INTRODUCTION

Geologic time, the extensive interval of time occupied by the Earth's geologic history. It extends from about 3.9 billion years ago (corresponding to the age of the oldest known rocks) to the present day. It is, in effect, that segment of Earth history that is represented by and recorded in rock strata. 
The geologic time scale is the "calendar" for events in Earth history. It subdivides all time since the end of the Earth's formative period as a planet (nearly 4 billion years ago) into named units of abstract time: the latter, in descending order of duration, are eons, eras, periods, and epochs. 

The fossil forms that occur in these rocks provide the chief means of establishing a geologic time scale. Because living things have undergone evolutionary changes over geologic time, particular kinds of organisms are characteristic of particular parts of the geologic record. 

By correlating the strata in which certain types of fossils are found, the geologic history of various regions (and of the Earth as a whole) can be reconstructed. The relative geologic time scale developed from the fossil record has been numerically quantified by means of absolute dates obtained with radiometric dating methods.

THE ORIGIN OF THE SOLAR SYSTEM

Early cosmogonic models
The problems faced by any theory on the origin of the solar system have become increasingly complex as astronomers' knowledge about the planets, satellites, comets, and asteroids has expanded. Beyond basic data of this sort, there is now an enormous amount of detailed information, including ratios of abundant isotopes, that must be incorporated in any comprehensive theory. 
The earliest of such theories were certainly much less constrained. A scientific approach to the origin of the solar system became possible only after the publication of Isaac Newton's laws of motion and gravitation in 1687. Even after this breakthrough, many years elapsed while scientists struggled with applications of Newton's laws to explain the apparent motions of planets, satellites, comets, and asteroids. Meanwhile, the first semblance of a modern theory for solar system origin was proposed by the German philosopher Immanuel Kant in 1755. Kant's central idea was that the system began as a cloud of dispersed particles. He assumed that the mutual gravitational attractions of the particles caused them to start moving and colliding, at which point chemical forces kept them bonded together. As some of these aggregates became larger than others, they grew still more rapidly, ultimately forming the planets. Because Kant was not highly versed in either physics or mathematics, he did not recognize the intrinsic limitations of his primitive approach. His model does not account for planets moving around the Sun in the same direction and in the same plane, as they are observed to do, nor does it explain the revolution of planetary satellites. 

A significant step forward was made by Pierre-Simon Laplace of France some 40 years later. Laplace was a brilliant mathematician who was particularly successful in the field of celestial mechanics. After publishing a monumental treatise on this subject, Laplace wrote a popular book on astronomy, with an appendix in which he made some suggestions about the origin of the solar system. Laplace's model begins with the Sun already formed and its atmosphere extending beyond the distance at which the farthest planet would be created. Knowing nothing about the source of energy in stars, Laplace assumed that the rotating Sun would start to cool as it radiated away its heat. In response to this cooling, as the pressure exerted by its gases declined, the Sun contracted. Owing to the law of conservation of angular momentum, the decrease in size would have to be accompanied by an increase in the Sun's rotational velocity. Centrifugal acceleration pushed the material in the atmosphere outward, while the gravitational attraction pulled it toward the central mass; when these forces just balanced, a ring of material was left behind. This process would have continued through the formation of several concentric rings, each of which subsequently coalesced to form a planet. The satellites are thought to have originated from similar rings produced by the forming planets. 

Laplace's model led naturally to the observed result of planets revolving around the Sun in the same plane and in the same direction as the Sun rotates. Because the theory of Laplace incorporated Kant's idea of planets coalescing from dispersed material, these two approaches for planet formation are often combined in a single model called the Kant-Laplace nebular hypothesis. This model for solar system formation was widely accepted for about 100 years. During this period, the apparent regularity of motions in the solar system was contradicted by the discovery of asteroids with highly eccentric orbits and satellites with retrograde orbits. Another problem with the nebular hypothesis was the fact that, while the Sun contains 99.9 percent of the mass of the solar system, the planets (principally the outer planets) carry more than 99 percent of the system's angular momentum. To conform to this theory, either the Sun would have to be rotating more rapidly or the planets would have to be revolving around it more slowly. 

In the early decades of the 20th century, several scientists independently decided that these deficiencies of the nebular hypothesis were so great that it was no longer tenable. The Americans Thomas Chrowder Chamberlin and Forest Ray Moulton, along with Sir James Jeans and Sir Harold Jeffreys, both of Britain, independently developed variations on the idea that the planets were formed catastrophically--i.e., by the close encounter of the Sun with another star. The basis of this model was that, when the two bodies passed at close range, material would be drawn out from one or both stars, and this material would later coalesce to form planets. A somewhat discouraging aspect of this theory was the implication that the formation of solar systems must be extremely rare, because sufficiently close encounters between stars occur very seldom, and thus very few would have taken place during the lifetime of the galaxy.

The next significant development occurred during the middle of the 20th century, as scientists became more aware of the processes by which stars themselves must form and acquired a more mature understanding of the behaviour of gases under astrophysical conditions. This perspective led to the realization that hot gases stripped from a stellar atmosphere would simply dissipate in space; they would not condense to form planets. Hence the basic idea of solar system formation through stellar encounters was physically impossible. Furthermore, the growth in knowledge about the interstellar medium--the gas and dust distributed in the space separating the stars--indicated that large clouds of such matter exist and that stars form in these clouds. Planets must somehow be created in the process that forms the stars themselves. This awareness prompted scientists to reconsider certain basic processes that resembled some of the earlier notions of Kant and Laplace. 

Encyclopedia Britannica

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