Eclipses

Eclipses
Because of the tilt of the Moon's orbit, the Moon usually passes above or below the Sun at new moon and above or below Earth's shadow at full moon. Sometimes, though, the full moon or new moon crosses the plane of Earth's orbit. By a coincidence of nature, even though the Moon is about 400 times smaller than the Sun, it is also about 400 times closer to Earth than the Sun is, so the Moon and Sun look almost exactly the same size from Earth. If the Moon lines up with the Sun and Earth at new moon (when the Moon is between Earth and the Sun), it blocks the Sun’s light from Earth, creating a solar eclipse. If the Moon lines up with Earth and the Sun at the full moon (when Earth is between the Moon and the Sun), Earth’s shadow covers the Moon, making a lunar eclipse.

A total solar eclipse is visible from only a small region of Earth. During a solar eclipse, the complete shadow of the Moon that falls on Earth is only about 160 km (about 100 mi) wide. As Earth, the Sun, and the Moon move, however, the Moon’s shadow sweeps out a path up to 16,000 km (10,000 mi) long. The total eclipse can only be seen from within this path. A total solar eclipse occurs about every 18 months. Off to the sides of the path of a total eclipse, a partial eclipse, in which the Sun is only partly covered, is visible. Partial eclipses are much less dramatic than total eclipses. The Moon’s orbit around Earth is slightly elliptical, or egg-shaped. The distance between Earth and the Moon varies slightly as the Moon orbits Earth. When the Moon is farther from Earth than usual, it appears smaller and may not cover the entire Sun during an eclipse. A ring, or annulus, of sunlight remains visible, making an annular eclipse. An annular solar eclipse also occurs about every 18 months. Additional partial solar eclipses are also visible from Earth in between.

At a lunar eclipse, the Moon is actually in Earth's shadow. When the Moon is completely in the shadow, the total lunar eclipse is visible from everywhere on the half of Earth from which the Moon is visible at that time. As a result, more people see total lunar eclipses than see total solar eclipses.

Comets and Asteroids

Comets and asteroids are rocky and icy bodies that are smaller than planets. The distinction between comets, asteroids, and other small bodies in the solar system is a little fuzzy, but generally a comet is icier than an asteroid and has a more elongated orbit. The orbit of a comet takes it close to the Sun, then back into the outer solar system. When comets near the Sun, some of their ice turns from solid material into gas, releasing some of their dust. Comets have long tails of glowing gas and dust when they are near the Sun. Asteroids are rockier bodies and usually have orbits that keep them at always about the same distance from the Sun.

Both comets and asteroids have their origins in the early solar system. While the solar system was forming, many small, rocky objects called planetesimals condensed from the gas and dust of the early solar system. Millions of planetesimals remain in orbit around the Sun. A large spherical cloud of such objects out beyond Pluto forms the Oort cloud. The objects in the Oort cloud are considered comets. When our solar system passes close to another star or drifts closer than usual to the center of our galaxy, the change in gravitational pull may disturb the orbit of one of the icy comets in the Oort cloud. As this comet falls toward the Sun, the ice turns into vapor, freeing dust from the object. The gas and dust form the tail or tails of the comet. The gravitational pull of large planets such as Jupiter or Saturn may swerve the comet into an orbit closer to the Sun. The time needed for a comet to make a complete orbit around the Sun is called the comet’s period. Astronomers believe that comets with periods longer than about 200 years come from the Oort Cloud. Short-period comets, those with periods less than about 200 years, probably come from the Kuiper Belt, a ring of planetesimals beyond Neptune. The material in comets is probably from the very early solar system, so astronomers study comets to find out more about our solar system’s formation.

When the solar system was forming, some of the planetesimals came together more toward the center of the solar system. Gravitational forces from the giant planet Jupiter prevented these planetesimals from forming full-fledged planets. Instead, the planetesimals broke up to create thousands of minor planets, or asteroids, that orbit the Sun. Most of them are in the asteroid belt, between the orbits of Mars and Jupiter, but thousands are in orbits that come closer to Earth or even cross Earth's orbit. Scientists are increasingly aware of potential catastrophes if any of the largest of these asteroids hits Earth. Perhaps 2,000 asteroids larger than 1 km (0.6 mi) in diameter are potential hazards.

Solar System Formation

Our solar system began forming about 5 billion years ago, when a cloud of gas and dust between the stars in our Milky Way Galaxy began contracting. A nearby supernova—an exploding star—may have started the contraction, but most astronomers believe a random change in density in the cloud caused the contraction. Once the cloud—known as the solar nebula—began to contract, the contraction occurred faster and faster. The gravitational energy caused by this contraction heated the solar nebula. As the cloud became smaller, it began to spin faster, much as a spinning skater will spin faster by pulling in his or her arms. This spin kept the nebula from forming a sphere; instead, it settled into a disk of gas and dust.

In this disk, small regions of gas and dust began to draw closer and stick together. The objects that resulted, which were usually less than 500 km (300 mi) across, are the planetesimals. Eventually, some planetesimals stuck together and grew to form the planets. Scientists have made computer models of how they believe the early solar system behaved. The models show that for a solar system to produce one or two huge planets like Jupiter and several other, much smaller planets is not unusual.

The largest region of gas and dust wound up in the center of the nebula and formed the protosun (proto is Greek for “before” and is used to distinguish between an object and its forerunner). The increasing temperature and pressure in the middle of the protosun vaporized the dust and eventually allowed nuclear fusion to begin, marking the formation of the Sun. The young Sun gave off a strong solar wind that drove off most of the lighter elements, such as hydrogen and helium, from the inner planets. The inner planets then solidified and formed rocky surfaces. The solar wind lost strength. Jupiter’s gravitational pull was strong enough to keep its shroud of hydrogen and helium gas. Saturn, Uranus, and Neptune also kept their layers of light gases.

The theory of solar system formation described above accounts for the appearance of the solar system as we know it. Examples of this appearance include the fact that the planets all orbit the Sun in the same direction and that almost all the planets rotate on their axes in the same direction. The recent discoveries of distant solar systems with different properties could lead to modifications in the theory, however.

Studies in the visible, the infrared, and the shortest radio wavelengths have revealed disks around several young stars in our galaxy. One such object, Beta Pictoris (about 62 light-years from Earth), has revealed a warp in the disk that could be a sign of planets in orbit. Astronomers are hopeful that, in the cases of these young stars, they are studying the early stages of solar system formation.

STARS

Stars are an important topic of astronomical research. Stars are balls of gas that shine or used to shine because of nuclear fusion in their cores. The most familiar star is the Sun. The nuclear fusion in stars produces a force that pushes the material in a star outward. However, the gravitational attraction of the star’s material for itself pulls the material inward. A star can remain stable as long as the outward pressure and gravitational force balance. The properties of a star depend on its mass, its temperature, and its stage in evolution.

Astronomers study stars by measuring their brightness or, with more difficulty, their distances from Earth. They measure the “color” of a star—the differences in the star’s brightness from one part of the spectrum to another—to determine its temperature. They also study the spectrum of a star’s light to determine not only the temperature, but also the chemical makeup of the star’s outer layers.


Kinds of Stars
Many different types of stars exist. Some types of stars are really just different stages of a star’s evolution. Some types are different because the stars formed with much more or much less mass than other stars, or because they formed close to other stars. The Sun is a type of star known as a main-sequence star. Eventually, main-sequence stars such as the Sun swell into giant stars and then evolve into tiny, dense, white dwarf stars. Main-sequence stars and giants have a role in the behavior of most variable stars and novas. A star much more massive than the Sun will become a supergiant star, then explode as a supernova. A supernova may leave behind a neutron star or a black hole.
In about 1910 Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently worked out a way to graph basic properties of stars. On the horizontal axis of their graphs, they plotted the temperatures of stars. On the vertical axis, they plotted the brightness of stars in a way that allowed the stars to be compared. (One plotted the absolute brightness, or absolute magnitude, of a star, a measurement of brightness that takes into account the distance of the star from Earth. The other plotted stars in a nearby galaxy, all about the same distance from Earth.) The resulting Hertzsprung-Russell diagram, also called an H-R diagram or a color-magnitude diagram (where color relates to temperature), is a basic tool of astronomers.

Black Holes

Black holes are objects that are so massive and dense that their immense gravitational pull does not even let light escape. If the core left over after a supernova explosion has a mass of more than about fives times that of the Sun, the force holding up the neutrons in the core is not large enough to balance the inward gravitational force. No outward force is large enough to resist the gravitational force. The core of the star continues to collapse. When the core's mass is sufficiently concentrated, the gravitational force of the core is so strong that nothing, not even light, can escape it. The gravitational force is so strong that classical physics no longer applies, and astronomers use Einstein’s general theory of relativity to explain the behavior of light and matter under such strong gravitational forces. According to general relativity, space around the core becomes so warped that nothing can escape, creating a black hole. A star with a mass ten times the mass of the Sun would become a black hole if it were compressed to 90 km (60 mi) or less in diameter.

Astronomers have various ways of detecting black holes. When a black hole is in a binary system, matter from the companion star spirals into the black hole, forming a disk of gas around it. The disk becomes so hot that it gives off X rays that astronomers can detect from Earth. Astronomers use X-ray telescopes in space to find X-ray sources, and then they look for signs that an unseen object of more than about five times the mass of the Sun is causing gravitational tugs on a visible object. By 1999 astronomers had found about a dozen potential black holes.

Religion !!

INTRODUCTION
Religion, sacred engagement with that which is believed to be a spiritual reality. Religion is a worldwide phenomenon that has played a part in all human culture and so is a much broader, more complex category than the set of beliefs or practices found in any single religious tradition. An adequate understanding of religion must take into account its distinctive qualities and patterns as a form of human experience, as well as the similarities and differences in religions across human cultures.

In all cultures, human beings make a practice of interacting with what are taken to be spiritual powers. These powers may be in the form of gods, spirits, ancestors, or any kind of sacred reality with which humans believe themselves to be connected. Sometimes a spiritual power is understood broadly as an all-embracing reality and sometimes it is approached through its manifestation in special symbols. It may be regarded as external to the self, internal, or both. People interact with such a presence in a sacred manner—that is, with reverence and care. Religion is the term most commonly used to designate this complex and diverse realm of human experience.

The word religion is derived from the Latin noun religio, which denotes both earnest observance of ritual obligations and an inward spirit of reverence. In modern usage, religion covers a wide spectrum of meanings that reflect the enormous variety of ways the term can be interpreted. At one extreme, many committed believers recognize only their own tradition as a religion, understanding expressions such as worship and prayer to refer exclusively to the practices of their tradition. Although many believers stop short of claiming an exclusive status for their tradition, they may nevertheless use vague or idealizing terms in defining religion—for example, “true love of God,” or “the path of enlightenment.” At the other extreme, religion may be equated with ignorance, fanaticism, or wishful thinking.

By defining religion as a sacred engagement with what is taken to be a spiritual reality, it is possible to consider the importance of religion in human life without making claims about what it really is or ought to be. Religion is not an object with a single, fixed meaning, or even a zone with clear boundaries. It is an aspect of human experience that may intersect, incorporate, or transcend other aspects of life and society. Such a definition avoids the drawbacks of limiting the investigation of religion to Western or biblical categories such as monotheism (belief in one god only) or to church structure, which are not universal. For example, in tribal societies, religion—unlike the Christian church—usually is not a separate institution but pervades the whole of public and private life. In Buddhism, gods are not as central as the idea of a Buddha (fully enlightened human being). In many traditional cultures the idea of a sacred cosmic order is the most prominent religious belief. Because of this variety, some scholars prefer to use a general term such as the sacred to designate the common foundation of religious life.

Religion in this understanding includes a complex of activities that cannot be reduced to any single aspect of human experience. It is a part of individual life but also of group dynamics. Religion includes patterns of behavior but also patterns of language and thought. It is sometimes a highly organized institution that sets itself apart from a culture, and it is sometimes an integral part of a culture. Religious experience may be expressed in visual symbols, dance and performance, elaborate philosophical systems, legendary and imaginative stories, formal ceremonies, meditative techniques, and detailed rules of ethical conduct and law. Each of these elements assumes innumerable cultural forms. In some ways there are as many forms of religious expression as there are human cultural environments.

The Geography of Religion

Religion is nearly universal and as ancient as human culture. Belief systems and traditions of worship have been common to nearly all societies since before the days of recorded history. Despite cultural diversity, these systems and traditions share many common elements. Religion typically involves faith in spiritual, or nonhuman, beings that can influence events, although some religions—notably some forms of Buddhism—do not stipulate belief in the supernatural.


Prayers, teachings, rituals, and rules of behavior are usually maintained by religious specialists. These religious leaders act as caretakers of tradition and sometimes as guides or disciplinarians for those within the faith. Religion is participatory—ordinary practicing members of a religious society help to perpetuate sacred traditions by attending religious services in community churches, mosques, synagogues, or temples. For an individual, a religion offers an explanation of reality and a framework for directing life decisions.


The impact of religions is also significant in a global context. Religious differences over customs, resources, and political policy are common, and throughout history have frequently led to conflict. Many borders and boundaries reflect religious differences as much as political divisions. Religions take many forms, ranging from large, institutionalized faiths that extend across national borders to localized belief systems practiced only within specific ethnic groups in small enclaves.

The Geography of Religion


The prevalence of religion throughout the world allows geographers to study how religions vary and how they are distributed from one region to another. Religions influence the landscape in many ways, directing settlement patterns, architectural styles, and daily activities such as farming and cooking practices.


Geographers are interested in the distribution of religions as a way of studying how religions start and spread, and as a means of providing statistical data about the religious beliefs of a country’s population. It is difficult to interpret the results of such studies because geographers from various countries collect data in different ways. Studies of religion may be biased by the politics of religion, particularly in countries where government and religion are closely tied. Despite these obstacles, some general patterns characterize the major world religions.


Islam, prevalent in North Africa and throughout much of Southwest Asia, also extends into South Asia and some areas of Southeast Asia. Europe is predominantly Christian—Protestant in the north, and Roman Catholic in the south. Both North and South America are mostly Christian, although Roman Catholicism is more common in South America. Australia, since its settlement by the British, has also had a large Christian population. Orthodox Christianity remains the main religion of Russia. The major religions of India include Hinduism, Islam, and Sikhism. Buddhism, which originated in India, is now found primarily in East Asia and in some areas of Central Asia and Southeast Asia. Judaism is centered in Israel, although the large cities of Europe and the United States also have significant Jewish populations.


It is important to note that these examples of the general and historical distribution of world religions have many exceptions, and that most countries are home to members of a diversity of faiths. There are many smaller-scale religions and distinct sects of larger religions within the distributions generalized above. The region of Africa south of the Sahara is home to many local religions, and shamanism is still practiced throughout Central Asia and in areas of South Asia. Some native peoples of North America, Central America, and South America still engage in religions they practiced before the arrival of Europeans.


Human activities such as evangelism, migration, and commerce can make the distribution of religions fluid. Some religions are spread by evangelists, who openly preach the virtues and beliefs of their religion, inviting others to join. Migration also serves as a means of changing the distribution patterns of religion—when people migrate, they take their religions with them. Countries with particularly rich religious diversity include those with large immigrant populations, such as Australia, Brazil, and the United States, and those encompassing ancient cultural crossroads, such as Egypt, India, Israel, and Turkey.


In the modern world, ancient religious traditions are frequently mixed with modern ones to form cross-cultural, or syncretic, religions. This phenomenon can be seen throughout South and Central America and Mexico, where the Roman Catholicism practiced by conquering European cultures has only partially subdued native religions. Many festivals are based on both Christian and pagan beliefs, and in some Andean villages, earth and water deities are still worshiped alongside images of the Virgin Mary. Likewise, people of African descent in the Americas have retained some of their traditional beliefs, which are sometimes mixed with Catholicism. An example is the Santería religion practiced in Cuba.


Most religions are complex, incorporating ideas, traditions, and technologies that may be specific to certain cultures or regions. It would be difficult to make a precise map outlining the range of religious practices, but general distributions and trends of change in religious traditions give geographers a valuable tool for studying the widespread movement of peoples and ideas throughout history.

Sacred Sites


Perhaps the most interesting aspect of the geography of religion is the study of sacred sites and spaces. Certain locations are viewed as sacred because they are associated with religiously significant events or because of their unique geographic characteristics. For example, Bodh Gaya in India acquired sacred significance because it was the site of the Buddha’s enlightenment. Mount Kailas, in western Tibet, is considered important because its slopes give rise to the four major rivers of South Asia. Collectively, the sacred sites, paths, and spatial relationships connected with a religion make up what can be called a “sacred geography,” in which these places are embodied with a specific meaning derived from the beliefs of that particular religion.


In many cultures mountains are viewed as links between humans and the spiritual world. In addition to Mount Kailas, Mount Catherine in Egypt, Mount Olympus in Greece, Mount Fuji in Japan, and Navajo Mountain in the western United States are examples of mountains that are considered sacred. Rivers and other natural features can likewise acquire religious significance. For example, Hinduism celebrates seven holy rivers, the holiest of which is the Ganges. In pre-Christian Europe, forests were regarded as the abode of gods, and the Celts worshiped at springs and pools throughout the British Isles and Europe. Similar beliefs about nature were held in Japan before the spread of Buddhism.


In addition to sacred natural places, humans have constructed religious shrines, churches, temples, and observatories—often on sites that were considered holy from prehistoric times. An example is Stonehenge in England, which is believed to have been built for the observation of stars, planets, eclipses, and the sun, possibly for religious purposes. Structures such as roadside shrines in Japan or rock cairns at mountain passes in the Himalayas are small-scale markers that identify sacred sites. All of these represent places that, to the religious believer, have a special spiritual quality that sets them apart from ordinary localities.


Beyond recognizing specific sacred sites, religion has played a central role in the development of secular aspects of the community. Religion has influenced architecture, commerce, and settlement patterns throughout history. In areas dominated by Islam, the mosque serves as an orientation point. In Hindu towns, the sacred space extends from the temple and includes neighboring territory. Likewise, in medieval Europe, monasteries and cathedrals were focal points for communities and trade.


Religious attitudes toward death also affect the landscape. Shrines, tombs, and groves are established according to religious tradition in honor of deceased ancestors. The Taj Mahal of India, the Egyptian pyramids, and the clay army of Xi’an in China are examples of tombs marked by striking structures that define their surroundings, drawing visitors of many beliefs. The beliefs of Judaism, Christianity, and other religions have led to the dedication of tracts of land as cemeteries. Some of these cemeteries have become integral parts of urban landscapes, serving as parks, historical sites, cultural treasures, and even neighborhoods. Examples include the City of the Dead in Cairo, the National Cemetery in Jerusalem, and Arlington National Cemetery in Washington, D.C.

Pilgrimage


To people of many faiths, the act of traveling to a sacred site for the purpose of religious observance is itself a religious experience. Each year, millions of faithful pilgrims go on religious journeys to sacred destinations all over the world. Pilgrimages have greatly influenced communities housing sacred sites as they strive to accommodate large numbers of visitors. Lodging, food, hygiene services, transportation, and security must be provided to support the religious crowds.


Lourdes, a town at the base of the Pyrenees in southern France, is one of the most popular pilgrimage sites in Europe. Catholics believe that the Virgin Mary appeared in visions here in 1858, and that the water from a nearby underground spring has healing powers. This site is an example of a location that became sacred because a divine being is believed to have been present there. The influence of this holy site extends well into the neighboring community—Lourdes has swelled with new hotels, restaurants, and facilities to support the many pilgrims visiting the shrine.


The largest religious pilgrimage takes place each year when more than a million devoted Muslims travel to Mecca (Makkah) in Saudi Arabia. Birthplace of Muhammad, the founder of Islam, Mecca is the spiritual hub of the Islamic world. All Muslims try to make this pilgrimage, called the hajj, at least once in their lives. The primary destination of the hajj is the main mosque, where a cube-shaped stone building, the Ka’abah, is dramatically situated in an immense, theater-like plaza. The ceremonies associated with the hajj take several days, during which most pilgrims stay in Mecca to take side trips to other sacred locations, placing heavy demands on regional food supplies, sanitation, and medical facilities. Another effect of the pilgrimage has been to shape transportation routes throughout the region, as Muslims arrive from all directions and by various means of transport.

Sacred Geographies


For many religious pilgrims, the arrival at a sacred site is the goal of the pilgrimage. But for others, the journey itself constitutes a personal spiritual transition. In such cases, the associated geography of such experiences may take on special spiritual meaning. Following the rituals of the Islamic hajj, for example, pilgrims make a series of journeys to a network of holy sites in the area. These sites form a spiritual landscape, and visiting them is a religious rite of passage for each pilgrim. Another case in which the pilgrimage route takes on sacred significance is the circuit of 88 Buddhist temples on the Japanese island of Shikoku. Legend tells that the circuit was established in the 9th century by Kobo Daishi, the founder of the Japanese Shingon sect. The pilgrims believe that Kobo Daishi accompanies them along the route, and his perceived presence sanctifies the circuit of the journey. The pilgrimage destination is not one individual site but rather a network—a sacred geometry covering the island.


This type of sacred visualization of space can also be found among the aboriginal people of Australia. They believe that their ancestors sang into being the physical characteristics of Earth, the myriad forms of life, and the codes governing conduct and relationships among humans. Through rituals, aboriginal people dissolve the temporal boundary between the present and the time of the ancestors, which they call the Dreamtime. These Australian indigenous groups define their landscape with symbolic markers that organize territory and give spiritual meaning to the land. The most famous of these markers is the massive red sandstone mound called Uluru, formerly Ayers Rock, in the heart of the Australian continent.


Spirit journeys in dreams enable aborigines who have been separated from their homelands to visit their sacred ancestral sites. The map of the aboriginal territories is drawn with descriptive musical “songlines” that must be sung, and physically marked by totemic rocks, trees, and caverns. Explanations of the landscape are given through allegory and by spiritual connections between the Dreamtime ancestors and the succeeding generations leading up to the present. This worldview of the Australian aborigines is another example of sacred geography—a way of ordering space that imbues the landscape with spiritual meaning.

Sacred geographies, these networks of spiritual spaces and places, are found in many forms around the world. From the small-scale layout of a religious structure to the broader webs of commerce surrounding pilgrimage sites, to the vast territories ascribed to the work of ancient ancestors—religion has a profound, ever-changing effect on the landscape and on our fundamental perception of the place of humanity in the world.

Internet !!

INTRODUCTION
Internet, computer-based global information system. The Internet is composed of many interconnected computer networks. Each network may link tens, hundreds, or even thousands of computers, enabling them to share information with one another and to share computational resources such as powerful supercomputers and databases of information. The Internet has made it possible for people all over the world to communicate with one another effectively and inexpensively. Unlike traditional broadcasting media, such as radio and television, the Internet does not have a centralized distribution system. Instead, an individual who has Internet access can communicate directly with anyone else on the Internet, make information available to others, find information provided by others, or sell products with a minimum overhead cost.

The Internet has brought new opportunities to government, business, and education. Governments use the Internet for internal communication, distribution of information, and automated tax processing. In addition to offering goods and services online to customers, businesses use the Internet to interact with other businesses. Many individuals use the Internet for communicating through electronic mail (e-mail), for news and research information, shopping, paying bills, and online banking. Educational institutions use the Internet for research and to deliver courses and course material to students.

Use of the Internet has grown tremendously since its inception. The Internet’s success arises from its flexibility. Instead of restricting component networks to a particular manufacturer or particular type, Internet technology allows interconnection of any kind of computer network. No network is too large or too small, too fast or too slow to be interconnected. Thus, the Internet includes inexpensive networks that can only connect a few computers within a single room as well as expensive networks that can span a continent and connect thousands of computers.

Internet service providers (ISPs) provide Internet access to customers, usually for a monthly fee. A customer who subscribes to an ISP’s service uses the ISP’s network to access the Internet. Because ISPs offer their services to the general public, the networks they operate are known as public access networks. In the United States, as in many countries, ISPs are private companies; in countries where telephone service is a government-regulated monopoly, the government often controls ISPs.

An organization that has many computers usually owns and operates a private network, called an intranet, which connects all the computers within the organization. To provide Internet service, the organization connects its intranet to the Internet. Unlike public access networks, intranets are restricted to provide security. Only authorized computers at the organization can connect to the intranet, and the organization restricts communication between the intranet and the global Internet. The restrictions allow computers inside the organization to exchange information but keep the information confidential and protected from outsiders.

USES OF THE INTERNET
Before the Internet was created, the U.S. military had developed and deployed communications networks, including a network known as ARPANET. Uses of the networks were restricted to military personnel and the researchers who developed the technology. Many people regard the ARPANET as the precursor of the Internet. From the 1970s until the late 1980s the Internet was a U.S. government-funded communication and research tool restricted almost exclusively to academic and military uses. It was administered by the National Science Foundation (NSF). At universities, only a handful of researchers working on Internet research had access. In the 1980s the NSF developed an “acceptable use policy” that relaxed restrictions and allowed faculty at universities to use the Internet for research and scholarly activities. However, the NSF policy prohibited all commercial uses of the Internet. Under this policy advertising did not appear on the Internet, and people could not charge for access to Internet content or sell products or services on the Internet.

By 1995, however, the NSF ceased its administration of the Internet. The Internet was privatized, and commercial use was permitted. This move coincided with the growth in popularity of the World Wide Web (WWW), which replaced file transfer as the application used for most Internet traffic. The difference between the Internet and the Web is similar to the distinction between a highway system and a package delivery service that uses the highways to move cargo from one city to another: The Internet is the highway system over which Web traffic and traffic from other applications move. The Web consists of programs running on many computers that allow a user to find and display multimedia documents (documents that contain a combination of text, photographs, graphics, audio, and video). Many analysts attribute the explosion in use and popularity of the Internet to the visual nature of Web documents. By the end of 2000, Web traffic dominated the Internet—more than 80 percent of all traffic on the Internet came from the Web.

Companies, individuals, and institutions use the Internet in many ways. Companies use the Internet for electronic commerce, also called e-commerce, including advertising, selling, buying, distributing products, and providing customer service. In addition, companies use the Internet for business-to-business transactions, such as exchanging financial information and accessing complex databases. Businesses and institutions use the Internet for voice and video conferencing and other forms of communication that enable people to telecommute (work away from the office using a computer). The use of electronic mail (e-mail) speeds communication between companies, among coworkers, and among other individuals. Media and entertainment companies use the Internet for online news and weather services and to broadcast audio and video, including live radio and television programs. Online chat allows people to carry on discussions using written text. Instant messaging enables people to exchange text messages in real time. Scientists and scholars use the Internet to communicate with colleagues, perform research, distribute lecture notes and course materials to students, and publish papers and articles. Individuals use the Internet for communication, entertainment, finding information, and buying and selling goods and services.

URL

URL (Uniform Resource Locator), method of naming documents or places on the Internet, used most frequently on the World Wide Web (WWW). A URL is a string of characters that identifies the type of document, the computer the document is on, the directories and subdirectories the document is in, and the name of the document.

HyperText Transfer Protocol (HTTP), http means the document is on the WWW. If, instead of http, that part of the URL was ftp, it would mean that that document could be accessed through File Transfer Protocol (FTP), a format that allows a user to list files on, retrieve files from, and add files to another computer on the Internet. Some other schemes are gopher, which indicates the document is on a Gopher system, a menu-driven document delivery system for retrieving information from the Internet; news, which means the document occurs on a Usenet newsgroup, a forum in which users can post and respond to messages; and telnet, which indicates Telnet, an access method in which the user logs on to a remote computer.

The next part of the URL, www2.whitehouse.gov, is called the hostname and represents the computer on which the document can be found: www2 is the name of a specific computer at the whitehouse.gov host computer. The .gov extension identifies the computer as belonging to the United States government. Some other common extensions are .com (commercial) and .edu (education —usually a college or university).

After the computer and host names come the path, or chain of directories, on which the document is found; in this case, the only directory is WH. The last item to be listed is the document name— in this case, Welcome.html.

URLs are case-sensitive, which means that uppercase and lowercase letters are considered different letters, so a user has to enter a URL with all letters in the correct case. URLs on the WWW are accessed with browsers, or computer programs that can connect to the Internet and display Web pages.

The Internet to carry data from one computer to another, each service follows a separate set of rules that define the messages used in the exchange. The Web uses the HyperText Transfer Protocol (HTTP), electronic mail uses the Simple Mail Transfer Protocol (SMTP), and file transfer uses the File Transfer Protocol (FTP). The application programs that users run to access the Internet often blur the distinction among these services. For example, an application program that can send e-mail also allows a user to transfer the contents of a file, and an application program used to access the Web also allows the user to process e-mail.

Cellular Radio Telephone

INTRODUCTION
Cellular Radio Telephone, also called cellular telephone or cell phone, low-powered, lightweight radio transceiver (combination transmitter-receiver) that provides voice telephone and other services to mobile users. Cellular telephones primarily operate like portable or cordless telephones. However, unlike conventional wire-based cordless phones, cellular telephones are completely portable and do not require proximity to a jack to access the wire-based networks operated by local telephone companies. A new generation of services for cell phones includes videoconferencing and Internet access with the ability to send e-mail. Cellular telephones have become very popular with professionals and consumers as a way to communicate while away from their regular, wire-based phones—for example, while traveling or when in remote locations lacking regular phone service. As cellular radio service proliferates and achieves greater market penetration, some users have begun to consider it an alternative to conventional wire-based services.

Cellular telephones work by transmitting radio signals to cellular towers. These towers vary in their capability to receive cellular telephone signals. Some towers can receive signals from distances of only 1.5 to 2.4 km (1.0 to 1.5 mi), while others can receive signals from distances as far as 48 to 56 km (30 to 35 mi). The area a tower can cover is referred to as a cell. However, more than one tower may exist in a given cell area. The cells overlap so that the system can handle increased telephone traffic volume. The towers within these cells are networked to a central switching station, usually by wire, fiber-optic cable, or microwave. The central switching station handling cellular calls in a given area is directly connected to the wire-based telephone system. Cellular calls are picked up by the towers and relayed to another cell telephone user or to a user of the conventional wire-based telephone network. Since the cells overlap, as a mobile caller moves from one cell into another, the towers “hand off” the call so communication is uninterrupted.

Cellular phone networks exist in nearly every metropolitan area throughout the world, and cellular coverage is expanding in rural areas. Due to the convenience and mobility of cellular telephones, users typically pay a higher fee than they would for normal telephone use. A newer generation of cellular radio technology, called Personal Communications Services (PCS), operates much like earlier cellular services, but at higher frequencies, the number of times a radio wave oscillates or completes a cycle, which is measured in a unit known as a hertz (Hz). (The higher frequencies of PCS operate at around 1900 megahertz [MHz] in the United States.) PCS also utilizes completely digital transmissions, rather than both the analog and digital transmissions that many current cellular telephones use. Digital transmissions convert sound into digital form, which can be transmitted more efficiently than analog signals. Digital technologies can also generate more channel capacity over the same amount of the radio spectrum.

MECHANICS OF CELLULAR RADIO TELEPHONES

Both cellular radio and PCS use high-frequency radio waves to transmit calls. High-frequency waves have short wavelengths that pass by a given point at a very high rate. High-frequency waves can provide better sound quality and more reliable short-distance transmission than lower-frequency waves (such as AM radio) as they are less susceptible to sound degradation caused by the noise generated by weather, such as lightning which causes static, and other noise generators such as motors. However, high-frequency signals cannot effectively travel as far as low-frequency signals can.

For cellular networks, the limited range of high-frequency waves is actually advantageous because it means the same frequencies can be reused at nearby locations. Cell phone calls connect with short-range antennas known as towers. If there were only one tower for a large area, more customers would be trying to use the same high-frequency waves, and these waves would tend to overlap and cause interference. But because cell phone networks establish many towers covering small areas, a smaller number of customers access a given tower, and frequencies can be reused when a cell phone call is handed off from one tower to another as a mobile cell phone user travels. This ability to reuse frequencies is helpful because there are a limited number of radio frequencies available to cell phone companies. It also allows cellular network providers to accommodate a larger number of users.

The transceiver inside a cellular phone is a much more complex device than a conventional phone used over the wire-based network. A cellular telephone has circuitry that creates a unique identity code that is used to locate and track the telephone. This identity code is necessary for coordinating calls to and from the telephone, and for billing such calls. Because a cellular telephone user may move quite a distance during the duration of a call, the cellular radio network must manage calls from different tower sites as the telephone moves out of the range of one tower and into the range of another tower.

Current cellular telephones offer such features as a memory database for storing frequently called numbers and a lock to deter theft. Most cell phones, whether old or new, also have a small liquid crystal screen to display the telephone number being called or the number from which an incoming call originated. Many newer cell phones can display a short text message, much like a pager displays this information. Some cellular phones can also access the Internet and display text from Web sites, such as stock quotes and news stories. Internet-capable cell phones can also send and receive e-mail. Because mobile telephones use radio waves to send and receive calls, the device must include a power source. Rechargeable batteries provide the usual source of power, but most cell phones can also be attached to the cigarette lighter in a vehicle or to some other external power device.

BRITNEY SPEARS

HAIRY TOUR PROBLEMS FOR BRITNEY SPEARS
Britney Spears' Circus tour is taking its toll on the pop superstar - high-energy dance routines are leaving her dizzy and nauseous, and costing her clumps of her hair. In a new YouTube.com video of one of her most recent shows in North America, a suspended dancer clearly yanks off a handful of hair from Spears' wig as he attempts to pull her into the air with him. Meanwhile, insiders claim Spears is struggling with her energetic dance routines. A source tells Britain's The Mirror, "The choreography is insane - it's so fast it is really taking its toll on her. There is one move during (the song) Touch Of My Hand, where two guys dangle from the ceiling and pick her up upside down, which is really getting to her. "She spins around for nearly five minutes some nights. The blood rushes to her head and when she gets down she complains of nausea and dizziness. There have been a couple of times when she ran off stage and vomited."