The study of the motions of the celestial bodies required only that they be regarded as mere points of light. But already in the 4th century B.C., Aristotle proposed in his De caelo (On the Heavens) a theory of the physical nature of these bodies, which conferred on them the properties of perfection and unchangeability thought appropriate to the divine celestial regions, in contrast to the ever-changing Earth. In the next century, Aristarchus of Samos, again exhibiting the mathematical penchant of the Hellenistic era, offered an observationally based estimate, although about ten times too low, of the sizes of the Sun and Moon. These size estimates were universally admired, and Aristotle's physical hypothesis only sporadically challenged, by medieval commentators.

Moon and Planets. The physical similarity of the Earth and planets became a matter of significant inquiry only after Copernicus showed that the Earth and all of the planets are in motion around the central Sun. The question might have remained forever unresolved had not Galileo Galilei constructed a telescope, although not the first in Europe, which he turned toward the heavens in 1609. The results, announced in the Sidereus nuncius (Sidereal Messenger, 1610), were shattering to the Aristotelian view. The Moon was found to be a mountainous body "not unlike the face of the Earth." Galileo's further discovery of the moons of Jupiter and the phases of Venus was more evidence that the planets had Earthlike characteristics.

Such discoveries - extremely important as verification for the physical reality of the Copernican theory - slowly accumulated throughout the 17th and 18th centuries. The nature of the Moon was discussed in increasing detail in Kepler's Somnium (1634), Johannes Hevelius's Selenographia (1647), and G. B. Riccioli's Almagestum novum (1651). Christiaan Huygens, the greatest observational astronomer of the 17th century, first correctly interpreted the rings of Saturn in his Systema Saturnium (1659), observed dark markings on Mars, and belts of clouds on Jupiter, and speculated that Venus was shrouded in clouds. With more refined telescopes, such as those built by Sir William Herschel in England, the details of the solar system became better known. Herschel himself made the spectacular discovery of the planet Uranus in 1781. In 1846, the presence of yet another planet (Neptune), predicted by J. C. Adams and U. J. J. Leverrier, was confirmed observationally - a triumph for both theory and observation.

With the invention of spectroscopy in the 1860s, and with the work of Sir William Huggins and Norman Joseph Lockyer in England, P. A. Secchi in Rome, César Janssen in Paris, Lewis Rutherfurd (1816-92) in the United States, and Hermann Vogel (1842-1907) in Germany, the science of astrophysics was born. By spreading the light of the celestial bodies into the constituent colors of the spectrum, each interspersed with lines characteristic of the elements present, a powerful new tool was given to the astronomer. The ability to determine the chemical composition of planetary atmospheres and even of the stars, a task which the positivist philosopher Auguste Comte had offered less than 30 years before as the paradigm of what science could never achieve, now became possible.

Sun and Stars. Astrophysics yielded its most substantial results in the study of the Sun and stars, where the myriads of observed spectral lines were gradually interpreted as a precise set of chemical fingerprints. With spectroscopy and the almost simultaneous invention of photography, astronomers compiled great catalogs mapping the solar spectrum. Knowledge of the Sun, which had not advanced substantially since Galileo's discovery of sunspots, now outstripped planetary astronomy. The means were at hand to determine the temperature, composition, age, and structure of the Sun and to compare this data with that of the other stars, which were now for the first time proved to be other suns. In conjunction with advances in nuclear physics the investigations of the latter half of the 19th century led to the building of a firm foundation for a discussion in the first quarter of the 20th century of the internal constitution and evolution of stars. This program was pioneered by A. S. Eddington and Karl Schwarzschild. By providing a wealth of data previously unavailable, astrophysics fueled the controversy over the possible existence of extraterrestrial life not only in our solar system, but throughout a universe of other possible solar systems.