![]() ![]() No distances are given by Copernicus for Uranus, Neptune and Pluto, because these planets had not yet been discovered.Ĭopernicus' heliocentric model clearly provided a much simpler description of planetary motion than Ptolemy's epicycle model. The distances in Table 1 are in Astronomical Units (1 AU=distance from the Sun to Earth, 1.49 × 10 11 m). The average distances of the planets from the Sun obtained by Copernicus are given in Table 1, along with the currently accepted values and various other planetary characteristics such as their orbital periods and rotation periods. Using simple geometric arguments and existing data he established the now accepted ordering of the planets outward from the Sun, and their approximate distances from the Sun. This model is called the heliocentric model. Although Copernicus was a practicing medical doctor, at one time the personal physician of the Duke of Prussia, he developed an interest in astronomy and began exploring the idea that the planets revolve in circular orbits around the Sun. Because of the church's influence, no other model was even considered for over twelve centuries, until a Polish physician, Nicholas Copernicus, who lived from 1473 to 1543, critically examined the issue. Although complex, the epicycle model of Ptolemy accounted for planetary motions reasonably well, especially considering the rather imprecise astronomical measurements available at the time, and was endorsed by the Roman Catholic Church as the official dogma of the church. To account for the apparent non-uniform looping motions of the planets across the sky, the motions were regarded as a combination of two circular motions that revolved at different rates. Ptolemy's model was basically geometric and involved clock-like motions of the stars and planets around Earth. This view was based on the model of the Greek philosopher Ptolemy, who lived from about 127-151 AD. ![]() Although the basic picture of planets orbiting the Sun is known to all school children, prior to the seventeenth century the widely held view was that Earth was at the center of the universe, and that the stars and planets all revolved around Earth. ![]() The solar system consists of the Sun, the planets, the moons and all the other minor objects that orbit the Sun, such as comets and asteroids. Finally, I will discuss recent spacecraft observations of the moons of Jupiter and Saturn that suggest life may be possible in the interiors of these seemingly hostile moons. Next, I will discuss early ground-based observations that led many to believe that Venus and Mars had living things, and will show how modern spacecraft observations have radically modified our views regarding the possibility of life at these planets. I will begin by reviewing the historical developments that led to our current understanding of the solar system, and discuss the concept of a habitable zone, which is the region around the Sun where the temperature is predicted to be favorable for life. As you will see, the history of this subject is riddled with tremendous conflicts and radical revisions in our thinking. In this paper I will discuss the history of the search for life in the only region that is currently accessible to direct in situ investigation, namely our solar system. Although no extraterrestrial life has yet been found, most would agree that the discovery of life elsewhere, even in its simplest microbial form, would be one of the most significant scientific advances in human history. On the other hand, Ward and Brownlee ( 2) have recently argued that the conditions for life are so specialized that Earth is most likely entirely alone in having complex life. Ever since man gazed at the stars, the question must certainly have arisen, are we alone, does life exist somewhere else in the universe? Because of the enormous number of stars in the universe, 300 billion in our galaxy alone, and the likelihood that many of them probably have planetary systems similar to our own, some investigators, such as Sagan and Drake ( 1), have concluded that life, including intelligent life, should be very common throughout the universe.
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