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"Late every autumn around November 17 th or 18 th, in the still, silent hours before dawn, dedicated meteor watchers have long kept a vigil. In the cold early morning darkness the sky glimmers with a preview of early spring constellations. Leading them up in the east is Leo. Its familiar Sickle asterism, a backward question mark, seems especially full of meaning these nights, for on the cutting edge of the Sickle's curved hook is the radiant of the greatest of all meteor showers" (Roe). The Leonid's are called the "Kings of meteor showers" for good reason; they light up the sky brilliantly with white streaks, and occasionally a ball of fire that seems to fall toward earth. First, to understand the Leonid's, you must understand meteoroids and comets.
Comets are large pieces of ice, rock, and metal. They form in the outer reaches of our solar system, near the distances of Uranus and Neptune. They formed where there was plenty of water and temperatures were cold enough for water to freeze into balls of ice. These pieces of ice then condensed under the force of gravity from the nearby planets to form comets.
Often referred to as "dirty snowballs" or "dirty icebergs", these comets are held in two belts in our solar system, the Kuiper Belt and the Oort cloud. The Kuiper Belt has a plane of approximately that of our earth; it is about 500 AU from our sun. The Oort cloud is a vast holding tank for the majority of our solar systems comets; it is projected to be about 50, 000 AU from our sun (Kaufmann). Once in a while a comet will come out of one of these reserves (most likely from the Kuiper Belt) and enter into a highly elliptical orbit around the sun. When a comet approaches the sun, the immense heat of our sun will begin to melt a portion of the comet and sends debris off into space, leaving a trail of small pieces of rock and ice behind. This trail left by the comet, called the meteoritic stream, more or less remains in the original orbit of the comet for a lengthy period of time.
These small pieces of debris are meteorites; they range in size from a few millimeters to a few meters across. If they would be any larger, they could be classified as asteroids (Kaufmann). When the comet returns back towards aphelion it leaves this trail of meteorites in its orbit. The force of gravity pulls this thin stream of debris on different paths, and when the orbit of the earth about the sun takes us into debris field we see a meteor shower. A meteor is a light phenomenon when a meteorite enters the earth's atmosphere. The brilliant flash of light is the piece of debris burning up in the upper atmosphere of earth due to the intense heat caused by friction.
These particles are moving very fast, velocities range from 11 to 72 km / sec (ASA). Larger meteors are known as fireballs, these can be classified as such if they are brighter than any star in the sky. This is exactly what happens during the Leonid's meteor showers, the earth's orbit plows into the meteoritic stream of 55 P/ Tempel-Tuttle comet. This particular comet is responsible for the Leonid showers. It returns to perihelion from its highly elliptical orbit every 33. 25 years. It was at perihelion, the closest point to the sun in its orbit, in 1998 (ASP).
The most spectacular showers are in the years following the return of Tempel-Tuttle. The showers are always in late autumn, usually the 17 th or 18 th of November. They are most visible during the early hours of the morning. The Leonid's are named so because of the direction in the sky that they seem to come, the constellation Leo, hence the Leonid's (Roe).
The Leonid's have an impressive history of being the most spectacular showers. They have produced many great showers, most of them being in years that were near the 33 year cycle. One of the most famous of these was the great shower of 1833. This was a year on the 33 year cycle.
It was a spectacular sight; the sky was lit up with streaks of light, and fireballs that rivaled the brightness of the full moon. This particular shower was a new phenomenon to many of the people who witnessed this event. Many had thought of "shooting stars" as an atmospheric phenomenon, like lighting. No one knew what was going on and so many accounts on that early morning entail fear, terror, and panic.
A South Carolina plantation owner account of that night: "I arose and, taking my sword, stood at the door. At this moment I heard the same voice still beseeching me to rise, and saying, 'O, my God, the world is on fire!' I then opened the door, and it is difficult to say which excited me most -- the awfulness of the scene, or the distressed cries of the negroes. Upwards of one hundred lay prostrate on the ground, some speechless, and others uttering the bitterest moans, but with their hands raised, imploring God to save the world and them. The scene was truly awful, for never did rain fall much thicker than the meteors fell toward the earth; east, west, north, and south, it was the same (Devens). " This plantation owner was right, "for never did rain fall much thicker than meteors fell toward earth", experts estimate the at peak hours, 200, 000 meteors fell per hour. That number is outstanding, most likely illuminating the sky with bright flashes and streaks of light. Another famous shower was that of 1966, again this year lay in the path of the 33 year cycle.
At peak hours, some data came in of 25, 000 meteors in only fifteen minutes! This is way above average for a Leonid. Many showers have been predicted, but many do not live up to the expectations. It is very hard to predict these storms, but now scientists can come pretty close. The 2001 Leonid show was a great surprise to amateur viewers and veteran astronomers. The ZHR (meteors / hr ) was recorded as 800 in the Americas, comparing to 480 in Europe and N.
Africa. This next year's showers are expected to be a magnificent astronomical event. The predictions for November 19, 2002, in the Americas is supposed to be 10, 000 ZHR (JPL). The meteor showers we see here on earth are the meteorites burning in our atmosphere.
Just like the earth, the moon passes through this same debris field, but unlike earth the moon has no atmosphere for the body to create friction, hence burning up. These meteorites strike the moon with impressive force, leaving small craters with immense power. These meteorites strike the moon with a velocity of 72 m / s or 160, 000 mph. With this tremendous velocity, the impact is as strong as 10, 000 times the force of a stick of TNT.
We know of this because of visual observations, with confirmation from lunar seismic detectors, placed on the moon in the seventies by Apollo 17 Lunar missions. The Leonid meteor showers are a magnificent astronomical event that has mesmerized generations of people, ranging from astronomers to small children. This rite of fall is a delight to young and old. These meteors are on a celestial time-table, returning every 33. 25 years with the return of their compliment comet, Temple Tuttle. We enjoyed the recent show, but now look to future shows to excite and enthuse a new generation of star gazers.
Works Cited web web 1. htm? list 11516 web www / info sheets / leonid's . html web web web web
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