If you wear too much, you can always take
something off. If your dress too lightly, then you are just cold and uncomfortable. Even
in the summer, it can become somewhat chilly just before dawn, especially if there is a
light wind.
Some Significant Meteor Showers
Quadrantids
Lyrids
Eta Aquarids
Arietids
Zeta Perseids
(Early January)
(Late April)
(Early May)
(Mid June)
(Mid June)
South Delta Aquarids
Perseids
Orionids
Leonids
Geminids
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(Late July)
(Mid August)
(Late October)
(Mid November)
(Mid December)
METEOR OBSERVATION LABORATORY
Equipment Needed:
Red Filtered Flashlight
Clipboard (or other way of writing)
Pen or Pencil
Correctly Set Timepiece
(You can set watch to the US Naval
Observatory Website Clock.)
Prior to doing the laboratory, some preliminary work is needed. It is important that you do
this in order to do the best job with doing the laboratory.
1)
Determine what meteor shower you are observing, and what constellation the radiant will
be coming from.
Meteor Shower: ______________________
2)
3)
4)
Radiant Constellation:___________
Use a star chart or star wheel to determine several easily identifiable stars visible at the
time that you will be observing the meteor shower. Use a star chart, almanac, computer
software, or other means to determine the approximate magnitudes of these stars.
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
Next, you will observe the meteors. Record your observations on the enclosed observing
sheet. If you should fill up the sheet, then use your own paper to continue the
observations. For each meteor that you observe, note the time of observation, the
direction that the meteor was moving (eg: southwest), and you must estimate the
magnitude of the meteor by comparing to the stars that you have previously identified.
You may write the name of the star that is most nearly the same magnitude as the star,
and later look up the magnitude, if you wish, or if you know the magnitude, then write
down the magnitude directly. During a meteor shower, you will likely see the usual
sporadic meteors as well as the meteors of the shower. Note on your observation sheet
whether the meteor is part of the shower or is a sporadic meteor (shower meteors will be
the ones coming from the radiant). Also, you need to note the color of the meteor.
Finally, note any interesting things that you might observe, such as a train or “sparkling”
effect.
Finally, you wish to determine the hourly rate of the meteor shower. To do this, pick
two reference times some time apart (preferably over an hour apart). Then count how
many shower meteors occurred between the reference times. Divide the number of
meteors by the time in hours to get the average hourly rate.
Reference Time One: ________________
Reference Time Two: ________________
Time Difference ___________ hours
Number of Shower Meteors: ______________ Hourly Rate :_____________
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METEOR OBSERVATIONS
Time
Shower/
Sporadic?
Direction
Color
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Mag.
Comments
METEOR OBSERVATIONS
Time
Shower/
Sporadic?
Direction
Color
8
Mag.
Comments