The ancient Greeks, especially during the Hellenistic* period, accomplished important work in many different areas of science, in particular astronomy and physics. They are most noted for developing mathematical models of natural processes. After A.D. 200 Greek science declined, and many of the scientific advances of the Hellenistic Greeks were not surpassed for more than a thousand years. Although the Romans made some contributions to the advancement of scientific knowledge, such as cataloging the variety of plants in the Roman empire, their achievements did not approach those of the Greeks.

Although Greek and Roman scientists observed the natural world to find evidence to support their theories, little scientific experimentation was conducted. One of the few experimenters was the philosopher* Aristotle, who devised an experiment to demonstrate that seawater becomes fresh water after it evaporates. In scientific fields in which experimentation was impossible, such as astronomy, scientists based their work on comparisons of situations or events that were similar.

Because ancient people used the movements of the stars and planets for navigation and timekeeping, the science of astronomy was especially important in the ancient world. Starting in the 400s B.C., Greek philosophers and astronomers advanced several significant concepts, including the notion that the earth is spherical and that the moon receives its light from the sun. The Greeks also determined the cause of eclipses. However, the greatest contribution of the ancient Greeks to astronomy was the development of mathematical models of the movements of the stars and planets. Around A.D. 150 the astronomer Ptolemy developed a model of the universe that was almost universally accepted for more than a thousand years.

* Hellenistic referring to the Greek-influenced culture of the Mediterranean world during the three centuries after Alexander the Great, who died in 323 B.C.

* philosopher scholar or thinker concerned with the study of ideas, including science

The science of physics originated in Greece during the 500s B.C. Philosophers such as Thales of Miletus looked for a single theory to explain the natural events in the world around them. According to the four-element theory developed in the 400s B.C. by the philosopher Empedocles, the physical universe is composed of just four substances—air, water, fire, and earth. The actions and interactions of these substances explain all natural events. At about the same time, the philosopher Democritus developed the rival atomic theory. According to this theory, the physical universe is composed of empty space and minuscule particles called atoms. All natural events are a result of the movements of atoms in the empty space. These two theories dominated science for almost 2,000 years. (See also Archimedes; Astronomy and Astrology; Calendars; Euclid; Galen; Hippocrates; Mathematics, Greek; Medicine, Greek; Medicine, Roman; Philosophy, Greek and Hellenistic; Plato; Pliny the Elder; Pythagoras; Technology.)

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