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Gravity is responsible not only for the existence of stars and planets; it also creates the weirdest objects imaginable. A body with mass greater than 1.4 solar masses cannot remain a white dwarf and will collapse into a neutron star. But if the mass is greater than about two and a half solar masses, the collapse will continue until it becomes a black hole. This is the strangest object in the universe. Its gravity is so strong that even light cannot get out of it. Anything near it is sucked in, crushed to a point, and approaches infinite density. The laws of physics as now known do not apply at the centre of a black hole and the very meaning of its existence is in doubt.

This chapter looks at climate change in the broader planetary sense. It examines evolutionary process in planetary atmospheres, with an application to the modelling of the evolution of the Earth's climate from the Precambrian to the present time. It examines the comparative climatology of the terrestrial planets and looks at the atmospheres of the giant planets. The photochemical and climate modelling techniques developed in the earlier chapters is then applied to Titan's haze formation and atmosphere. A brief look is given to extrasolar planets.

This concluding chapter discusses some of the lessons that can be learned from studying the planets and planetary climates. It first considers the general principles that turned out to be right; for example, size and distance from the Sun matter. The larger objects are able to hold on to their atmospheres better than the small objects. The outer solar system is hydrogen rich and the inner solar system is oxygen rich; as one moves away from the Sun different substances take on different roles. There are also assumptions that proved inaccurate; such was the case for Venus, Mars, and the moons of the giant planets. The chapter also asks whether the study of planetary climates provides lessons for Earth, whether the study of planets has informed us about the likelihood of extraterrestrial life, and whether it has made the development of extraterrestrial life seem more likely.

Photosynthesis — both anoxygenic and oxygenic — allows access to new sources of energy. Oxygenic photosynthesis has the potential to create an oxygen-rich atmosphere and so allow aerobic respiration, which yields much higher amounts of energy than anaerobic respiration. The amount of oxygen added to the atmosphere is intimately linked to the burial of organic matter in sediments, therefore marine phytoplankton are crucially important in maintaining the levels of atmospheric oxygen on Earth. Anoxygenic photosynthesis will have a positive Gaian effect by providing an important source of energy. Oxygenic photosynthesis is more problematical; as with anoxygenic photosynthesis it provides an energy source, but the oxygen given off is likely to be toxic to organisms evolved in anoxic conditions. It is currently impossible to know if we should expect most biospheres to evolve oxygenic photosynthesis. However, improvements in telescope technology should allow us to look for oxygen-rich atmospheres around distant Earth-like planets.

This chapter examines the first novel in the series, Out of the Silent Planet. It is based on the premise that the physically damaged but “unfallen” world of Malacandra (Mars) is a sublimated or “beatific” version of the Darwinian view of life as a relentless “struggle for existence.” Lewis’s would-be conquerors of Mars—the physicist Weston and the businessman Devine—use the presumption of their own evolutionary superiority to justify the conquest or outright eradication of other beings. In contrast to our own self-divided species, Mars possesses three rational species—each with its own body-type—who nevertheless live peacefully (if separately) in a condition of mutual equality. In the footsteps of his eminent predecessor H.G. Wells, Lewis employs interplanetary conflict as a means of exploring the perpetual strife within humankind itself, especially as it appears in modern European imperialism and in the virulent nationalism, racism, and genocidal mania of the 1930s.

The conclusion brings together the various concerns of this study by focusing on the relations among three pivotal scenes that take place at precisely the same point in each of the three novels—the hunting expedition in Out of the Silent Planet (chapter 13); the call to combat with the Satanic Un-man in Perelandra (chapter 11); and the reckoning with imminent death in That Hideous Strength (chapter 11). Each of these scenes involves commitment to violent action and the risk of annihilation. Each is also closely tied to a particular version of the modern developmental paradigm—material, vitalist, spiritual, respectively—and turns on the tension between the developmental model and its “beatific” transfiguration in the novel. Taken together, these scenes map a narrative progression—Adamic (or tragic), Christic (or salvific), and Ecclesiastic (or agapic)—that spans the three-volume saga and demonstrates its formal and thematic unity.

The ideal of wisdom—and citizenship in the cosmos—is explored in detail, both regarding ideal human agents and gods. The view that the sage is strictly omniscient is rejected; rather, the sage's knowledge is knowledge relevant to the leading of a lawful life. The reconstruction of the conception of citizen—gods leads into a core question of Stoic theology: how a plural conception of ‘gods’ fits into Stoic physics, which considers God (in the singular) the first principle. It is argued that the gods of the Stoics are portions of pneuma, and some of them are celestial bodies. The life of the gods exemplifies the Stoics' ideal of an easy flow of life: the planets are considered models of perfect deliberation, being—through their perfect action—fully integrated into the movements of the universe as a whole.

This chapter begins with a discussion of spherically symmetric spacetimes, the Schwarzschild metric, and other coordinates. It then covers Schwarzschild spacetime, the motion of the planets and perihelion precession, stability of circular orbits, deflection of light rays, red shift and time delay, spherically symmetric interior solutions, the Schwarzschild black hole, spherically symmetric gravitational collapse, the Reissner–Nordström solution, and Schwarzschild spacetime in dimension n + 1.

This chapter begins by highlighting the three basic ingredients for life: energy, the chemical components that make up cells, and water. It then shows that the availability of each of these is linked by special properties of planet Earth. It concludes that Earth is a pretty terrific place for life. It sits comfortably within the habitable zone of the Sun. In addition, its active tectonics both control the temperature of the surface environment, providing a continuous supply of liquid water, and recycle the basic components required to fuel abundant life. The same tectonics may have also provided optimal conditions for the earliest biosphere.

Part I is devoted to Newton’s argument for proposition 5 and his important Rule 4 for reasoning in natural philosophy. Rule 4 is a very informative characterization of theory acceptance for Newton and for scientific method today. Part II is devoted to Newton’s argument for proposition 6 and his important Rule 3. The phenomena cited count as agreeing measurements of equal acceleration components toward planets for all bodies at any equal distances from their centers. These measure equal ratios of weight to mass for attracted bodies at equal distances. They include absence of polarization toward the sun of orbits of moons about planets. The agreement of all these measurements supports an interpretation of Rule 3 which informs the role of theory-mediated measurements in supporting scientific inferences today. The appendix gives details of polarization calculations.

Part I reviews Newton’s argument for his conceptual transition from gravity as centripetal forces of attraction toward planets to gravity as a universal force of pair-wise interaction between bodies. Part II features Newton’s measurements of masses of the sun and planets from orbits. Part III reviews Newton’s initial application of gravity to the solar system and his project of applying universal gravity to account for motions of solar system bodies by successively more accurate approximations. Appendix 1 gives details of a theorem about Law 3 for attractions. Appendix 2 reviews theorems about attraction to spherical bodies. It extends an integral of Chandrasekhar’s to show that Newton’s inference to inverse-square attraction to particles from inverse-square attraction toward whole solid spheres is backed up by systematic dependencies. Appendix 3 gives details of Newton’s proofs about attraction toward spherical shells. Appendix 4 reviews details of Newton’s measurements of properties of planets from orbits.

This book examines the fundamental physical processes that control planetary climates, from convection and radiation to escape of atmospheres, evaporation, condensation, atmospheric chemistry, and the dynamics of rotating fluids. It looks at the climates of the planets in order of distance from the Sun, starting with Venus and ending with planets around other stars. The greenhouse effect and climate evolution are discussed, along with basic physical processes such as convection, radiation, Hadley cells, and the accompanying winds. It also considers the “faint young Sun paradox,” illustrated by Mars, and the effect of planetary rotation on climate, the influence of sunlight and rotation on weather patterns, and Neptune's extraordinary strong winds.

This chapter focuses on the climates of Uranus, Neptune, and exoplanets. Uranus spins on its side, which allows a comparison between sunlight and rotation for their effects on weather patterns. In contrast to Venus, Uranus is only weakly affcted by tides from the Sun because it is so far away. Models of planet accretion give a gradual clumping of small bodies into medium-sized bodies and then into large bodies, until finally only a few large bodies are left. The final collisions, which involved these large bodies, would have been quite violent and were capable of knocking Uranus on its side. After providing an overview of Uranus's rotation, insensitivity to seasonal cycles, and wind profile, the chapter considers Neptune's winds, effective radiating temperature, and Great Dark Spot. It also explains the radial velocity method and the transit method of detecting extrasolar planets.

Does Aristotle have a scala naturae in which humans are ranked higher than the other animals? Is it better to be human than to be something else? This chapter shows that Aristotle's sequence of functions of the soul is not ordered in terms of honour but in terms of distribution. Greater complexity is not a mark of superiority. The animals' lack of what Aristotle called eudaimonia does not mean that they are lacking in happiness or success in their own pursuits. Rather, simplicity is an ideal; psychological complexity is (for Aristotle) a mark of things that fall short of perfection, as is evident from his discussion of the complex motions of the planets. Texts discussed include Metaphysics A, De anima, Nicomachean Ethics X, De partibus animalium, De Caelo, De incessu animalium, and Aristotle's discussion of slaves and women in Politics.

The chapter discusses the first Book of Manilius' Astronomica, his description of the universe, before the background of ancient cosmology. After examining general notions about the cosmos that were widespread in antiquity (especially the idea that the world is indeed a kosmos, a thing of order and beauty), it provides a brief history of ancient astronomy and cosmology up to Roman times, with a particular focus on the ‘two-sphere universe’, the model of the cosmos generally accepted from the fourth century BC to the time of Copernicus. This is followed by a detailed exploration of Manilius' own portrait of the universe, including, among other things, the poet's debt to the Hellenistic astronomical poet Aratus. The chapter concludes with a discussion of a striking feature of Manilius' astronomy, his near-neglect of the topic of the planets.

The chapter dicusses the system of astrology that Manilius describes in Books 2-5 of his Astronomica. It begins with an exposition of the tenets of ancient astrology, pointing out that Manilius' astrology is both ‘strong’ (the stars hold the key to every aspect of our fate) and ‘hard’ (the stars themselves cause this fate). A brief history of ancient astrology from Mesopotamia via Hellenistic Greece to Rome is followed by a detailed examination of Manilius' text: the poet treats first the three astrologically significant circles (zodiac, fixed circle of the observer, and circle of lots) and then the influences of the heavenly bodies—in particular the signs of the zodiac and the paranatellonta—on human beings. The chapter concludes with an examination of the idiosyncrasies of Manilius' astrology, a topic that harks back to the puzzle of the planets discussed at the end of Chapter 1.

This chapter undertakes a reading of the longest astrological poem in the Welsh language, the Puritan writer Morgan Llwyd's idiosyncratic 172-stanza Gwyddor Vchod, ‘Heavenly Science’. Llwyd's poem is read in the context of 16th and 17th century manuscripts containing astrological material, largely for practical, medical purposes, looking in particular at Elias Gruffydd's mid-16th century translation of the Compost Ptolomeus, which probably holds the status of the most sophisticated work of practical astrology in Welsh, giving detailed readings for the meaning of the various planets through the zodiacal signs. It is shown that Llwyd's grandfather was almost certainly familiar with astro-medical material of this type, and may very well have belonged to the class of person known as a planedydd, literary ‘planet-ist’, or ‘astrologer’. Llwyd's poem takes the tradition of the Welsh astro-medical treatise and rewrites it as a form of evangelical exhortation; the poet discusses the characteristics of seven types of human beings, ruled by each of the planets; influenced by the mysticism of Jakob Böhme, he prescribes spiritual remedies for the defects of character that these entail.

This chapter sets out to define the boundaries and explore the limits of the globe. After establishing the planet Earth as the central pivot around which the discussion will revolve, the chapter turns to a more extensive purview of the Earth's “neighborhood.” It argues that the Earth's surface alone does not define the boundaries of the planet itself, as the Earth's atmosphere must be considered, as well as its neighboring planets, Venus, Mars, and still others. Moreover, there are other celestial bodies that are situated around the “home planet”—the sun, moon, and stars—to name a few. Furthermore, while the Earth is indeed central to this discussion, the chapter also situates the globe in the periphery of these other celestial bodies.