Before portable cellular phones, people like police officers or taxi drivers communicated from their cars using two-way radios. All of the radios in one city transmitted signals via a large, central antenna located on top of a tall building. With all callers sharing one antenna, the number of calls that could be made at any one time was very limited.
Consumers, like you and me, had an old type of cell phone that was permanently attached to the car and powered by its battery. The first transportable cellular phones had their own battery packs that allowed owners to detach them from the car and carry them in a pouch. However, most weighed about 5 pounds (2.25 kilograms) and were not very practical when used this way.
Known for its cold, desert-like conditions, Arctic tundra is located in the Northern Hemisphere, encircling the North Pole. The stark landscape is frozen for most of the year, and water is unavailable. Temperatures during the arctic winter can dip to –60 Fahrenheit (–51 Celsius).
In the summer the ground thaws for just a few weeks, and the region’s plants and animals—which include geese, sea birds, polar bears, caribou, and shrews—fight for survival. Because the tundra is not usually exposed to human activity it is most susceptible to change and damage from human use or pollution. Oil spills damage the plants, land, and animals that live along the coast.
Although a helicopter doesn’t have wings like an airplane, it uses the same principle of lift to rise and maneuver in the air. The blades of a helicopter’s propeller-like top rotor are shaped just like a plane’s wings—flat on the bottom and rounded on the top—and are likewise adjustable. Instead of rushing forward through the air like a plane does to gather enough lift to fly, a helicopter moves only its (three to six) rotor blades, which are attached to a central shaft driven by an engine.
The rotor blades slice through enough air—creating the changes in surrounding air pressure that produce lift—to achieve flight. Adjusting the angle at which the rotor blades are set helps control a helicopter’s lift and manner of flight. Because the angle of the rotor is adjustable, too, a helicopter has far greater maneuverability than an airplane: besides moving up, down, and forward, it can fly backward and hover in the air.
Television works through a series of complicated processes. It starts with a television camera, which takes pictures of scenes. Photo cells inside the camera change the pictures to electrical signals. At the same time, a microphone records sounds that are occurring during the scenes. A vibrating magnet in the microphone changes these sounds into electrical signals, too. Some television shows, like news reports, are recorded live, which means that they are broadcast to homes as they occur. But most of the television programs that we watch are recorded, which means that they are put on videotape and sent out later. The electrical signals of sound and pictures are stored as magnetic signals on videotape, which are converted back to electrical signals when played. Before a program is broadcast, its electrical picture and sound signals are run through a device called a television transmitter. With the help of strong magnets, the transformer turns the electrical signals into invisible bands of energy called radio waves (similar to visible light waves), which can travel great distances through the air. They can travel directly to outdoor television antennae, which catch the waves and send them to television sets that change them into pictures and sounds again.
Cable companies send electrical picture and sound signals through cables directly to homes. When broadcasting to distant places, communication satellites that orbit Earth are used to bounce or return the waves back to Earth, extending their travel distance. Satellites are necessary because radio waves move in straight lines and cannot bend around the world. When an antenna or satellite dish receives radio waves, it changes them back into electrical signals. A speaker in a television set changes some of the signals back into sound. The pictures are reproduced by special guns at the back of a television set that shoot electron beams at the screen, causing it to glow with tiny dots of different colors. Viewed together, the dots look like a regular picture. The individual pictures that make up a scene are broadcast and received, one after another, at a pace so quick that it looks like continuous action is occurring on the screen. The entire process happens very fast because television stations and broadcast towers are all around and because radio waves travel very quickly, at the speed of light. Radio programs broadcast talk and music across the airwaves using the same technology.
Air is a mixture of gases that circle Earth, kept in place by gravity. Air makes up Earth’s atmosphere. The air we breathe is 78 percent nitrogen gas, 21 percent oxygen, 0.9 percent argon, and 0.03 percent carbon dioxide, along with water vapor (floating molecules of water).
Also present are traces of other gases and tiny bits of dust, pollen grains from plants, and other solid particles. As our atmosphere extends higher and higher above Earth, toward outer space, air becomes thinner and the combination of gases in the air changes.
Scientists believe that the solar system in about 4.6 billion years old. Earth and the rest of the solar system formed from a giant cloud of gas and dust. Gravity and rotational forces caused the cloud to flatten into a disc and much of the cloud’s mass to drift into the center. This material became the Sun. The leftover parts of the cloud formed small bodies called planetesimals.
These planetesimals collided with each other, gradually forming larger and larger bodies, some of which became the planets. This process took approximately 25 million years, according to scientific estimates.
A veil is a piece of cloth that is usually worn to hide a person’s hair or face. Women have worn veils since ancient times—mostly in Middle Eastern countries— primarily to keep men from looking at them. Many women of the Muslim faith still wear veils of some type when they are out in public. In some Muslim countries, only a woman’s eyes are allowed to show.
Although Westerners (people from North America and Western Europe) may find these veils symbolic of women’s restricted freedoms in many Muslim societies, Muslim women wear them to honor long-held traditions of modesty and to show respect for their religion and the men in their lives.
Because animals are unpredictable and because schools have rules, it is never okay to bring animals out in public without the permission of both a parent and teacher. Even if you have permission, these creatures—such as snakes, rabbits, hamsters, gerbils, lizards, frogs, and other small critters—should be contained in their habitats and handled very carefully. Sometimes in public animals are outside of their natural habitats because they play a role in helping humans.
Guide dogs (also called seeing-eye dogs) help the blind or visually impaired move around cities and neighborhoods. These dogs are trained to behave properly on public transportation, in restaurants, in stores, and in any other place their masters go. Other times people carry their dogs and cats in pet carriers to the vet, to the park, or on shopping trips. However, they must have the store’s permission to enter, since many businesses (like restaurants and supermarkets) do not allow pets or require them to be contained and/or on leashes.
The greenhouse effect was first described in theoretical terms by a Swedish researcher, Svante Arrhenius, in the late 1800s. However it wasn’t until the following century that Arrhenius’s theory was observed. In the 1930s, scientists realized that parts of the globe had warmed during the previous half-century.
Then in the early 1960s scientists discovered that the level of carbon dioxide in the atmosphere was rising. Researchers began to take an interest and found a strong relationship between the increasing levels of carbon dioxide and average global temperature.
Earth Day is a national holiday that was first celebrated on April 22, 1970. It was created by Senator Gaylord Nelson, a Democrat from Wisconsin who was elected to the U.S. Senate in 1962. Senator Nelson decided to set aside one day aside for the entire nation to focus on environmental issues, learn about ways to improve the environment, and protest against the federal government’s unwillingness to help solve problems such as air pollution and the widespread destruction of forests.
After lots of hard work and publicity, on the first Earth Day 20 million Americans gathered at different places from the East to West coasts to hear speeches, participate in community-wide cleanup efforts, and demonstrate to the government that the environment is a major national issue. Ever since then, April 22 has been the date for celebrating Earth Day—a time when the United States (and now many countries all over the world) could participate in educational activities that celebrate Earth and think of new ways to preserve our natural resources. On Earth Day 2008, over 100 million people joined in the effort to celebrate and protect our planet.