Cosmology

I

Introduction

Cosmology, study of the universe as a whole, including its distant past and its future. Cosmologists study the universe observationally—by looking at the universe—and theoretically—by using physical laws and theories to predict how the universe should behave. Cosmology is a branch of astronomy, but the observational and theoretical techniques used by cosmologists involve a wide range of other sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used to mean the study of the origin of the universe but now usually refers only to the study of the origin of the solar system.

II

Evolution of Cosmological Theories

Humans have been examining and wondering about the sky for many millennia. As scientific discoveries have been made, ideas about the origin of the universe have changed and are still changing.

A

Ancient Cosmologies

As far back as 1100 bc, Mesopotamian astronomers drew constellations, or formations of stars perceived to form shapes. Some of today’s constellation names date back to that time. Mesopotamian and Babylonian cultures mapped the motion of the planets across the sky by observing how they moved against the background of stars.

Until the 16th century, most people (including early astronomers) considered Earth to be at the center of the universe. Greek philosopher Aristotle proposed a cosmology in about 350 bc that held for thousands of years. Aristotle theorized that the Sun, the Moon, and the planets all revolved around Earth on a set of celestial spheres. These celestial spheres were made of the quintessence—a perfect, unchanging, transparent element. According to Aristotle, the outermost sphere was made of the stars, which appear to be fixed in position. Early astronomers called the stars “fixed stars” to differentiate between stars and planets. The spheres inside the sphere of the fixed stars held the planets, which astronomers called the “wandering stars.” The Sun and Moon occupied the two innermost spheres. Four elements (earth, air, fire, and water) less pure than the quintessence made up everything below the innermost sphere of the Moon. In about 250 bc, Greek astronomer Aristarchus of Sámos became the first known person to assert that Earth moved around the Sun, but Aristotle’s model of the universe prevailed for almost 1,800 years after that assertion.

---

---

Early astronomers called the planets wandering stars because they move against the background of the stars. Astronomers noted that the planets sometimes moved ahead with respect to the stars but sometimes reversed themselves, making retrograde loops. In about ad 140, Greek scientist Ptolemy explained the retrograde motion as the result of a set of small circles, called epicycles, on which the planets moved. Ptolemy hypothesized that the epicycles moved on larger circles called deferents and that the combination of these motions caused the dominant forward motion and the occasional retrograde loops.

B

Sun-Centered Universe

The ideas of Ptolemy were accepted in an age when standards of scientific accuracy and proof had not yet been developed. Even when Polish astronomer Nicolaus Copernicus developed his model of a Sun-centered universe, published in 1543, he based his ideas on philosophy instead of new observations. Copernicus’s theory was simpler and therefore more sound scientifically than the idea of an Earth-centered universe. A Sun-centered universe neatly explained why Mars appears to move backward across the sky: Because Earth is closer to the Sun, Earth moves faster than Mars. When Mars is ahead of or relatively far behind Earth, Mars appears to move across Earth’s night sky in the usual west-to-east direction. As Earth overtakes Mars, Mars’s motion seems to stop, then begin an east-to-west motion that stops and reverses when Earth moves far enough away again. Copernicus’s model also explained the daily and yearly motion of the Sun and stars in Earth’s sky. Scientists were slow to accept Copernicus’s model of the universe, but followers grew in number throughout the 16th century. By the mid-17th century, most scientists in western Europe accepted the Copernican universe.

In the 16th century, Danish astronomer Tycho Brahe made the most scientific and accurate observations of the universe to that time. Brahe discovered discrepancies between astronomical predictions and the actual events, and built a set of large instruments that enabled him to record the positions of the planets and stars with unprecedented accuracy. He moved to Prague, and, after his death, his observations were taken over by German astronomer Johannes Kepler. Kepler discovered that the planets orbited around the Sun in ellipses (elongated circles) with the Sun a bit off-center at one focus. This discovery was Kepler’s first law, and he developed two more laws about how the speeds and periods of the planets changed (see Kepler’s Laws). The first two laws were published in 1609 and the third was published in 1619.

The Italian scientist Galileo Galilei lived and worked during the same time period as Kepler. Galileo was the first astronomer to use a telescope to observe the sky and to recognize what he saw there. He saw that the Moon had craters, that Venus went through a full set of phases like the Moon, and that Jupiter had satellites, or moons, of its own. His discoveries, published in 1610, marked the scientific end of the cosmological systems of Ptolemy and Aristotle, though it took some time for his findings to be generally accepted.

	More from Encarta
	•  7 tips for funding an online degree •  How to succeed in the fashion industry without being a top designer •  Presidential Myths Quiz •  Coffee break: Recharge your brain