Astronomy

I

Introduction

Astronomy, science dealing with all the celestial bodies in the universe, including the planets and their satellites, the dwarf planets, the comets and meteors, the stars and interstellar matter, the star systems known as galaxies, clusters of galaxies, and quasars. Modern astronomy is divided into several branches: astrometry, the observational study of the positions and motions of these bodies; celestial mechanics, the mathematical study of their motions as explained by the theory of gravitation; astrophysics, the study of their chemical composition and physical condition from spectrum analysis and the laws of physics; and cosmology, the study of the universe as a whole.

II

Ancient Origins

The curiosity of ancient peoples concerning day and night and the Sun, Moon, and stars led eventually to the observation that the heavenly bodies appear to move in a regular manner that is useful in defining time and direction on the Earth. Astronomy grew out of problems originating with the first civilizations, the need to establish the proper times for planting and harvesting crops and for religious celebrations and to find bearings and positions on long trading journeys or voyages. see Archaeoastronomy.

To ancient peoples the sky showed many regularities of behaviour. The bright Sun, which divided day from night, rose every morning from one direction, the east, moved steadily across the sky during the day, and set in a nearly opposite direction, the west. At night the stars could be seen to follow a similar course, seeming to rotate in permanent groupings, called constellations, around a fixed point in the sky, which is known as the celestial pole.

People noticed that daytime and night-time were unequal in length. On long days, seen from the North Temperate Zone, the Sun rose north of east and climbed high in the sky at noon; on days with long nights the Sun rose south of east and did not climb so high. Observation of the stars that appear in the west after sunset or in the east before sunrise showed that the relative position of the Sun among the stars changes gradually. The Egyptians may have been the first to discover that the Sun moves completely around the sphere of the fixed stars in approximately 365 days and nights. see Ecliptic.

Further study showed that the Sun, Moon, and five bright planets move around the star sphere within a narrow belt called the zodiac. The Moon traverses the zodiac quickly, overtaking the Sun about once every 29.5 days, the period known as the synodic month. Star-watchers in ancient times attempted to arrange the days and either the months or the years into a consistent time system, or calendar. Because neither an entire month nor an entire year contains an exact number of days the calendar-makers assigned to different months or years different numbers of days, having a long-range average that is close to the true value. The modern calendar provides for 97 leap years in every 400-year period, so that the average number of days per year is 365.2425, very close to the astronomically determined number, which is 365.24220.

The Sun and Moon always traverse the zodiac from west to east. However, the five bright planets Mercury, Mars, Venus, Jupiter, and Saturn, which also have a generally eastward motion against the background of the stars, sometimes move westward, or retrograde, for varying durations. The planets seem to follow an eastward course erratically, with periodic loops in their paths. Since ancient times, people have imagined that celestial events, especially the planetary motions, were connected with their own fortunes. This belief, called astrology, encouraged the development of mathematical schemes for predicting the planetary motions, but has no scientific basis.

III

Babylonian Astronomy

Interesting constellation maps and useful calendars were developed by several ancient peoples, notably the Egyptians, the Maya, and the Chinese, but the Babylonians went further. To perfect their calendar, they studied the motions of the Sun and Moon. It was their custom to designate as the beginning of each month the day after the new moon, when the lunar crescent first appears after sunset. Originally this day was determined by observations, but later the Babylonians wanted to calculate it in advance. About 400 bc they realized that the apparent motions of the Sun and Moon from west to east around the zodiac do not have a constant speed. These bodies appear to move with increasing speed for half of each revolution to a definite maximum and then to decrease in speed to the former minimum. The Babylonians found how to represent this cycle arithmetically and could predict the time of the new moon and the day on which the new month would begin. As a by-product, they knew the daily positions of the Moon and Sun for every day during the month.

In a similar manner the planetary positions were calculated, with both their eastward and retrograde motions represented. Archaeologists have unearthed hundreds of cuneiform tablets that show these calculations. A few of these tablets, which originated in the cities of Babylon and Erech (Uruk) on the Euphrates River, bear the name of Naburiannu, who flourished around 491 bc, or Kidinnu, who flourished around 379 bc—astrologers who may have invented the systems of calculation.

IV

Greek Astronomy

The ancient Greeks made important theoretical contributions to astronomy. The Odyssey of Homer refers to such constellations as the Great Bear, Orion, and the Pleiades and describes how the stars may serve as a guide in navigation. The poem Works and Days by Hesiod informs the farmer which constellations rise before dawn at different seasons of the year to indicate the proper times for ploughing, sowing, and harvesting.

Scientific contributions are associated with the names of the Greek philosophers Thales of Miletus and Pythagoras of Samos, but none of their own writings survive. About 450 bc the Greeks began a fruitful study of planetary motions. Philolaus, who flourished during the 5th century bc and was a follower of Pythagoras, believed that the Earth, Sun, Moon, and planets all moved around a central fire hidden from view by an interposed counter-Earth. According to his theory, the revolution of the Earth around the fire every 24 hours accounted for the daily motions of the Sun and stars. About 370 bc the astronomer Eudoxus of Cnidus explained observed motions by the supposition that a huge sphere bearing the stars on its inner surface revolved around the Earth, rotating daily. In addition, to account for solar, lunar, and planetary motions, he assumed that inside the star sphere were many interconnected transparent spheres that revolved in various ways.

Another Greek, Aristarchus of Samos, believed that motions in the sky could be explained by the hypothesis that the Earth turns on its axis once every 24 hours and, along with the other planets, moves around the Sun. This explanation was rejected by most Greek philosophers, who regarded the large, heavy Earth as a motionless globe around which the light, incorporeal celestial bodies revolve. This theory, known as the geocentric system, remained largely unchallenged for about 2,000 years.

The Greeks combined their celestial theories with carefully planned observations. The astronomers Hipparchus, in the 2nd century bc, and Ptolemy, in the 2nd century ad, determined the positions of about 1,000 bright stars and used this star chart as a background for measuring the planetary motions. They postulated that the planets, Sun, and Moon moved around a series of eccentric circles, with the Earth near a common centre. To explain the periodic variations in the speed of the Sun and Moon and the retrograde motions of the planets, they proposed that each of these bodies revolved uniformly around a second circle, called an epicycle, the centre of which moved around the first circle. By proper choice of the diameters and speeds for the two circular motions ascribed to each body, its observed motion could usually be represented. In some cases a third circle was required. This technique was described by Ptolemy in his great work the Almagest (see Ptolemaic System). Another thinker, who, like Ptolemy, kept the tradition of Greek astronomy alive in Alexandria in the early centuries of the Christian era, was Hypatia, a follower of Plato. She wrote commentaries on mathematical and astronomical topics and is regarded today as the first noteworthy female scientist and philosopher of the West.

Greek astronomy was later transmitted eastward to the Syrians, the Hindus, and the Arabs. The Arabian astronomers compiled new star catalogues in the 9th and 10th centuries and subsequently developed tables of planetary motion. The Arabs were good observers, but they made few useful contributions to astronomical theory. Medieval European astronomy began to flourish in the 13th century, when Arabic translations of Ptolemy’s Almagest filtered into Western Europe. Initially, Europeans were content to make tables of planetary motions, based on Ptolemy’s system, or to write short popular digests of his theory. Later the German philosopher and mathematician Nicholas of Cusa and the Italian artist and scientist Leonardo da Vinci questioned the basic assumptions of the centrality and immobility of the Earth.