Astronomy is the study of the heavenly bodies—the sun, moon, planets, and stars. Astrology is the belief that the position and movement of those heavenly bodies influence life on earth. Today, astronomy is a science, while astrology is considered an occult, or magical, practice. The two were not always sharply separated, however. In ancient times, they often were closely linked and regarded with equal respect.
Both astronomy and astrology had their roots in the skywatching practices of the ancient Babylonians and other civilizations of the ancient Near East. Babylonian astronomers watched as the heavenly bodies seemed to rise, move across the sky, and set. They saw that these motions occurred in cycles or patterns that were repeated daily, monthly, yearly, or at longer intervals. From their observations, they created orderly systems of timekeeping. Their calendars advised people when to plant and sow crops and when to hold religious celebrations. Certain celebrations were linked to celestial* events, such as solar and lunar eclipses or the longest and shortest days of the year. The Babylonians also began the practice of giving names from myth or legend to various constellations, or groups of stars. Greek and Roman astronomers later adopted this practice.
A Science of the Skies. The early Greeks watched the sky very closely. Greek farmers used the positions of the sun and stars to plan and organize agricultural chores for each season. Greek sailors used the stars to guide their navigation. The Greeks’ main contribution to astronomy, however, was their effort to explain what they saw. They did more than merely observe and record the movements of the heavenly bodies. They sought to understand why and how those bodies moved in such an orderly and predictable way. In doing so, the Greeks transformed stargazing into a science.
Greek astronomers spent little time wondering about what the planets were made of or how they came into existence. Instead, they wanted to find a logical, orderly system for predicting the movements of the sun, moon, and planets. They looked to mathematics and geometry to help discover such a system. In the 300s B.C., a mathematician named Eudoxus proposed a theory about the movement of the sun, the moon, and the planets. Eudoxus suggested that each heavenly body was fastened to the inside of a series of concentric spheres, with the Earth at the center. The spinning of the spheres made the planets move. Eudoxus’s theory failed to explain all celestial movements, however, and other Greek thinkers tackled the problem.
Around 270 B.C., the mathematician and astronomer Aristarchus correctly suggested that the sun, not the Earth, is at the center of the planetary system. He also suggested that the Earth moves around the sun like the other planets, and that it rotates, or spins, on an axis. Most ancient thinkers did not believe that the Earth moved, so they rejected Aristarchus’s ideas.
More than a century later, in about 150 B.C., the astronomer Hipparchus developed a theory of celestial movement based on the geometry of circles. He suggested that the sun and the moon moved around the Earth in circular paths, or orbits. To explain why celestial movements are not perfectly regular or symmetrical, Hipparchus theorized that the Earth was not exactly at the center of the orbits, or that the orbits themselves moved in complicated patterns. Hipparchus also compiled a star catalog that listed 850 stars and gave the location of each in the sky.
The most influential astronomer of the ancient world was Ptolemy, who lived and worked in the A.D. 100s. A keen observer, Ptolemy accepted Hipparchus’s theory and used his own observations to expand it.
* celestial relating to the heavens
Ancient Romans were free to study astrology, but professional astrologers were not always free to cast horoscopes. During the time of the Roman Empire, it was treason to cast the emperor's horoscope, for to "know" his time of death might prove a political advantage. Astrologers were particularly feared during times of internal strife and disorder. Roman authorities drove them out of Rome and the Italian provinces at least nine times between 139 B.C and A.D. 93. Each time, however, they returned to satisfy the Romans' love of divination.
He developed an elaborate theory that explained celestial movement as a series of interlocking circular orbits. Ptolemy’s system was detailed and difficult to understand, and it was based on some inaccurate notions, including the idea that the sun revolves around the Earth. Nevertheless, the system enabled astronomers to account for and predict the movement of all known heavenly bodies at any moment. Ptolemy also produced a star catalog that listed over 1,000 stars. His view of the universe was accepted by astronomers throughout the Mediterranean world, and it remained the foundation of astronomy for more than 1,000 years after his death.
Foretelling the Future. One reason that the sun, the moon, the planets, and the stars attracted so much attention in the ancient world was that these heavenly bodies were believed to have an affect on human lives and earthly events. This notion, the basis of astrology, was widespread among ancient peoples of the Near East. Beginning in the 300s B.C., astrology spread throughout the Greek and Roman civilizations.
In ancient Greece, astrology was based on the belief that the heavens and the Earth were connected in some mysterious way. This idea eventually had a great effect on Greek culture and philosophy. For example, the followers of Stoicism, who believed that each person’s destiny is determined from birth, supported the astrological notion that celestial movement shapes human lives. The astronomers Hipparchus and Ptolemy also believed in astrology.
Astrology took many forms. Among the most popular and well-known aspects of astrology was divination, or fortune-telling. Astrologers most commonly performed divination by calculating, or casting, horoscopes. A horoscope supposedly revealed the pattern of a person’s life based on the position of the stars and planets at the time of his or her birth. Astrologers often were called upon to cast horoscopes for newborn royal or noble infants.
Casting horoscopes required considerable astronomical knowledge and mathematical skill. As a result, many astrologers also were astronomers. Astrology actually may have helped to create the science of astronomy. It certainly helped keep astronomy alive, since both astronomy and astrology used the same framework of observation and theories about the heavens.
Astrology as a means of foretelling the future was widely accepted among the ancient Romans, who also believed strongly in omens and oracles. By the 100s B.C., astrological ideas influenced all levels of Roman society. Astrology even played a part in affairs of state. Tiberius and other Roman emperors frequently relied upon the advice of astrologers when faced with important decisions.
With the rise of Christianity in the Roman Empire, the importance of astrology declined. Christians regarded divination as a form of pagan magic and therefore unchristian. The Christian church banned divination, and Christian emperors enforced this ban. In A.D. 357, the emperor Constantius II made fortune-telling a crime punishable by death, thus ending the widespread practice of astrology. It survived as a series of superstitious beliefs and practices until its rebirth in later centuries. (See also Science.)