Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page
Search the Web





The Contents Introduction
 From our small world we have gazed upon the cosmic ocean for thousands of years.Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers, and named them after Roman deities- Jupiter, king of the gods; Mars, the god of war;Mercury, messenger of the gods; Venus, the god of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky.
Ancient astronomers believed that the Earth was the center of the universe, and that the sun and all the other stars revolved around the earth .Copernicus proved that earth and the other planets inour solar system orbit our sun. Little by little, we are charting the universe, and an obvious question arises
Are there other planets around other stars? Are there other planets where life might exist? Only recently have astronomers had the tools to indirectly detect large planets, around other stars in nearby galaxies.
In 1610,Galileo Galilei aimed his telescope at Jupiter and spotted four points of light orbiting the planet. For the first time , humans had seen the moons of another world. In honor of their discoverer, these four bodies would become known as the Galilean satellites or moons.
Since the invention of telescope, there more planets have been discovered in our solar system:
Uranus (1781), Neptune (1846), and Pluto (1930). In addition ,there are thousands of small bodies such as astroids and comets.From 1610 to 1977, Saturn was thought to be the only planet with rings. We now know that Jupiter, Uranus, and Neptune also have ring systems.
Back to top

What is astronomy
 Astronomy, study of the universe and the celestial bodies, gas, and dust within it. Astronomy includes observations and theories about the solar system, the stars, the galaxies, and the general structure of space. Astronomy also includes cosmology, the study of the universe and its past and future. People who study astronomy are called astronomers, and they use a wide variety of methods to perform their research. These methods usually involve ideas of physics, so most astronomers are also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas of astronomy also use techniques of chemistry, geology, and biology. Astronomy is the oldest science, dating back thousands of years to when primitive people noticed objects in the sky overhead and watched the way the objects moved. In ancient Egypt, the visibility of certain stars for the first time each year marked the onset of the seasonal flood, an important event for agriculture. In 17th-century England, astronomy provided methods of keeping track of time that were especially useful for accurate navigation. Astronomy has a long tradition of practical results, such as our current understanding of the stars, day and night, the seasons, and the phases of the Moon. Much of today's research in astronomy does not address immediate practical problems. Instead, it involves basic research to satisfy our curiosity about the universe and the objects in it. One day such knowledge may well be of practical use to humans
Back to top

History of astronomy
introduction to the history
H istory of the science that deals with 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. The sky exhibited many regularities of behavior, which ancient astronomers noticed and recorded. The bright Sun, which divided daytime from nighttime, 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 more than 1,000 visible stars followed a similar course. The stars appeared to rotate in permanent groups, called constellations, around a fixed point in the sky. In the middle latitudes of the northern hemisphere, people noticed that daytime and nighttime were unequal in length for most of the year. On long days in the summer, the Sun rose north of east and climbed high in the sky at noon; the winter had long nights and the Sun rose south of east and did not climb so high at noon . People also observed that the stars that appear in the west after sunset or in the east before sunrise changed gradually from night to night. The pattern of stars repeated itself every 365 days. Ancient astronomers saw that the sky also held the Moon and five bright planets. These bodies, together with the Sun, move around the star sphere within a narrow belt called the zodiac. The Moon moves around 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. Neither an entire month nor an entire year contains an exact whole number of days; 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 approximate the true value. The Sun and Moon always move along the zodiac from west to east. The five bright planets-Mercury, Mars, Venus, Jupiter and Saturn-also have a generally eastward motion against the background of the stars. However, the planets move westward, or retrograde, for varying durations during each synodic period. Thus, the planets 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 progress of astronomy during ancient times.
Back to top

Babylonian astronomy
I nteresting constellation maps and useful calendars were developed by several ancient peoples, notably the Egyptians, the Maya, and the Chinese, but the Babylonians accomplished even greater achievements. The Babylonian civilization thrived from the 18th to the 6th century BC in what is now Iraq . To perfect their calendar, they studied the motions of the Sun and Moon. They designated the day after the new moon as the beginning of each month. The new moon occurs when the side of the Moon away from Earth is lit by the Sun, so the side of the Moon facing Earth is dark. Originally calendar makers determined this day by observations, but later the Babylonians wanted to calculate it in advance. About 400 BC, by which time Babylonia was a part of Persia, Babylonian astronomers 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 attempted to represent this cycle arithmetically by giving the Moon, for example, a fixed speed for its motion during half its cycle and a different fixed speed for the other half. Later they refined the mathematical method by representing the speed of the Moon as a factor that increases linearly from the minimum to maximum during half of its revolution and then decreases to the minimum at the end of the cycle. With these calculations of the lunar and solar motions, Babylonian stargazers 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 Uruk (Erech) on the Euphrates River, bear the name of Naburiannu or Kidinnu, astrologers who may have invented the systems of calculation.
Back to top

Greek astronomy
T he civilization of ancient Greece extended from about 1400 to about 300 BC. The ancient Greeks made important theoretical contributions to astronomy. The Odyssey, an epic poem traditionally attributed to Greek author Homer and probably written in the 8th century BC, refers to star groups such the Great Bear, Orion, and the Pleiades and describes how the stars serve as a guide in navigation. The poem Works and Days by Greek poet Hesiod informs farmers about which constellations rise before dawn at different seasons of the year, indicating the proper times for plowing, sowing, and harvesting. Scientists associate many important scientific contributions with Thales of Miletus and Pythagoras of Sámos, but none of the writings of these Greek philosophers survive. The legend that Thales correctly predicted a total solar eclipse on May 28, 585 BC, is of dubious origin. About 450 BC the Greeks began a fruitful study of planetary motions. Philolaus, a follower of Pythagoras, believed that Earth, the Sun, the Moon, and the planets all moved around a central fire. People on Earth could not see the fire because a body called counterearth moved around the fire between the fire and Earth. According to his theory, the revolution of 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 moved around Earth at its center in a daily rotation. 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. 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 explanation was rejected by most Greek philosophers, who regarded the big, heavy Earth as a motionless globe around which the light, incorporeal bodies revolve. This theory, known as the geocentric system, remained virtually unchallenged for about 2,000 years. In the 2nd century AD, at the beginning of the Hellenistic period of Greek civilization, the Greeks combined their celestial theories with carefully planned observations. The astronomers Hipparchus and Ptolemy determined the positions of about 1,000 bright stars and used this star chart as a background for measuring the planetary motions. Abandoning the spheres of Eudoxus for a more flexible system of circles, they postulated a series of eccentric circles with Earth near a common center to represent the general eastward motions at varying speeds of the Sun, Moon, and planets around the zodiac. To explain the periodic variations in the speed of the Sun and Moon and the backward movement of the planets, they postulated that each of these bodies revolved uniformly around a second circle, called an epicycle, the center of which was situated on the first. By proper choice of the diameters and speeds for the two circular motions ascribed to each body, its observed motion usually could be represented.In some cases a third circle was required. This technique was described by Ptolemy in his great work the Almagest . Hypatia, a follower of Plato, wrote commentaries on mathematical and astronomical topics and is regarded today as the first female astronomer.
Back to top

Medieval astronomy
G reek astronomy was transmitted eastward to the Syrians, the Hindus, and the Arabs. The Arabian astronomers compiled new star catalogs in the 9th and 10th centuries and subsequently developed tables of planetary motion. Although the Arabs were good observers, they made few useful contributions to astronomical theories. In the 13th century, Arabic translations of Ptolemy's Almagest filtered into western Europe, stimulating interest in astronomy. 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 Earth.
Back to top

The Copernican theory
T he history of astronomy took a dramatic turn in the 16th century as a result of the contributions of the Polish astronomer Nicolaus Copernicus. He was educated in Italy and was a canon of the Roman Catholic church. He spent most of his life pursuing astronomy, however, and he made a new star catalog from personal observations . He is most famous for his great work On the Revolution of Heavenly Bodies (1543), in which he analyzed critically the Ptolemaic theory of an Earth-centered universe and showed that the planetary motions can be explained by assuming a central position for the Sun rather than for Earth. Little attention was paid to the Copernican, or heliocentric, system until Italian astronomer Galileo discovered evidence to support it. Long a secret admirer of Copernicus's work, Galileo saw his chance to test the Copernican theory of a moving Earth when the telescope was invented in the Netherlands. In 1609 Galileo made a small refracting telescope, turned it skyward, and discovered the phases of Venus, indicating that this planet revolves around the Sun; he also discovered four moons revolving around Jupiter, as well as the rings of Saturn. Convinced that some bodies, at least, do not circle Earth, he began to speak and write in favor of the Copernican system. His attempts to publicize the Copernican system caused him to be tried by the ecclesiastical authorities. Although he was forced to repudiate his beliefs and writings, the powerful theory could not be suppressed.
Back to top

Kepler's laws and the newtonian theory
F rom the scientific viewpoint, the Copernican theory was only a rearrangement of the planetary orbits, as conceived by Ptolemy. The ancient Greek theory of planets moving around circles at fixed speeds was retained in the Copernican system. From 1580 to 1597 Danish astronomer Tycho Brahe observed the Sun, Moon, and planets at his island observatory near Copenhagen and later in Germany. Based on the data compiled by Brahe, his German assistant, Johannes Kepler, formulated the laws of planetary motion, stating that the planets revolve around the Sun, not in circular orbits with uniform motion but in elliptical orbits at varying speeds, and that their relative distances from the Sun can be determined from the observed periods of revolution. British physicist Sir Isaac Newton advanced a simple principle to explain Kepler's laws of planetary motion. By mathematical reasoning, he argued that an attractive force exists between the Sun and each of the planets. This force, which depends on the masses of the Sun and planets and on the distances between them, provides the basis for the physical interpretation of Kepler's laws. Newton's mathematical discovery is called the law of universal gravitation.
Back to top

Toward modern astronomy
A fter Newton's time, astronomy branched out in several directions. With his law of gravitation, the old problem of planetary motion was studied anew as celestial mechanics. Improved telescopes permitted the scanning of planetary surfaces, the discovery of many faint stars, and the measurement of stellar distances. In the 19th century a new instrument, the spectroscope, yielded information about the chemical composition and motions of heavenly bodies. During the 20th century, increasingly larger reflecting telescopes were built, with mirrors as large as 390 in (1,000 cm) in diameter. Studies with these instruments revealed the structure of huge distant assemblages of stars, called galaxies, and of clusters of galaxies. In the second half of the 20th century, developments in physics led to new classes of astronomical instruments, some of which have been placed on Earth-orbiting satellite observatories. These instruments were sensitive to a wide variety of radiation wavelengths, including the gamma-ray, X ray, ultraviolet, infrared, and radio regions of the electromagnetic spectrum. Astronomers began to study not only planets, stars, and galaxies but also plasmas (hot, ionized gases) surrounding double stars, interstellar regions that are the birthplaces of new stars, cold dust grains that are invisible in the optical regions, energetic nuclei of galaxies that may contain black holes, and photons originating from the big bang that may yield information about the early history of the universe.
Back to top

Homepage|First astronomers|First journey
Second journey|Solar system|Satellites|Space stations