General Astronomy/The Early Origins of Astronomy

From Wikibooks, open books for an open world
< General Astronomy
Jump to: navigation, search
General Astronomy
Coordinate Systems The Early Origins of Astronomy The First Physics


Stonehenge Inside Facing NE April 2005.jpg

Thousands of years ago, the night sky was much more a part of everyday life than it is today. Without light pollution from streetlights, anyone could see thousands of stars or the path of the Milky Way through the sky. The motion of the stars provided an essential calendar for predicting the weather, so ancient people were accordingly very attentive and observant of the heavens and the rest of their surroundings. These ancient observations were the foundation for what would become modern astronomy, and were the first manifestations of scientific thought in society.

A central principle in scientific reasoning is that the world is inherently orderly and capable of being understood, and that observation is the means by which we can learn about how the universe works. There is, however, nothing that requires the universe to make sense or be accessible to human understanding. Humanity has placed trust in the ideas behind the scientific method because science has been so successful, and so much of the world really does obey a set of rules.

The ingredients that make up a successful philosophy of science were not conceived all at once, but rather in a gradual process of development over the entire course of history. The ancients could never have known in advance which principles of scientific method would eventually prove to be reliable. In its adolescence, society stumbled through the dark to learn about the world, to discover how their experiences could teach them what the universe was like, and to identify the boundaries between reason and superstition. The story of the development of science in cultures throughout the world is revealing not only of the nature of the world and of truth, but also of human nature more generally.

Ancient artifacts going as far back as 20 thousand years, to the paleolithic age, may arguably be related to astronomy, although the interpretation of such artifacts is a subject of controversy. The best-known examples are found in the caves of Lascaux in France, where a variety of paleolithic artwork are found. Some specimens found in the caves might possibly depict the Pleiades star cluster or the zodiac. Animal bones nearby have markings such that they could have been used as a lunar calendar. Although artifacts from the paleolithic era are scarce and ambiguous, this scarcity does not necessarily imply that the peoples of the period took no interest in the heavens. Modern aboriginal cultures, for example, pass down ceremonies with strong astronomical symbolism that would not reflect themselves in archaeological artifacts.

The Stonehenge monument is one of the oldest observatories in the world. Ancient builders arranged the stones of the monument to align with important directions, such as the northernmost and southernmost rising and setting positions of the Sun and Moon.

The character of artifacts of the neolithic period is markedly different, since understanding of the skies and the calendar clearly bear deep meaning for agricultural cultures in even the earliest times. Observers gained the ability to manage the planning of significant annual events. The motions of the heavens develop a profound influence on the affairs of humanity, and this influence comes to manifest itself in religious interpretations of heavenly phenomena and worship of the planets, practices that form the origins of both astrology and astronomy.

Ancient monuments and markers demonstrate societies' early interest in the heavens to archaeologists and historians. Many graves of this period are aligned with the cardinal directions. Ancient monuments and altars of sacrifice face east, south, or west to within a few degrees — a clear indication that neolithic peoples had begun very early to identify the most fundamental concepts of astronomy. The most famous of the ancient monuments is Stonehenge. The stones of the monument mark the positions of sunrise on the summer solstice, among other things.

Several of the planets have been known since prehistoric times. These bodies are very bright, and they wander among the fixed stars, through the constellations. These were known to the Greeks as planētēs, "wanderers". The ancients knew of 7 "planets": the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The names for the seven days of the week come from the Norse gods: Sunday (the Sun), Monday (the Moon), Tuesday (Mars/Tiw), Wednesday (Mercury/Wodan), Thursday (Jupiter/Thor), Friday (Venus/Frigg), and Saturday (Saturn/Soeternes).

Ancient travelers knew that the Earth was round because their view of the stars changed as they traveled. Observers A and B can see the star, but observer C can't.

Highly systematic and carefully recorded observation of the heavens appears as early as the third millennium BC, in the ancient Mesopotamian civilization of Babylon. The Babylonians also developed the earliest timekeeping instruments. Of perhaps even greater significance, however, was the birth of astrology in Mesopotamia. The astrologers of Babylon were the first to suppose a cause and effect relationship between heavenly and earthly events, and the first to imagine an underlying order in nature — though no real connection between human and celestial events existed, the prophecies of astrology were the ancestors of modern scientific prediction.

The early astronomers understood a great deal about the world around them. By Aristotle's time, many people believed that the world was round. They knew this from several sources of evidence:

  1. When ships sailed beyond the horizon, those watching from land would see the hull disappear first, and then would watch the sail disappear. Sailors see the land sink below the horizon bottom-first. From this, they gathered that the surface of the world was curved, and that the ship was moving around the curvature of the Earth.
  2. Ancient people supposed eclipses of the Moon to be the shadow of the Earth. Shadows seen during lunar eclipses were always round. The only shape that always casts a round shadow is a sphere. This suggested that the Earth was spherical.
  3. Travelers noticed that new stars became visible as they moved in their journeys. When travelers move north, the North Star and the northern constellations gain altitude in the sky. The ancients recognized that this implied that the Earth is curved.

Aristotle mentions in his writings that some thinkers of his time hold that the Earth is flat, while others believe it to be spherical. He himself argues firmly that the Earth is a sphere. After Aristotle's time, nearly all Western writers claim that the Earth is a sphere.

Eratosthenes knew that obelisks in the city of Syene cast no shadow at midday on the summer solstice. In Alexandria, however, obelisks cast a shadow. This reveals that the Earth is not flat and makes it possible to measure its circumference.

In the third century BC, the Hellenistic Egyptian scholar Eratosthenes served as head of the library of Alexandria, where he studied and wrote on a variety of subjects. During his studies, Eratosthenes learned that the Sun cast no shadow on midday of the summer solstice from the city of Syene. From Alexandria, Eratosthenes observed that the Sun did cast a shadow, and he measured that the angle between the Sun and the zenith was about seven degrees when the Sun was highest in the sky.

Eratosthenes knew he could measure the circumference of the Earth by measuring the distance between Alexandria and Syene, assuming that the Sun was at the highest point in both cities at the same time. Since 7 degrees is 7/360 of a circle, the distance between Syene and Alexandria must be 7/360 (or, equivalently, about 1/50) of the circumference of Earth.

Eratosthenes estimated the distance between Syene and Alexandria to be about 5,000 stadia, and concluded that the circumference of the Earth was about 50 times that, or 250,000 stadia. The stadium was a common unit of distance in the ancient world, comparable to kilometers or miles in modern usage; the exact length meant by a stadium varied over history and from region to region, so it is difficult to compare Eratosthenes's estimate with modern values. Most values of the stadium, however, translate to a circumference in the area of 40,000 kilometers, which is remarkably close to the accepted circumference of 40,070 kilometers.

As science was beginning to grow in the cultures of Western civilization, it was likewise taking root elsewhere in the world. Ancient and medieval China, India, and America in particular have been centers of the scientific world.

The history of science in India is one of the longest and best documented in the world. Largely isolated from the rest of the world through much of its earliest history, India was free to develop its unique flavor of astronomical practice at its own pace and without external influence. Ancient India had no system of astrology, but celestial timekeeping became popular very early. The first astronomers monitored the progression of the year and tracked the movement of the Sun, Moon, and planets through the nakshatra, the Indian zodiac.

The quality of these observations would make it possible for later Indian astronomers to detect the effects of precession, just as Hipparcos had done for Greek astronomy. Indian astronomers of the Middle Ages estimated the speed of precession to be 54 seconds per year. Although this is much more accurate than Hipparcos's measurement, we must note that the number 54 was chosen for mystical rather than scientific reasons. Ancient people throughout the world mixed mysticism with arithmetic in this way, but we see this much more consistently in the Indian culture than in any other.

The Christian era brought great change to Indian astronomy. By about the year AD 519, Babylonian conquest had spread into the Indus basin, and many Greek scholars had already visited or settled. Naturally, cultural exchange took root.

The ancient Chinese valued astronomy very highly. Systematic observation of the heavens may have begun in China as early as the third millennium BC. Science was viewed as a powerful tool and therefore guarded very closely by the state. Under dynastic rule, the perceived purpose of astrology and astronomy was to remind the people of the connection between the emperor and celestial events. Accordingly, astrology was charged specifically to an Astronomical Bureau appointed by the emperor and carried out according to strict rules.

The Chinese regarded celestial events as important omens, particularly in the case of "guest stars," stars that temporarily appeared in the sky and quickly faded away. Today we know that these are stellar explosions called supernovae. In the year AD 1054, a new supernova became bright enough to be visible even during the day. Chinese and Arab astronomers recorded the event, and it appears to be depicted in an American cave painting. Because of the importance the Chinese assigned to supernovae, their observations of ancient supernovae are by far the most complete in the world, and remain useful as a source of scientific data even today.

Mayan astronomers used observatories such as this one, located in the ancient city of Chichén Itzá. Windows at the top level, now partially destroyed, were strategically placed to observe the positions of sunrise and sunset and the motions of the Moon and Venus.

Meanwhile, astronomy was thriving in pre-Columbian America, where interest in the heavens was widespread. Astronomy held a high position in the Mayan civilization, which dominated Mesoamerica for two thousand years prior to the arrival of Europeans. The Maya people believed that patterns in nature were very important, and that it should be possible to use astrology to predict the future based on past experience. This made the observation of celestial cycles very important, since they would give advance knowledge of future events, and Mayans dutifully kept track of motions of the Sun and Moon. With the accumulation of centuries of observation, Mayan astronomers were able to predict eclipses and measure the lengths of heavenly cycles very well.

Similar ideas were sprouting throughout the Americas. Throughout the Americas, ancient people built monuments, buildings, mounds, and temples aligned with rising and setting points of important stars, or with the northernmost and southernmost positions of the Moon. Calendars and monuments like Stonehenge are found here. Clearly, interest in the heavens was common throughout the ancient world, as were the rudimentary beginnings of science.

The painting on this rock, made by Anasazi Americans in the 11th century, might depict the rare appearance of a bright explosion of a dying star. The supernova of AD 1054 (of which the Crab Nebula is a result) and the Moon were in this configuration when the supernova was near its brightest. An imprint of a hand at the top signifies that this is a sacred place.

Scientific thought has appeared throughout history and all over the world, but it's clear that the scientific enterprise enjoyed more success in some areas and time periods than in others. The reasons for this are entirely unclear. What factors of culture, economics, and philosophy affect the rise scientific thought? The Babylonian culture, for example, did not permit deviation of any kind from tradition or conventional practice. Such a culture might be expected to hinder the progress of science, yet science and mathematics flourished there. In ancient China, only nobles were permitted to practice science. Some historians have suggested that this culture prevented a renaissance that might otherwise have occurred in China long before the European Renaissance. This question of the interplay between science and sociology has been the subject of great interest in the study of the history of science.

While magic and superstition were a significant part of early thought and difficult to isolate from the emerging pursuit of science, it would be unfair to say that spiritualism completely permeated the first ideas about the nature of the universe. The scientific spirit, the principle of learning about the universe by observing it, was clearly present even from the earliest times, and scientific thought has been remarkably widespread.

General Astronomy
Coordinate Systems Motion and Gravity The First Physics