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Earth (planet), one of nine planets in the solar system , the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans. Earth’s surface is surrounded by a layer of gases known as the atmosphere , which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form. Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury , the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets— Jupiter , Saturn , Uranus , Neptune , and tiny Pluto . Earth’s

Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation . The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit. Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it woul

The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy ). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 1030 kg, or 4.4 × 1030 lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth. For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back f

Nine major planets are currently known. They are commonly divided into two groups: the inner planets ( Mercury , Venus , Earth , and Mars ) and the outer planets ( Jupiter , Saturn , Uranus , and Neptune ). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto does not belong to either group, and there is an ongoing debate as to whether Pluto should be categorized as a major planet. Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477°C (about 890°F). Earth is the only planet k

The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1,003 km (623 mi), to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s Moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters. S

If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury and Pluto have obviously inclined orbits. Pluto’s orbit is so elliptical that it is sometimes closer than Neptune to the Sun. The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, and Neptune each have at least one satellite that moves around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. (Some satellites of Saturn have done the same with smaller bodies.)

Sun, closest star to Earth. The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the other planets in the solar system in orbit. The Sun’s light and heat influence all of the objects in the solar system and allow life to exist on Earth. The Sun is an average star—its size, age, and temperature fall in about the middle of the ranges of these properties for all stars. Astronomers believe that the Sun is about 4.6 billion years old and will keep shining for about another 7 billion years. For humans, the Sun is beautiful and useful, but also powerful and dangerous. As Earth turns, the Sun rises over the eastern horizon in the morning, passes across the sky during the day, and sets in the west in the evening. This movement of the Sun across the sky marks the passage of time during the day (see Sundial ). The Sun’s movement can produce spectacular sunrises and sunsets under the right atmospheric conditions. At night, reflected sunlight makes t

The distance between the sun and the earth varies because the earth travels in an elliptical rather than circular orbit. The distance is roughly 100 times the sun’s diameter. Turbulence in the photosphere forms granules of various sizes and sunspots. Temperature is a measure of kinetic energy . The dense plasma in the center of the sun is roughly 2500 times hotter than the surface. Gases in the corona have escaped from the sun’s surface and have a very high velocity. The sun’s spectral type, G2, indicates that it is composed of hydrogen , helium , calcium , iron and other metals.

Regions of the Sun include the core, radiation zone, convection zone, and photosphere. Gases in the core are about 150 times as dense as water and reach temperatures as high as 16 million degrees C (29 million degrees F). The Sun’s energy is produced in the core through nuclear fusion of hydrogen atoms into helium. In the radiation zone, heat flows outward through gases that are about as dense as water. The radiation zone is cooler than the core, about 2.5 million degrees C (4.5 million degrees F). In the convection zone, churning motions of the gases carry the Sun’s energy further outward. The convection zone is slightly cooler, about 2 million degrees C (3.6 million degrees F), and less dense, about one-tenth as dense as water. The photosphere is much cooler, about 5500° C (10,000° F) and much less dense, about one-millionth that of water. The turbulence of this region is visible from earth in the form of sunspots, solar flares, and small patches of gas called granules.

The material in the Sun farther out from the center than the photosphere makes up the Sun’s atmosphere. The atmosphere extends far beyond the disk we see in the sky. Very diffuse solar gases extend all the way to Earth and beyond. The solar atmosphere consists of, from the innermost part outward, the photosphere, the chromosphere, the corona, and the expanding outer layers of the corona that astronomers call the solar wind. The photosphere is the visible part of the Sun. We look right through the chromosphere, the corona, and the solar wind, just as we see through Earth’s atmosphere at night. The chromosphere and corona are visible during total solar eclipses, when the Moon lines up between the Sun and Earth, completely blocking the main disk of the Sun from view. The thin chromosphere becomes visible a few seconds before or after a solar eclipse, creating a narrow pink, rose, or ruby-colored band at the edge of the Sun. For up to eight minutes during an eclipse, the corona is visib

Mercury (planet), one of the planets in the solar system . Mercury orbits closest to the Sun of all the planets, at an average distance of approximately 58 million km (about 36 million mi). The planet’s diameter is 4,879 km (3,032 mi), and its volume and mass are about one-eighteenth that of Earth . Mercury’s mean density is approximately equal to that of Earth and is higher than that of any of the other planets. The force of gravity on the planet's surface is about one-third of that on Earth's surface or about twice the surface gravity on the Moon . Mercury revolves once about the Sun every 88 days. Radar observations of the planet show that it rotates only once every 58.7 days, two-thirds of its period of revolution. Only three of the planet’s days, therefore, occur during every two of its years. The side facing the Sun gets very hot, while the side facing away quickly cools to frigid temperatures. The point in Mercury's orbit at which the planet is closest to the Sun

Venus (planet), one of the planets in the solar system , the second in distance from the Sun . Except for the Sun and the Moon , Venus is the brightest object in the sky. The planet is called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because of the distances of the orbits of Venus and Earth from the Sun, Venus is never visible more than three hours before sunrise or three hours after sunset. When viewed through a telescope, the planet exhibits phases like the Moon. Maximum brilliance (a stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when Venus is closer to Earth. Venus’s full phase appears smaller and dimmer because it occurs when the planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeat every 1.6 years (see Time; Year). T

Venus's complete cloud cover and deep atmosphere make it difficult to study from Earth. Most knowledge of the planet has been obtained through the use of space vehicles, particularly those carrying probes that descend through the atmosphere. The first flyby was that of Mariner 2, launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner 10 in 1974. The former Union of Soviet Socialist Republics developed several entry probes, some combined with flybys or orbiters: Venera 4 and 5 (1967), 6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15 and 16 (1983); Vega 1 and 2, sent toward Halley's comet in 1984, also flew by Venus and released descent capsules. Several of these probes successfully reached the planet's surface. The United States sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere. Pioneer Venus 1, an orbiter, mea

The atmosphere of the planet consists of 97 percent carbon dioxide and is so thick that the surface pressure is 96 bars (compared with 1 bar on Earth). The surface temperature on Venus varies little from place to place and is extremely hot, about 462°C (736 K/864°F). The high surface temperature is explained by an intense greenhouse effect . Even though only a small percentage of the solar energy that falls on Venus reaches the surface, the planet stays hot because the thick atmosphere prevents the energy from escaping. That nearly all of Venus's atmosphere is carbon dioxide is not as strange as it might seem; in fact, the crust of Earth contains almost as much carbon dioxide chemically bound in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas. By contrast, 78 percent of Earth's atmosphere is nitrogen . Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the Sun, was subjected to a so-called ru

Venus rotates very slowly on its axis, and the direction is retrograde (opposite to that of Earth). Curiously, the periods of Venus's orbit and rotation cause the same side of the planet to always face Earth when the two planets are closest. At such times, the side facing Earth can be viewed and mapped by Earth-based radar. In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and Earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from Earth at closest approach.

Moon, name given to the only natural satellite of Earth . The Moon is the second brightest object in Earth’s sky, after the Sun , and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons. Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 7 percent of the light that falls on it. This reflectivity, called albedo , of 0.07 is similar to that of coal dust. The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Mo

The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. The Moon is said to be waning as it pr

Ancient observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge. The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Scientists have also recently discovered evidence of ice in permanently shadowed areas of the surface. See: Craters ;  Volcanic Features ;  Ice

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface. The bigge

Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. In 1975, having studied moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation: a giant, p

The Moon has no global magnetic field as does Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in the presence of a magnetic field. The Moon has small, local magnetic fields that seem to be strongest in areas that are on opposite hemispheres from large basins. The origin of these local magnetic fields is unknown. Some scientists speculate that the magnetic fields were induced by the extreme shock pressures associated with the large asteroid collisions that created the basins. Others believe that the Moon originally had a global magnetic field generated by the movement of liquid metal in the core as on Earth. This global field shut down for some reason and only remnants of it exist in certain places on the lunar surface, preserved in material ejected by the asteroid collisions. The “fossil” magnetism found in some lunar rocks supports the former global field model, whereas the regional distribution of the magnetic surface anomalies tends to support the local

Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon's visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt , a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide. Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon's crust is thicker on the far side than on the near side, and therefore the lavas that form the maria wer

Temperatures on most of the Moon’s surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). In 1996 a team working with data gathered by the Clementine spacecraft announced that frozen water may exist in one of these shadowed areas near the Moon’s south pole. Clementine was a joint venture by the Department of Defense and the National Aeronautics and Space Administration (NASA). The spacecraft’s radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months. NASA launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data confirming the Clementine discovery and suggesting that a significant amount of water exists in the dar

Eclipse ->> SOLAR ECLIPSES The length of the moon’s umbra varies from 367,000 to 379,800 km (228,000 to 236,000 mi), and the distance between the earth and the moon varies from 357,300 to 407,100 km (222,000 to 253,000 mi). Total solar eclipses occur when the moon’s umbra reaches the earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the earth, so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower. The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4828 km (about 3000 mi). At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon. The shadow of the moon moves across the surface of the earth in an easterly direction. Because the ea

Eclipse ->> FREQUENCY OF ECLIPSES If the earth’s orbit, or the ecliptic , were in the same plane as the moon’s orbit, two total eclipses would occur during each lunar month, a lunar eclipse at the time of each full moon, and a solar eclipse at the time of each new moon. The two orbits, however, are inclined, and, as a result, eclipses occur only when the moon or the sun is within a few degrees of the two points, called the nodes, where the orbits intersect. Periodically both the sun and the moon return to the same position relative to one of the nodes, with the result that eclipses recur at regular intervals. The time of the interval, called the saros, is a little more than 6585.3 days or about 18 years, 9 to 11 days, depending on the number of intervening leap years, and 8 hours. The saros, known since the time of ancient Babylonia, corresponds almost exactly to 19 returns of the sun to the same node, 242 returns of the moon to the same node, and 223 lunar months. The disparity

Eclipse ->> OBSERVATION OF ECLIPSES Many problems of astronomy can be studied only during a total eclipse of the sun. Among these problems are the size and composition of the solar corona and the bending of light rays passing close to the sun because of the sun’s gravitational field (see Relativity ). The great brilliance of the solar disk and the sun-induced brightening of the earth’s atmosphere make observations of the corona and nearby stars impossible except during a solar eclipse. The coronagraph, a photographic telescope, permits direct observation of the edge of the solar disk at all times. Today, scientific solar eclipse observations are extremely valuable, particularly when the path of the eclipse traverses large land areas. An elaborate network of special observatories may provide enough data for months of analysis by scientists. Such data may provide information on how minute variations in the sun affect weather on earth, and how scientists can improve predictions of s