Brief History Of Astronomy

Astronomy, which literally means "the study of the stars," involves the study of the universe and every type of object that exists in the universe, including our own planet. Astronomers study the universe by investigating the origins, evolution, composition, motions, relative positions, and sizes of celestial bodies.

Unlike astrology, which alleges that the positions of the Sun, Moon, and stars affect human affairs, astronomy is a scientific discipline based upon an understanding of physical principles (the so-called Laws of Nature) and how their operation produces phenomena that may be observed. These principles can often be summarized in simple mathematical equations. Learning some of the simpler and more fundamental principles allows you both insight into how astronomers learn about the universe as well as an understanding of why the objects we observe have the properties that they do.

The number of physical factors that astronomers can measure is small. These factors include masses, sizes, densities, temperatures, and brightnesses, as well as the time over which changes may occur. Often, these factors are measured in the metric system based on kilograms, meters, and seconds (see Chapter 2). Many quantities can be numerically large; therefore, astronomers also use other units specific to the discipline of astronomy, such as astronomical units for distances within the solar system, parsecs for distances to stars, solar masses (the mass of the Sun) for masses of stars, and solar luminosities (the luminosities of the Sun) for the energy-per-second emitted by stars (see Chapter 8).

Astronomy has had a long, rich history, from ancient peoples' interpretations of celestial phenomena to modern studies of the universe. Many individuals have made significant contributions to the development of astronomy throughout human history.

Archaeoastronomy

Archaeoastronomy is the study of ancient (pre-technological) humankind's awareness of celestial phenomena and its influence on their societies. A number of these influences in ancient societies have been identified, including how structures were built, early forms of calendars, and the development of mathematical concepts.

Ancient structures

Ample evidence exists that in ancient times, many cultural groups all over the world built single structures, buildings, and even cities facing in astronomically significant directions. For example, the Newgrange passage grave in Ireland (circa 3200 B.C.) faces the midwinter sunrise, and Stonehenge, in southern England (circa 2800 B.C.), points toward the mid-summer sunrise. Similarly, various Mayan buildings in Mesoamerica show relationships to significant horizon rise and set positions for the planet Venus.

Early calendars

Before ancient humans recognized that the yearly cycle predictably repeats itself, they regulated agricultural, hunting, and religious activities by directly observing celestial phenomena. These phenomena included solar position against the background of stars, the cyclic north-south oscillation of the Sun's rise and set positions on the horizon, or the pattern of nighttime constellations. Evidence of early peoples' celestial timekeeping include the orientation of Stonehenge, which suggests a calendrical awareness, and the Sun Circle (circa 1100 A.D.) built by the people of the Indian city of Cahokia, Illinois.

Navigation

When societies embarked on extended travel where natural features on land were not available, the sky provided an alternative means to mark one's position during journeys. The Polynesians were experts at using the apparent positions of stars in the sky to navigate across the vast expanses of the southern Pacific Ocean. In the same manner, traders crossing the featureless tracks of the Sahara Desert used the sky for their directions. It has been argued that our oldest constellations may date from the delineation of stellar patterns as mnemonic devices for Mediterranean navigation as early as 3000 B.C.

Mathematics

In due course, as societies accumulated extensive observations of the sky, there developed the desire to understand celestial events in a quantitative fashion. The 360 degrees of the circle is the most tractable approximation that the Babylonians could make for the annual circuit of the Sun around the sky in 365.25 days. Islamic scholars were inspired to develop spherical trigonometry for navigational reasons as well as for the Islamic tenet to pray toward the city of Mecca.

Other cultural influences

Awareness of phenomena in the sky has affected other aspects of cultures. Descendancy from the celestial deities of the Sun and Moon was used in many past societies to justify political supremacy. The development of mythologies, astrologies, and religions also contain elements that help societies obtain a sense of order regarding the workings of celestial events.

Greek Astronomy

The greatest influence on western astronomy comes from ancient Greece. Greek philosophers were the first to apply mathematics to attempt to understand the universe more deeply than for simple predictive purposes. Although the civilization of ancient Greece declined, the Greek ideas that were passed to Indian and Asian colonies were subsequently adopted by Islamic scholars, and were later introduced into Europe by Arab invaders in the fifteenth century Table 1-1 gives a listing of the Greek alphabet. These letters note scientific quantities and are used in the naming of stars.

Table 1-1: The Greek Alphabet

Letter

Upper Case

Lower Case

Letter

Upper Case

Lower Case

Alpha

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