A meteor stream (meteor stream: 流星群)is composed of dust particles that have been ejected from a parentcomet at a variety of velocities. These particles follow the same orbitas the parent comet, but due to their differing velocities they slowlygain on (gain on: v.逼近, 超过, 侵蚀)or fall behind the disintegrating comet until a shroud of dustsurrounds the entire cometary orbit. Astronomers have hypothesized thata meteor streamshould broaden with time as the dust particles’ individual orbits areperturbed by planetary gravitational fields. A recent computer-modelingexperiment tested this hypothesis by tracking the influence ofplanetary gravitation over a projected 5,000-year period on thepositions of a group of hypothetical dust particles. In the model, theparticles were randomly distributed throughout a computer simulation (computer simulation: 计算机模拟) of the orbit of an actual meteor stream, the Geminid. The researcher found, as expected, that the computer-model streambroadened with time. Conventional theories, however, predicted that thedistribution of particles would be increasingly dense toward the centerof a meteor stream. Surprisingly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe.
Whenever the Earth passes through a meteor stream, a meteor shower (meteor shower: n.[天]流星雨)occurs. Moving at a little over 1,500,000 miles per day around itsorbit, the Earth would take, on average, just over a day to cross thehollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteoractivity during the shower would be observed, one as the Earth enteredthe thick-walled “pipe” and one as it exited. There is no reason whythe Earth should always pass through the stream’s exact center, so the time interval between the two bursts of activity would vary from one year to the next.
Has the predicted twin-peaked activity been observed for the actual yearly Geminid meteor shower? The Geminid data between 1970 and 1979 show just such a bifurcation, a secondary burst of meteoractivity being clearly visible at an average of 19 hours (1,200,000miles) after the first burst. The time intervals between the burstssuggest the actual Geminid stream is about 3,000 years old.
Could anyone help me to analyse the overall structure of the passage and the function of each paragraph/? Thank you a lot ~~~~~~作者: greates 时间: 2010-4-24 21:38
文章中先说 Astronomers have hypothesized thata meteor streamshould broaden with time as the dust particles’ individual orbits areperturbed by planetary gravitational fields. A recent computer-modelingexperiment tested this hypothesis by tracking the influence ofplanetary gravitation over a projected 5,000-year period on thepositions of a group of hypothetical dust particles...............
后来紧接着又说The researcher found, as expected, that the computer-model streambroadened with time. Conventional theories, however, predicted that thedistribution of particles would be increasingly dense toward the centerof a meteor stream. Surprisingly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe.
