Astronomy, History of

In ancient times, people watched the sky and used its changing patterns throughout the year to regulate their planting and hunting. The Sun seemed to move against the background of stars. A few bright objects (Mercury, Venus, Mars, Jupiter, and Saturn) wandered against the same background. Greek philosopher Aristotle (384-322 B.C.E.) tried to make sense of all this by proposing a system of the universe with Earth in the center (known as a geocentric system). Revolving around Earth were the Sun, the five known planets, and the Moon. This system satisfied the Greek desire for uniformity with its perfectly circular orbits as well as everyone's common sense of watching sunrise and sunset.

Greek astronomer and mathematician Ptolemy refined Aristotle's theory in 140 C.E. by adding more circles to obtain better predictions. For over a thousand years people used his scheme to predict the motions of the planets. Polish astronomer Nicholas Copernicus (1473-1543) was dissatisfied with its increasingly inaccurate predictions. He looked for a method that would be both accurate and mathematically simpler in structure. Although he did not achieve great accuracy, he was able to produce a beautiful scheme with the Sun in the center of the universe (known as a heliocentric system). His system improved on Ptolemy's plan by determining with fair accuracy the relative distance of all the planets from the Sun. However, it still used circles. The plan became a matter of religious controversy because some people did not want to displace humankind from the important spot as the center of the universe.

In 1609, German astronomer Johannes Kepler (1571-1630) showed with careful mathematical calculations that the orbits were not circles but ellipses. (An ellipse is a mathematically determined oval.) Also in 1609, Italian mathematician and astronomer Galileo Galilei (1564-1642) first used a telescope to observe celestial objects. He discovered moons orbiting Jupiter, phases of Venus, sunspots, and features on the Moon that made it seem more like a planet. None of these discoveries proved that the Copernican heliocentric theory was correct, but they offered evidence that Aristotle was wrong. For example, the phases of Venus indicated that Venus orbited the Sun (but did not prove that Earth did also). The discovery of sunspots and lunar surface features proved that the Sun and Moon were not perfect unblemished spheres. Galileo also did experiments to explore gravity and motion. English physicist and mathematician Isaac Newton (1642-1727) articulated the laws of gravity and motion. He also used a prism to split light into its component colors (spectroscopy).

In 1860 Italian astronomer Angelo Secchi (1818-1878) first classified stellar spectra. In the twentieth century, astronomers used spectra to find temperatures and line-of-sight motions of stars and galaxies. Stellar temperature and distance, when combined with the theory of how stars are powered by fusion , provide the basis for the current theory of stellar evolution. American astronomer Edwin P. Hubble (1889-1953) discovered that galaxies are moving away from each other as the universe expands. These motions of galaxies and changes of their component stars and gas over time indicate the evolution of the universe. It has thus become clear that although we can map our location with respect to the galaxies, we live in the midst of an expanding universe for which no center can be measured.

see also Age of the Universe (volume 2); Cassini, Giovanni (volume 2); Copernicus, Nicholas (volume 2); Cosmology (volume 2); Einstein, Albert (volume 2); Exploration Programs (volume 2); Galaxies (volume 2); Galilei, Galileo (volume 2); Gravity (volume 2); Herschel Family (volume 2); Hubble Constant (volume 2); Hubble, Edwin P. (volume 2); Huygens, Christiaan (volume 2); Kepler, Johannes (volume 2); Kuiper, Gerard Peter (volume 2); Newton, Isaac (volume 2); Planetary Exploration, Future of (volume 2); Sagan, Carl (volume 2); Shapley, Harlow (volume 2); Shoemaker, Eugene (volume 2); Stars (volume 2); Tombaugh, Clyde(volume 2).

Mary Kay Hemenway

Bibliography

Kuhn, Thomas S. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, MA: Harvard University Press, 1957.

Nicolson, Iain. Unfolding Our Universe. Cambridge, UK: Cambridge University Press,1999.