Most people think that Mercury is the hottest planet because it is nearest to the Sun. However, Venus, the second nearest planet, is the hottest because it has an atmosphere. Its atmosphere is primarily composed of carbon dioxide, which acts like a greenhouse.
The solar heat enters Venus’s atmosphere, but it cannot leave, heating the planet’s surface to about 900 degrees Fahrenheit (482 degrees Celsius). This temperature is hot enough to melt several metals, including lead, tin, and zinc.
Oxygen is necessary for all humans, animals, and plant life to survive. When Earth was first formed, its atmosphere had no oxygen—the colorless, odorless and tasteless gas that makes up about 20 percent of the air we breathe. It had only a deadly combination of hydrogen, methane, ammonia, and hydrogen cyanide.
The hydrogen escaped into space and ultraviolet radiation from the Sun broke down the mixture, leaving only nitrogen and carbon dioxide. Only when life began and photosynthesis (the conversion of light energy into chemical energy by living organisms) occurred did oxygen first appear—about 3.4 billion years ago.
Located at 0 degrees longitude, the prime meridian passes through Greenwich, England. Halfway around the world in the middle of the Pacific Ocean (180 degrees from Greenwich) is the International Date Line (IDL), where the date changes across the boundary of the time zone.
The entire world is on the same date only at the instant when it is noon in Greenwich, England, and midnight at the IDL. At all other times, there are different dates on each side of the IDL.
Earth is more active, in terms of both geology and weather, which makes it hard for craters to remain. Even those craters scientists can see on the surface—which may be millions of years old—have been overgrown by vegetation, weathered by wind and rain, and changed by earthquakes and landslides. The Moon, meanwhile, is geologically quiet and has almost no weather, so its hundreds of thousands of craters are easy to see.
The craters are the result of both meteorites and volcanic activity. Interestingly, some of the oldest Earth rocks might be awaiting discovery on the Moon, having been blasted there billions of years ago by asteroid impacts that shook both worlds.
Many volcanoes occur near the area where two ridges or plates of Earth’s crust meet. Circling the Pacific Ocean—where crust plates meet—is a group of volcanoes known as the Ring of Fire. Plate movement in such regions may allow liquid rock, called magma (it’s called “lava” only after it rises to the surface), that is located in chambers in Earth’s interior to rise, resulting in volcanic activity. (Such conditions often result in earthquakes as well.)
Volcanic activity can take place under the ocean as well as on land, and when this happens the formation of islands sometimes results.
At least two ancient Greek athletes would have done well in the modern games; their Olympic records stood until the twentieth century. Twenty-six hundred years ago, an athlete named Protiselaus threw a cumbersome primitive discus 152 feet from a standing position. No one exceeded that distance until Clarence Houser, an American, threw the discus 155 feet in 1928. In 656 BC, a Greek Olympian named Chionis leapt 23 feet, 1.5 inches, a long jump record that stood until 1900, When an American named Alvis Kraenzlein surpassed it by 4.5 inches.
Dust is made up of particles of all sorts of things. In places where people live, a great deal of dust comes from flakes of dead skin, which are being shed all the time. Dust mites, tiny microscopic creatures that feed on this dead skin, make up dust, too (including their waste and tiny skeletons). Particles of the environment contribute to dust as well: grit from the sidewalk, salt from the sea, dry earth, pollen from plants, pet dander, molds, and smoke from burning materials. And Earth gets 10 tons of dust from outer space every day, from the meteors that burn up as they enter our atmosphere. Sometimes these ingredients cause allergic reactions, such as sneezing and coughing.
In 1960, astronomers discovered some mysterious space objects and called them quasars because they were discovered to be a strong source of radio waves. In fact, the term “quasar” comes from the words “quasi-stellar radio source.” Quasars are sources of light or radio waves, just like galaxies, that emit enormous amounts of energy.
They are the most distant objects scientists have discovered. They are very bright (as bright as hundreds of galaxies, burning with the energy of 1 trillion Suns) and much smaller than most galaxies. Today, many astronomers refer to these objects as quasi-stellar objects, or QSOs.
The Soviet cosmonaut Yuri Gagarin became the first man in space when he made a full orbit of Earth in Vostok I on April 12, 1961. Although he was in space less than two hours, he became an international hero.
The United States launched the first American into orbit on February 20, 1962: Astronaut John Glenn completed three orbits of Earth in Friendship 7, traveling about 81,000 miles (130,329 kilometers).
The largest muscle is the buttock muscle (gluteus maximus), which moves the thighbone away from the body and straightens out the hip joint. It is also one of the stronger muscles in the body. The smallest muscle is the stapedius, in the middle ear.
It is thinner than a thread and 0.05 inches (0.127 centimeters) in length. It activates the stirrup that sends vibrations from the eardrum to the inner ear. The longest muscle is the sartorius, which runs from the waist to the knee. Its purpose is to flex the hip and knee.