Calendrical Systems
The calendar today is based on the Sun and its annual circuit around the sky against the background of the stars. The fundamental unit is the tropical year, the time from the summer solstice to the next summer solstice. Because one year is not exactly equal to an integral number of days, it is necessary to vary slightly the number of days in a year so that the calendar remains synchronized with the seasons over time. Thus every four years we add one extra day to the year. By contrast, the...
Formulation Of The Big Bang Theory
Both Friedmann and Lemaitre had devised cosmological models on the basis of general relativity that hypothesized that the universe had expanded outward from some initial time when the scale factor R was zero. In such a universe the density of matter and energy will increase as one goes backward in time, reaching infinite values at time zero. The general relativistic models were geometric structures satisfying the gravitational field equations, and the analysis of the physical conditions that...
Osianders Preface To Revolutions
It is asserted in the preface of Revolutions that the aim of the book is to advance a mathematical scheme to aid in the prediction of planetary motions and not to develop a theory of the planetary system as it actually exists in nature. Hypotheses such as that of the Earth's motion need not be true nor even probable, but provide only a reliable basis for computation. Although the purpose of the preface was apparently to forestall criticism of the philosophical or irreligious implications of the...
Special Theory of Relativity
By the middle of the nineteenth century the wave theory had become the accepted explanation for such optical phenomena as the reflection, refraction, and transmission of light. It was believed that space was filled with a universal luminiferous ether, through which light waves propagated, in the same way that waves from a stone dropped in a pond propagated through water. In the last decades of the century, physicists devised experiments to detect the motion of the Earth as it moved through the...
Large Telescopes
Conclusive evidence that would settle the great debate was obtained through observations made with a new generation of powerful telescopes constructed on high stations in the American West. Astronomy had entered a stage where advanced technology and large-scale science would dominate the frontiers of research. Although theorists continued to play an important role, they were overshadowed by the continuous stream of stunning findings coming from the mountaintop observatories. The primary place...
Gravitational Lens
Both Newtonian and Einsteinian theories of gravity predict that light will be bent in the presence of strong gravitational fields. Einstein's original prediction of such an effect and the value he obtained for its magnitude were confirmed in the 1960s by experiments in which radar signals were bounced off the planets. When a signal is reflected off Venus or Mercury while this planet is located on the other side of the Sun along the line of sight from the Earth to the Sun, the trajectory of the...
Tycho Brahe
Brahe was a Danish nobleman who carried out observations from 1576 to 1597 at his castle observatory Uraniborg on the island of Hveen between Denmark and Sweden. As a result of a dispute with the King of Denmark, he moved, at the end of 1597, to Prague to become court astrologer to the Emperor Rudolph II. He was assisted in the last years of his life by the young astronomer Johannes Kepler, who took possession of Tycho's treasure of observations when the latter died unexpectedly of a bladder...
Isaac Newton And The Newtonian Synthesis
Newton studied at Cambridge University and was appointed professor of mathematics there in 1669. In 1687 his greatest work was published, the Mathematical Principles of Mathematical Philosophy, a work that is commonly known as the Principia from the first word of its Latin title. The Principia was the crowning achievement of the revolution in cosmology and physics that began one and a half centuries earlier with Copernicus. It contained the Newtonian synthesis, a mathematical dynamics of forces...
Copernicus
In much popular writing on the history of science Copernicus has been portrayed as a heroic genius who overturned 2000 years of prejudice and launched a revolution in science that continues to the present day. The word Copernicus has entered the Western vocabulary to refer to that rare individual who initiates a truly fundamental change in some subject or field of investigation. Immanuel Kant was the Copernicus of philosophy for his introduction of the method of critical philosophy Nikolai...
Henrietta Swan Leavitt And Cepheid Variables
In the period 1880-1910 a group of assistants at the Harvard Observatory, under the direction of Edward Pickering (1846-1919), carried out an extensive program of photometry, involving the measurement of the brightness of stars from the images they produced on photographic plates. Included in this survey were stars in the Magellanic Clouds. The latter are visible as two patches of light in the southern sky and are resolved by telescope into two systems of stars. Pickering's assistants, most of...
Medieval Cosmology
In the centuries surrounding the decline and fall of the Roman Empire the level of understanding of astronomy and cosmology in Western Europe was very low, hardly rising above the literal interpretation of a few biblical pronouncements. The Earth was believed to be flat and situated at the bottom of the universe. Above the heavens were the upper waters mentioned in the Figure 4.2 Al-Shatir's model of the motion of the Moon. Figure 4.2 Al-Shatir's model of the motion of the Moon. book of...
Hubble And Extragalactic Nebulae
Many people were impressed by Curtis's arguments in the debate with Shapley, and opponents of the island-universe theory were increasingly put on the defensive. Still, there was no definite winner, and the question of the island-universe hypothesis was an open one in the early 1920s. As an advocate of the big-galaxy model, Shapley continued to question evidence for the hypothesis. Van Maanen's work on the rotation of the spirals also found an influential supporter in the British astrophysicist...
Kepler And Physical Astronomy
Kepler learned about heliocentric astronomy at the University of Tubingen, where he studied first as a student in the faculty of arts and later as a clergyman in training in the faculty of theology. Tubingen was a leading center of Lutheran theology, and Kepler planned to pursue a career in the church. One of his professors in Tubingen was Michael Maestlin, a supporter of the Copernican system and a prominent astronomer in the last decades of the sixteenth century. From his first encounter with...
Glossary
Acceleration In 1998, astronomers found that the red shifts of distant galaxies were smaller than predicted by Hubble's law. This fact implied that the expansion of the universe was slower in the past and that the rate of expansion has increased with time. The discovery of acceleration has caused cosmologists to revise their models and to posit the existence of dark energy to account for the acceleration. adaptive optics A system used in large telescopes to cancel the disturbing effect of the...
The Tetrabiblos
Ptolemy published another work relevant to cosmology, the Tetrabiblos, which deals with the influence of the celestial bodies on events on Earth. Astrology as a subject emerged in the later part of Greek antiquity, during the Roman Imperial period in the second century b.c. Earlier Greek thinkers such as Aristotle evinced no interest at all in astronomical prognostication. The popularity of astrology in the time of Ptolemy was connected to the emergence of Stoic philosophy and the Stoic concept...
Conclusion
Chinese astronomy of the Han period never reached a level of development comparable to its Babylonian and Hellenistic Greek counterparts. Han astronomers failed to detect the variable motion of the sun, used the assumption of a flat Earth to calculate the variation in the length of the Sun's shadow at different locations, and were unsuccessful in predicting solar eclipses. The later, more advanced development of Chinese astronomy, which reached its highest stage during the Yung Dynasty of the...
Islamic Astronomy And Cosmology
The emergence and spread of Islam in the seventh and eighth centuries was followed by the establishment of enlightened institutions that actively encouraged the study of mathematics, science, and philosophy. In the Middle East, and later in Sicily, northern Africa, and Moorish Spain, the Arabic language became the medium for scholarship. The pursuit of astronomy fulfilled both astrological and religious purposes. In religion the Islamic calendar required a very accurate lunar-solar calendar,...
Cosmology Before Einstein
Discoveries in observational astronomy would provide the basis for the revolution in cosmology that took place after 1900. It should nevertheless be noted that the study of cosmology existed before this time, largely as the study of general questions about the universe as a whole. These investigations reduced, in some cases, to speculative intellectual exercises, but they prepared the way for new mathematical descriptions of the universe and would become important when the revolutionary...
Babylonian Astronomy
Although the Babylonians did not contribute to cosmology, their astronomy is of great interest because the astronomical data they accumulated would later be of the utmost importance in the development of Greek geometric astronomy and cosmology. The emergence of Babylonian astronomy was preceded many centuries earlier by the appearance of a very advanced mathematics, documented in cuneiform clay tablets dating back to 1700 b.c. and earlier. This mathematics was based on a base-60 positional...
Chinese Astronomy And Cosmology
In ancient China, astronomy was a state-sponsored activity, and astronomers were members of the imperial bureaucracy. The demands of the emperor included the construction of accurate calendars and the keeping of a complete record of celestial events. Because detailed histories were produced for each dynasty, we have an unusually complete record of the activities of Chinese astronomers. As in Babylonian astronomy, the Chinese relied on arithmetical-algebraic procedures to study the motions of...
Versus Realism
A new stage in the historical study of ancient exact science began in the nineteenth century with the preparation of reliable textual editions of the extant Greek scientific classics. Of Ptolemy's two notable works on astronomy, the Almagest was a work of positional mathematical astronomy, while the Planetary Hypotheses was devoted to an investigation of the physical structure and dimensions of the celestial system. The Almagest has survived relatively intact and represents a scientific...
