The problem with orreries-and with the theories of the cosmos then prevailing-was that they had to become successively more complex as more became known. The fixed stars required a simple solution-they could be considered stuck in an outermost shell, also moved by clockwork. The other celestial bodies were positioned on rings of metal, each moving by clockwork at its own rate. As the center of the universe, the earth was a sphere in the center of the orrery. The majority were called "fixed stars," the wanderers were called "planets."ĭuring the late Middle Ages, and especially in the Renaissance, beautiful brass models known as orreries were made to show the relative positions and movements of the sun, planets, and moon as they circled the earth. Although most move in majestic unity, a few others are "wanderers"-appearing now with one group of stars and a week later somewhere else. The ancient observers realized that not all stars move in unison. In fact, it takes a full year for the stars to return to their previous position, an interval of time that defines our year. Thus, the stars of summer are different from those visible in the winter. The stars remain almost the same on successive nights, but slowly it becomes obvious that they, too, are slowed in their movements compared to the sun. The moon is slower by about one hour per day. It begins, however, by discussing another scientific development that challenged long-held understandings and beliefs: the discovery of heliocentricism. This chapter looks at how science works in the context of our overall understanding of how biological evolution occurred. For our own phylum, the Chordata, this move away from the nurturing sea led to the appearance of amphibians, reptiles, birds, and mammals-the latter including, of course, our own species, Homo sapiens. Somewhat more than 400 million years ago, some marine plants and animals began one of the greatest of all innovations in evolution-they invaded dry land. By about half a billion years ago, evolution had resulted in a wide variety of multicellular animals and plants living in the sea that are the clear ancestors of many of the major types of organisms that continue to live to this day. Less than a billion years ago, much more complex organisms appeared. For more than two billion years after that, life was housed in the bodies of many kinds of tiny, single-celled organisms, some of which produced the oxygen that now makes up more than a fifth of the earth's atmosphere. The best available evidence suggests that life on earth began more than three and a half billion years ago. The history of life on earth is a fascinating subject that can be studied through observations made today, and these observations have led to compelling accounts of how organisms have changed over time. Explanations that cannot be based on empirical evidence are not a part of science. Anything that can be observed or measured is amenable to scientific investigation. In science, explanations are restricted to those that can be inferred from confirmable data-the results obtained through observations and experiments that can be substantiated by other scientists. Science is a particular way of knowing about the world.
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