A meteor stream is composed of dust particles that have been
ejected from a parent comet at a variety of velocities. These particles follow
the same orbit as the parent comet; butt due to their differing velocities they
slowly gain on or fall behind the disintegrating comet until a shroud of dust
surrounds the entire cometary orbit. Astronomers have hypothesized that a
meteor stream should broaden with time as the dust particles’ individual orbits
are perturbed by planetary gravitational fields. A recent computer –modeling
experiment tested this hypothesis by tracking the influence of planetary
gravitation over a projected 5000 –ear- period on the positions of a group of
hypothetical dust particles. In the model, the particles were randomly
distributed throughout a computer simulation of the orbit of an actual meteor
stream, the Geminid. The researcher found, as expected, that the computer –
model stream broadened with time. Conventional theories, however, predicated
that his distribution of particles would be increasingly dense toward the
centre of 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 occurs. Moving at a little over 1,500.000 miles per day around its
orbit, the Earth would take, on average, just over a day to cross the hollow,
computer –model Geminid stream if the stream were 5,000 years old. Two brief
periods of peak meteor activity during the shower would be observed, one as the
Earth entered the thick-walled “ pipe” and one as it exited. There is no reason
why the Earth should always pass- through the stream’s exact centre, 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 meteor activity being clearly
visible at an average of 19 hours (1,200,000 miles) after the first burst. The
time intervals between the bursts suggest the actual Geminid stream is about 3,
000 years old.
17.
The primary focus of the passage is on which of
the following?
A.
Comparing two scientific theories and
contrasting the predictions that each would make concerning a natural
phenomenon
B.
Describing a new theoretical model and noting
that it explains the nature of observations made of particular natural phenomenon
C.
Evaluating the results of a particular
scientific experiment and suggesting further areas for research
D.
Explaining how two different natural phenomena
are related and demonstrating a way to measure them.
18.
According
to the passage, which of the following is an accurate statement concerning
meteor streams?
A.
Meteor streams and comets start out with similar
orbits, but only those of meteor streams are perturbed by planetary
gravitation.
B.
Meteor streams grow as dust particles are
attracted by the gravitations fields of comets.
C.
Meteor streams are composed of dust particles
derived from comets.
D.
Comets may be composed of several kinds of
materials, while meteor streams consist only of large dust particles.
Answers
and Explanations
17.
B To
answer a question about the primary focus of a passage, look at the passage as
a whole. The first paragraph describes the new theoretical model. The second
paragraph discusses the data relevant to actual observation, and the third
paragraph shows that actual observations were consistent with the theoretical
model.
18.
C The
phrase according to the passage indicates that the answer is explicitly stated
in the passage; however, it may be stated in slightly different language. The
passage begins with a definition of a meteor stream, which is composed of dust
particles that have been ejected from a parent comet at a variety of
velocities.
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