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To answer this question the inquisitive person must be prepared for
a stark reality about the nature of these celestial wonders. No romance
or magic; Ready? Meteors are nothing more than space dust!
You can
imagine the solar system as being something like a disk of material. In
the center you will find the sun along with many planets and small rocky
asteroids orbiting around. In addition and far more numerous are the
unnumbered dust particles within the same plane. In fact, it is possible
to see evidence of this dust from a dark site (such as Kitt Peak) in the
form of a glow of light in the sky. This very dim glow is called
"zodiacal light" and caused by this dust which scatters sunlight. It is
called "zodical" because the zodiac represents the "ecliptic" which is
the plane of the the solar system projected on to the sky (which is where
you find the dust).
So, what in the world does all of this have to do with falling stars? Well as the Earth orbits the Sun, it may run into many of these dust particles as it travels. What happens is: a dust particle (also meteoroid), no larger than your fingernail and no more dense than dust in your home, enters the Earth's atmosphere. It may be travelling some 60 kilometers per second relative to the Earth and enter the atmosphere some 100 kilometers over our head. When it does so the dust undergoes ablation and quickly erodes away due to friction. But when the dust is colliding with the rareified gas atoms at the top of the Earth's atmosphere it gives them alot of energy and causes them to be excited. This process releases light and is the glow that is seen from the ground.
If a meteroid reaches the ground intact, it is called a "meteorite". However, the meteoroid would have to be something much more dense than dust to survive the passage through the Earth's atmosphere.
2. "Is a telescope or binoculars necessary to observe meteors?"
No. Telescopes and binoculars will restrict your field of view. Since meteors can been seen in any portion of the sky, the best way to observe them is with your unaided eyes. The only requirements are that you find a place to observe away from bright city lights. If the moon is in the sky, it can also diminish the number of meteors you observe.
3. "How many meteors can you see on a given night?"
On average, you can see 7 random (or sporadic) meteors per hour from a dark site without the moon in the sky. However, this would mean you would stare at the entire sky for an hour only to be rewarded with a single meteor every 10 minutes. The chances of looking in just the time and place are not good. However, your chances increase with the advent of a "meteor shower."
4. "What is a meteor shower?"
Well, as explained above meteors are caused by dust that the Earth runs
into in its orbit about the Sun. Thus, during a meteor shower the Earth
is plowing through a greater number of dust particles than on a
non-shower evening. Famous meteor showers include the Perseids in August
(11/12th) and the Geminids in December (12/13th).
But what causes this
increase in dust? Comets! Comets are something like dirty snowballs that
populate the very outer parts of the solar system. However, sometimes
they fall in towards the sun and orbit in large elliptical paths. When a
comet "nears" the sun (around the orbit of Jupiter) the
ices on its surface can evaporate (sublimate) and turn to gas. The result
can be spectacular; I am cerain comet Hale-Bopp is still fresh in
everyone's
memory from last year. In addition to the gas, dust is also carried off the
surface of the comet (hence "dirty" snowball). So comets leave tiny
particles in their wake, and should the Earth pass through one of these
streams of dust in its orbit around the Sun, many meteors will be seen.
Every year the Earth passes through a stream on the night of August 11th
which is responsible for the Perseids meteor shower. The rate of meteors
per hour is greatly increased to something like 30-40 at peak activity!
5. "Why do meteor showers have the names of constellations?"
As the Earth orbits the sun, the direction it faces at a given time (say
midnight or away from the sun) changes through the year. It takes 365
days to go through 360 degrees so that each night at the same time the
Earth faces about one degree to the East further than the previous night.
With this in mind, as the Earth moves through a meteor stream on a single
night, the meteors will all appear to come from a particular place in the
sky, called the "radiant." The meteors are actually entering the
atmosphere in parallel trajectories.
Do you remember what it looked
like
when Han Solo made the jump to light speed in Star Wars? But perhaps a
better example is the perspective of two mice running along railroad
tracks towards you. Way off in the distance the mice seem to come from
one point (radiant). As they near their separation seems to grow; but
this is an angular separation, not a mouse-to-mouse change in distance.
Thus, whatever constellation the radiant for a particular shower is in
determines the name. The Perseids seem to come from the direction of the
constellation of Perseus; and the Leonids, Leo. You might notice that the
radiant generally rises very early in the morning on a night of a meteor
shower. Meteor activity is greater for a particular observer when the
observer's location on Earth is facing directly into the meteor stream.
This is generally after
midnight and before sunrise.
So, the constellation that rises during
this
time of night on a particular date (position in the Earth's orbit) that
contains the radiant is the name of the shower.
However, you do not want to stare at the constellation in the hopes of seeing more meteors. Scan the entire sky. Once a meteor is spotted, trace the path backwards and determine if it leads to the constellation (radiant) or if it was just a sporadic meteor.
6. "What comet is the progenitor of the Leonid Meteor Shower?"
Comet 55P/Tempel-Tuttle.
7. "Why is the Leonid Meteor Shower spectacular every 33 years?"
There are a number of factors which influence the activity (hourly rate) of a meteor shower. In certain years the Leonids sported thousands of meteors per hour rather than tens. One of the most important factors is the density of the dust in the meteor stream. In the case of the Leonids you can imagine that there exists a "clump" of dust in the meteor stream that the Earth runs through every 33 years. The clump orbits the sun in the same way the parent comet does. So the meteor "storm" is not seen every year; just near the years when the clump and Earth meet.
8. "Are the Leonids dangerous?"
Yes and no. They are not dangerous to anything on the ground. Space dust is very fragile stuff. No Leonid meteors would ever make it to the ground. However, even something the size of a fleck of paint travelling 70 kilometers per second can be dangerous in space. Thus, some scientists are concerned that the meteoroids might harm satellites orbiting the Earth. It remains to be seen whether this will be true or not. In 1966 there were very few satellites in orbit; now there are many. So, no one really knows!
9. "It has been predicted that the Leonids are best viewed from Asia and not the United States; why is this?"
By analyzing data from past Leonid showers, scientists believe they know the size and orbit of the greatest density of the meteor stream. This "clump" is not very large and the Earth will pass through it very quickly. If it took the Earth 24 hours to pass through the clump, everyone on Earth would have a chance to see the display. But, unfortunately the Earth will pass through the clump in just a few hours which means only the people facing into the meteor stream at the time (local time 2am - 5am) will see the show. The best guess is that Asia (or that longitude) will be the lucky witnesses. However, these estimates have very large uncertainties and everyone in the world has reason to look and hope for a heart stopping meteor storm!
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