History of Astronomy

I

Introduction

History of Astronomy, history of the science that studies all the celestial bodies in the universe. Astronomy includes the study of planets and their satellites, comets and meteors, stars and interstellar matter, star systems known as galaxies, and clusters of galaxies. The field of astronomy has developed from simple observations about the movement of the Sun and Moon into sophisticated theories about the nature of the universe. See also Cosmology.

II

Overview

Advances in astronomy over the centuries have depended to a great extent on developments in technology. Initially, ancient peoples could only view the sky with their eyes. With careful attention to the changing positions of the Sun, Moon, planets, and stars, they were able to develop calendars and ultimately predictions of rare events, including eclipses. Instruments that allowed the measurement of the precise positions of celestial objects were the first major technological development, and those measurements formed the basis of models of the solar system.

The invention of the telescope in the early 1600s completely changed scientists’ ideas about the structure of the solar system and led to the discovery of new planets around our own sun. The telescope was also key to the measurement of distances to nearby stars and thereby provided the first clues to just how vast the universe is. The invention of the spectroscope combined with photography led to the discovery that the stars are made of the same elements found here on Earth.

Astronomy is different from most other sciences in that, apart from the planets we have visited by spacecraft, researchers cannot do experiments in the laboratory with the objects that they want to study. Instead, astronomers must learn about these distant objects by relying entirely on the visible light and other forms of energy—electromagnetic radiation—that are given off by them. The great breakthroughs of the 20th century were the development of spacecraft that allowed scientists to observe the universe from outside the distorting effects of Earth’s atmosphere, and the development of new sensors sensitive to forms of energy our eyes cannot detect. Examples are X rays , gamma rays, infrared or heat energy, and radio waves. These new windows on the universe have greatly expanded astronomical knowledge. See also Space Exploration.

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III

Ancient Origins

Ancient astronomers had only their eyes with which to view the sky, but they had a very practical reason for studying the skies. Thousands of years ago, changes in the heavens were the only available clocks and calendars. The stars could also be used for navigation. See also Archaeoastronomy.

Ancient Babylonian, Assyrian, and Egyptian astronomers all knew the approximate length of the year. The Egyptians of 3,000 years ago adopted a calendar with a year that was 365 days long, very near the modern value of 365.242 days. The Egyptians also used the rising of the star Sirius in the pre-dawn sky to mark the time when the Nile River could be expected to flood. The Chinese determined the approximate length of the year at about the same time as the Egyptians. The Maya of Central America kept a continuous record of days from day zero, which occurred on our equivalent of August 13, 3114 bc. They also kept track of years, eclipses, and the motions of the visible planets. Their year consisted of 18 months, each 20 days long, plus one 5-day month to total 365 days. Occasional adjustments were made to allow for the extra quarter of a day.

The adjustments required in the Maya calendar illustrate a common problem faced by ancient astronomers. Neither an entire month nor an entire year contains an exact whole number of days; to keep calendar years in step with the seasons, which were important for planting crops, the calendar makers assigned different numbers of days to successive months or years. Even though individual months or years were not the same length, they averaged out to approximately the true value. See also Calendar.

In the British Isles, ancient people used stone circles to keep track of the motions of the Sun and Moon. The best-known example is Stonehenge, a complex array of massive stones, ditches, and holes laid out in concentric circles. Stonehenge was built over an extended period of time lasting from about 2800 to 1500 bc. Some of the stones are aligned with the directions in which the Sun rises and sets at critical times of the year, such as when it reaches its most northerly and southerly points in the sky (the summer and winter solstices).

Ancient astronomers also observed five bright planets (the ones we call Mercury, Venus, Mars, Jupiter, and Saturn). These bodies, together with the Sun and Moon, move relative to the stars within a narrow band called the zodiac. The Moon moves around the zodiac quickly, overtaking the Sun about once every 29.5 days. The Sun and Moon always move along the zodiac from west to east. The five bright planets—Mercury, Venus, Mars, Jupiter, and Saturn—also have a generally eastward motion against the background of the stars. However, ancient astronomers in many different places around the globe noted that Mars, Jupiter, and Saturn sometimes move westward, in a backwards or retrograde direction. These planets, therefore, appear to have an erratic eastward course, with periodic loops in their paths.

In ancient times, people 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 and thus furthered the early progress of astronomy. However, none of the systems of astrology has been shown to be at all effective in making verifiable predictions.

Stars provide the background against which the motions of the planets are measured. Ancient Chinese, Egyptians, Greeks, and others gave names to patterns of stars. We call these patterns constellations. Some are very familiar, such as the Big Dipper, the Pleiades, and Orion. Few constellations look like their namesakes. Rather, ancient astronomers probably simply named areas of the sky with prominent groupings of stars after important characters in their mythology.

IV

Greek Astronomy

Modern astronomy can trace its heritage directly back to the ancient Greeks, who began to develop explanations for their observations of the sky. The writings of Aristotle summarize the knowledge of that era. He attributed the phases of the Moon—that is, the changes in its apparent shape—to the fact that we see different portions of its sunlit surface during the month. He also knew that the Sun is farther away from the Earth than the Moon because the Moon occasionally passes between the Sun and Earth and blocks the Sun’s light (a solar eclipse).

Aristotle cited two observations to show that Earth is a sphere. The first is that the shadow of Earth, which is seen during an eclipse of the Moon (when Earth is directly between the Sun and Moon), is always round. Only a sphere always has a round shadow no matter how it is viewed. If the Earth were a disk, we would sometimes see the shadow edge-on, and it would look like a straight line. The second observation was that travelers who journeyed a long distance south reported seeing stars not visible from Greece. If Earth were flat, all travelers anywhere would see the same stars. On a spherical Earth, travelers at different latitudes (different distances north or south) view the sky from different angles and see different constellations.

The Greek astronomer and mathematician Eratosthenes measured the size of the spherical Earth in about 200 bc. He noticed that on the first day of summer in Syene, Egypt, the Sun was directly overhead at noon. On the same date and time in Alexandria, Egypt, the Sun was about 7 degrees south of zenith. With simple geometry and knowledge of the distance between the two cities, he estimated the circumference of the Earth to be 250,000 stadia. (The stadium was a unit of length, derived from the length of the racetrack in an ancient Greek stadium. We have an approximate idea of how big an ancient Greek stadium was, and based on that approximation Eratosthenes was within 20 percent, and possibly within 1 percent, of the correct answer.)

Probably the most original ancient observer of the heavens was Aristarchus of Sámos, a Greek. He believed that motions in the sky could be explained by the hypothesis that Earth turns around on its axis once every 24 hours and, along with the other planets, revolves around the Sun. This theory, however, makes an important prediction that ancient Greeks could not verify. If Earth moves in an orbit around the Sun, then we look at the stars from different directions at different times of the year. As Earth moves along, nearby stars should shift their positions in the sky relative to more distant ones. The Greeks tried to measure this effect for the stars but were unsuccessful. It was only in 1838 that astronomers’ equipment could make measurements with the accuracy required to measure the very small shift of the stars, which turn out to be much, much farther away than the Greeks could imagine.

Perhaps the greatest of the ancient astronomers was Hipparchus, who lived around 150 bc and did most of his work at an observatory he built in Rhodes. There he recorded accurate positions of about 850 bright stars and classified them according to their brightness. The brightest stars he said were of the first magnitude, a term astronomers still use today. Because our planet is not an exact sphere, but bulges at the equator, the gravitational pulls of the Sun and Moon cause it to wobble like a top. It takes about 26,000 years for Earth’s axis to complete one full circle. Hipparchus estimated that the Earth’s axis shifts its position relative to the stars by 46 seconds of arc per year, which is very close to the modern value of 50.26 seconds of arc per year. This is known as the precession of the Earth.

The last of the great ancient astronomers was Ptolemy, who worked in Alexandria in about the year ad 140. Ptolemy’s greatest contribution was a geometrical model of the solar system that made it possible to predict the positions of the planets at any date and time. His model was used for about 1,400 years, until the time of Copernicus. Ptolemy’s challenge was to explain the complex motions of the planets, including the fact that they sometimes appear to move westward or backward in their orbits. In order to explain the observation, he assumed that each planet revolved in a small orbit called an epicycle. The center of the epicycle then revolved about the Earth on a much larger circle. At the time, circles were thought to be the perfect shape. It was assumed that the heavenly bodies would follow the most perfect shape. See also Ptolemaic System.

Astronomers now know that the planets do not follow circular orbits but rather elliptical ones, and they orbit around the Sun, not Earth. The backward or westward motion is explained by the fact that Earth moves more rapidly in its orbit than do Mars, Jupiter, and Saturn. When the Earth overtakes them during its yearly circuit around the Sun, these planets appear to move backwards relative to the stars. For an analogy, think of passing a slowly moving car on the freeway. As you overtake it, the car appears to be moving backward relative to the scenery beyond the side of the road.

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