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Reading skills for academic study: Skimming for gist

Exercise 1

Read the headings in the following text.

The Origin of the Earth

The origin of the earth has puzzled man since ancient times. This problem is astronomical as well as geological, for the origin of the earth cannot be divorced from that of the component members of our solar system. The earth is one of nine planets which revolve about the sun and rotate in the same direction as the sun. The position of the earth among the planets is not conspicuously prominent. It is the third planet out from the sun and in size is intermediate, for three planets, Mercury, Venus, and Mars are known to be smaller and Jupiter, Saturn, Uranus, and Neptune are larger. About most of the planets revolve smaller bodies called moons or satellites. The earth and Neptune each have one, Mars two, Uranus four, and Jupiter and Saturn each have nine. In comparison with the 88,600-mile diameter of Jupiter, the largest planet, and that of the sun, 864,000 miles, the earth's diameter of 7,900 miles appears small. But, even the largest body and the greatest distance in our solar system fade into insignificance when compared with the distances to the fixed stars of our universe. To record these distances the astronomer uses the light year, which is the distance traveled by light in one year with a velocity of 186,000 miles a second. Light travels the 93,000,000 miles from the sun to the earth in about 8 minutes and the 2,800,000,000 miles to Neptune in about 4 hours. The nearest star is 4 light years away and the bright stars seen by us are 10 to 500 light years away. Beyond the Milky Way that comprises our stellar universe are distributed other nebulae or universes which have been reported to extend 150,000,000 light years into space. The immensity of it all baffles visualization by human imagination.
NEBULAR HYPOTHESIS. The first theory of the origin of the earth based upon astronomical observation was proposed by the French astronomer Laplace in 1796. It was probably suggested by the rings now present about the planet Saturn. According to this hypothesis our solar system was originally a vast nebula of highly heated gas, extending beyond the orbit of the outermost planet and rotating in the same direction as the planets now revolve. As this nebula, which was more than five billion miles in extent, lost heat, it contracted. Due to contraction the speed of rotation in-creased and resulted in flattening at the poles and a bulging at the equator. As further contraction continued, the speed of rotation increased until the centrifugal force at the equator of the spheroid was equal to the force of gravity and a ring of particles was left behind. The process continued until 10 successive rings were formed and the central mass became the sun. Each ring revolved as such for a time and then broke up to form a planet and its satellites. From one ring the 1200 or more planetoids between Mars and Jupiter were supposed to have formed. According to the hypothesis the earth was first a globe of highly heated gas, then it became liquid and with further cooling a crust formed over the liquid interior. From the gas of the original nebula an atmosphere collected around the earth and vapours condensed to form the water of the oceans. For more than one hundred years this was accepted as the most satisfactory explanation of the earth's origin, regardless of the increasing number of objections arising against it with advance in knowledge of Astronomy and Physics. Many of the objections cannot be stated here, but a few will suffice to show the nature of the difficulties that this simple theory presents. Laws of Physics indicate that the separation from the gaseous nebula would take place as individual molecules and not as rings. But, granted that rings could form, it is a mystery how contraction of a ring could produce a spheroid or yield other rings to form satellites. Since the parent and its satellites were travelling at the same speed and in the same direction at the time of separation and the parent kept on increasing its rotation by cooling, all the satellites should have a velocity slower than their parents. Some of the satellites move with a velocity too rapid or too slow and one in a direction opposite to that called for by the theory. Even more serious difficulties are encountered when the moment of momentum is calculated for each stage at which a planet separated or for the entire solar system expanded as a gas beyond the orbit of Pluto. Not only are the masses of the planets out of proportion to the moments of momenta, but also the original nebula with the momentum of the present solar system would not have a rapid enough rotation or a centrifugal force sufficient to form a ring until it had contracted within the orbit of the innermost planet.
PLANETESIMAL HYPOTHESIS. In 1905 Chamberlin and Moulton announced the planetesimal hypothesis in which our solar system is considered to have originated from a spiral nebula. Astronomic photography records many of these nebulae, each consisting of a central nucleus with two curved arms on opposite sides composed of masses of matter or knots separated by dark areas. Spectroscopic study indicates they are composed of solid or liquid particles. Because it was assumed that these particles revolved about the center of the nebula in elliptical orbits like planets, the name planetesimal was given to the hypothesis. The spiral arms were formed by explosive forces within the ancestral sun and by the tidal force of a passing star that approached closely enough to exert a pull on the gases shot out from the sun. Solar prominences in which gases rise above the sun's surface thousands of miles are evidence of the explosive forces within our modern sun. During such explosions a star passed close enough to the ancestral sun to pull out irregular bolts of gas as it reached critical positions. One bolt was shot out for each planet and one for the planetoids. The light material forming the large planets was drawn from the near side of the sun and carried farther away from the sun. The four smaller planets and the planetoids were formed from the tidal bulge on the opposite side of the sun. The amount of material disrupted to form the planets is calculated at a fraction of one per cent of the sun's mass. At no time was the passing star close enough to attract and capture any of the disrupted material. Its approach served to spread out the material far enough away from the sun that the planetesimals could start revolving in elliptical orbits without being drawn back into the sun. At this stage the erupted material arranged in two arms partly wound about the central mass may have resembled a spiral nebula, but on a much smaller scale. Later, the central mass formed the sun, the larger masses in the arms formed the planets, and smaller ones the satellites and the planetoids. In the 1928 version of the hypothesis given by Chamberlin the earth was first a bolt of gas erupted from the sun. Like the other bolts it expanded rapidly after leaving the sun and cooled throughout into solid planetesimals revolving in elliptical orbits so that by frequent collisions a planet was built up. Metallic constituents were segregated because of their weight and welded together to form the core, thus giving to the earth's in-tenor the high specific gravity it is known to possess. The nucleus of the earth grew by the capture of planetesimals; each collision, occurring whenever elliptical orbits crossed, modified the rate of revolution and resulted in a more nearly circular orbit. Growth took place slowly so that the earth was never molten. The heat produced by the impact between nucleus and planetesimals was largely lost during the long intervals between infalls of small planetesimals. At first the nucleus of the earth was too small to retain an atmosphere, but as it grew by accretion to about 1/10 of its present mass its gravitative attraction was strong enough to hold heavy gases. Its internal heat increased due to infall of planetesimals, self-compression, and radioactivity until local fusion of rock caused volcanic activity. Volcanic gases collected about the earth as the initial atmosphere. Finally, when saturation was reached the condensed water vapor collected in depressions on the earth to form the oceans. The gases now in the atmosphere and waters now in the oceans were originally contained in solid planetesimals. During continued growth segregation and preservation of the water-covered planetesimals led to a higher specific gravity for the oceanic segments than for the continental protuberances. With the development of the atmosphere and the oceans, deposiļæ½tion of sedimentary rocks commenced before planetesimal accumulation ceased and volcanic activity reached a climax. The founders of the hypothesis believe that conditions were favorable for life before the earth was full grown.
TIDAL-DISRUPTION HYPOTHESIS. Certain features of the planetesimal hypothesis were retained and others completely changed by Sir James Jeans, astronomer, and Harold Jeffreys, geophysicist, of England, in developing the tidal-disruption hypothesis. They assumed that the passing star causing our solar system approached nearer to our ancestral sun than Chamberlin postulated. The disruption of our sun was due entirely to the tidal force of the passing star and was aided in no way by explosive action of the sun. From the sun's side nearest the star a long filament of hot gas varying in size and density was pulled along by the passing star to the orbit of the outermost planet before the tidal force was released. This streamer was then unstable and broke up into ten segments which contracted into the nine planets and the planetoids. The original elliptical course of each segment about the sun was modified by a dense gaseous medium that later leaked away from the streamer after rounding the orbits. The satellites were produced by tidal strains as each segment made its first journey around the sun. According to this hypothesis the earth was first a highly heated gas. As it cooled and became liquid the heavy constituents were drawn toward the center forming shells that decreased in specific gravity outward. With further cooling a solid crust of light rocks formed over the liquid interior somewhat before the beginning of geologic time. The first atmosphere was dense and hot because the temperatures were so high that the chemical compounds inside and around the earth could not form. Finally, the temperature decreased so that water vapor could condense and fill the ocean basins.
ORIGIN OF MATTER. It is evident from these brief statements that one limitation applies to each of the hypotheses considered. None is complete in itself, for it does not explain the original matter of the universe from which the planets evolved. The question of the origin of the nebulae or stars taken as the starting point in these hypotheses remains unanswered. Scientists and philosophers have pondered over it. The best answer that has been offered is that it represents the work of an eternal God, who knows no beginning or end and who controls the orderly arrangement of the Universe.
(This earth of ours, by V. T. Allen, The Bruce Publishing Company, Milwaukee, 1936, pp. 216-222)


Notice how reading these headings gives you a good idea about the meaning of the text: a problem and several possible solutions. If you need more details, read the text again.