THE SOLAR SYSTEM

The discovery and description of the planets

To appreciate how impressive the night sky must have been to early man it is necessary today to go to a place remote from the distracting lights and pollution of urban centers. Viewed from the wilderness the firmaments appear to the naked eye as a canopy of shining points, fixed in space relative to each other. Early observers noted that the star pattern appeared to move regularly and used this as a basis for determining the timing of events. More than 3000 years ago, in about the thirteenth century BC, the year and month were combined in a working calendar by the Chinese, and about 350 BC the Chinese astronomer Shih Shen prepared a catalog of the positions of 800 stars. The ancient Greeks observed that several celestial bodies moved back and forth against this fixed background and called them the planetes, meaning “wanderers.” In addition to the Sun and Moon, the naked eye could discern the planets Mercury, Venus, Mars, Jupiter and Saturn.

Geometrical ideas were introduced into astronomy by the Greek philosopher Thales in the sixth century BC. This advance enabled the Greeks to develop astronomy to its highest point in the ancient world. Aristotle (384–322 BC) summarized the Greek work performed prior to his time and proposed a model of the universe with the Earth at its center.

In the ten years that have passed since the publication of the first edition of this textbook exciting advances have taken place in every discipline of geophysics. Computer-based improvements in technology have led the way, allowing more sophistication in the acquisition and processing of geophysical data. Advances in mass spectrometry have made it possible to analyze minute samples of matter in exquisite detail and have contributed to an improved understanding of the origin of our planet and the evolution of the solar system. Space research has led to better knowledge of the other planets in the solar system, and has revealed distant objects in orbit around the Sun. As a result, the definition of a planet has been changed. Satellite-based technology has provided more refined measurement of the gravity and magnetic fields of the Earth, and has enabled direct observation from space of minute surface changes related to volcanic and tectonic events. The structure, composition and dynamic behavior of the deep interior of the Earth have become better understood owing to refinements in seismic tomography. Fast computers and sophisticated algorithms have allowed scientists to construct plausible models of slow geodynamic behavior in the Earth's mantle and core, and to elucidate the processes giving rise to the Earth's magnetic field. The application of advanced computer analysis in high-resolution seismic reflection and ground-penetrating radar investigations has made it possible to describe subtle features of environmental interest in near-surface structures.

GEOCHRONOLOGY

Time

Time is both a philosophical and physical concept. Our awareness of time lies in the ability to determine which of two events occurred before the other. We are conscious of a present in which we live and which replaces continually a past of which we have a memory; we are also conscious of a future, in some aspects predictable, that will replace the present. The progress of time was visualized by Sir Isaac Newton as a river that flows involuntarily at a uniform rate. The presumption that time is an independent entity underlies all of classical physics. Although Einstein's Theory of Relativity shows that two observers moving relative to each other will have different perceptions of time, physical phenomena are influenced by this relationship only when velocities approach the speed of light. In everyday usage and in non-relativistic science the Newtonian notion of time as an absolute quantity prevails.

The measurement of time is based on counting cycles (and portions of a cycle) of repetitive phenomena. Prehistoric man distinguished the differences between day and night, he observed the phases of the Moon and was aware of the regular repetition of the seasons of the year. From these observations the day, month and year emerged as the units of time. Only after the development of the clock could the day be subdivided into hours, minutes and seconds.

THE EARTH'S SIZE AND SHAPE

Earth's size

The philosophers and savants in ancient civilizations could only speculate about the nature and shape of the world they lived in. The range of possible travel was limited and only simple instruments existed. Unrelated observations might have suggested that the Earth's surface was upwardly convex. For example, the Sun's rays continue to illuminate the sky and mountain peaks after its disk has already set, departing ships appear to sink slowly over the horizon, and the Earth's shadow can be seen to be curved during partial eclipse of the Moon. However, early ideas about the heavens and the Earth were intimately bound up with concepts of philosophy, religion and astrology. In Greek mythology the Earth was a disk-shaped region embracing the lands of the Mediterranean and surrounded by a circular stream, Oceanus, the origin of all the rivers. In the sixth century BC the Greek philosopher Anaximander visualized the heavens as a celestial sphere that surrounded a flat Earth at its center. Pythagoras (582–507 BC) and his followers were apparently the first to speculate that the Earth was a sphere. This idea was further propounded by the influential philosopher Aristotle (384–322 BC). Although he taught the scientific principle that theory must follow fact, Aristotle is responsible for the logical device called syllogism, which can explain correct observations by apparently logical accounts that are based on false premises.

INTRODUCTION

Seismology is a venerable science with a long history. The Chinese scientist Chang Heng is credited with the invention in 132 AD, nearly two thousand years ago, of the first functional seismoscope, a primitive but ingenious device of elegant construction and beautiful design that registered the arrival of seismic waves and enabled the observer to infer the direction they came from. The origins of earthquakes were not at all understood. For centuries these fearsome events were attributed to supernatural powers. The accompanying destruction and loss of life were often understood in superstitious terms and interpreted as punishment inflicted by the gods on a sinful society. Biblical mentions of earthquakes – e.g., in the destruction of Sodom and Gomorrah – emphasize this vengeful theme. Although early astronomers and philosophers sought to explain earthquakes as natural phenomena unrelated to spiritual factors, the belief that earthquakes were an expression of divine anger prevailed until the advent of the Age of Reason in the eighteenth century. The path to a logical understanding of natural phenomena was laid in the seventeenth century by the systematic observations of scientists like Galileo, the discovery and statement of physical laws by Newton and the development of rational thought by contemporary philosophers.

In addition to the development of the techniques of scientific observation, an understanding of the laws of elasticity and the limited strength of materials was necessary before seismology could progress as a science.

HISTORICAL INTRODUCTION

The discovery of magnetism

Mankind's interest in magnetism began as a fascination with the curious attractive properties of the mineral lodestone, a naturally occurring form of magnetite. Called loadstone in early usage, the name derives from the old English word load, meaning “way” or “course”; the loadstone was literally a stone which showed a traveller the way.

The earliest observations of magnetism were made before accurate records of discoveries were kept, so that it is impossible to be sure of historical precedents. Nevertheless, Greek philosophers wrote about lodestone around 800 BC and its properties were known to the Chinese by 300 BC. To the ancient Greeks science was equated with knowledge, and was considered an element of philosophy. As a result, the attractive forces of lodestone were ascribed to metaphysical powers. Some early animistic philosophers even believed lodestone to possess a soul. Contemporary mechanistic schools of thought were equally superstitious and gave rise to false conceptions that persisted for centuries. Foremost among these was the view that electrical and magnetic forces were related to invisible fluids. This view persisted well into the nineteenth century. The power of a magnet seemed to flow from one pole to the other along lines of induction that could be made visible by sprinkling iron filings on a paper held over the magnet. The term “flux” (synonymous with flow) is still found in “magnetic flux density,” which is regularly used as an alternative to “magnetic induction” for the fundamental magnetic field vector B.