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The Planet Song (Last Of All There's Pluto Too)

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Table of Contents

When was Uranus discovered?

How far is Uranus from the Sun?

What is the average temperature in Uranus's atmosphere?

How many moons and rings does Uranus have?

What is unusual about the axis of Uranus?

Uranus, seventh planet in distance from the Sun and the least massive of the solar system's four giant, or Jovian, planets, which also include Jupiter, Saturn, and Neptune. At its brightest, Uranus is just visible to the unaided eye as a blue-green point of light. It is designated by the symbol ♅.

Uranus is named for the personification of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 billion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant planets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour.
Planetary data for Uranus

*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.

**Calculated for the altitude at which 1 bar of atmospheric pressure is exerted.

mean distance from Sun

2,870,658,000 km (19.2 AU)

eccentricity of orbit

0.0472

inclination of orbit to ecliptic

0.77°

Uranian year (sidereal period of revolution)

84.02 Earth years

visual magnitude at mean opposition

5.5

mean synodic period*

369.66 Earth days

mean orbital velocity

6.80 km/sec

equatorial radius**

25,559 km

polar radius**

24,973 km

mass

8.681 × 10 ²⁵ kg

mean density

1.27 g/cm ³

gravity**

887 cm/sec ²

escape velocity**

21.3 km/sec

rotation period (magnetic field)

17 hr 14 min (retrograde)

inclination of equator to orbit

97.8°

magnetic field strength at equator

0.23 gauss

tilt angle of magnetic axis

58.6°

offset of magnetic axis

0.31 of Uranus's radius

number of known moons

27

planetary ring system

13 known rings

Most of the planets rotate on an axis that is more or less perpendicular to the plane of their respective orbits around the Sun. But Uranus's axis lies almost parallel to its orbital plane, which means that the planet spins nearly on its side, its poles taking turns pointing toward the Sun as the planet travels in its orbit. In addition, the axis of the planet's magnetic field is substantially tipped relative to the rotation axis and offset from the planet's centre. Uranus has more than two dozen moons (natural satellites), five of which are relatively large, and a system of narrow rings.

Uranus has been visited by a spacecraft only once-by the U.S. Voyager 2 probe in 1986. Before then, astronomers had known little about the planet, since its distance from Earth makes the study of its visible surface difficult even with the most powerful telescopes available. Earth-based attempts to measure a property as basic as the planetary rotation period had produced widely differing values, ranging from 24 to 13 hours, until Voyager 2 finally established a 17.24-hour rotation period for the Uranian interior. Since Voyager's encounter, advances in Earth-based observational technology have added to knowledge of the Uranian system.

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Uranus, seventh planet in distance from the Sun and the least massive of the solar system's four giant, or Jovian, planets. Uranus has more than two dozen moons, five of which (Umbriel, Miranda, Ariel, Titania, and Oberon) are relatively large, and a system of narrow rings.

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Saturn, second largest planet of the solar system in mass and size and the sixth nearest planet in distance to the Sun. In the night sky Saturn is easily visible to the unaided eye as a non-twinkling point of light. When viewed through even a small telescope, the planet encircled by its magnificent rings is arguably the most sublime object in the solar system. Saturn is designated by the symbol ♄.

Saturn's name comes from the Roman god of agriculture, who is equated with the Greek deity Cronus, one of the Titans and the father of Zeus (the Roman god Jupiter). As the farthest of the planets known to ancient observers, Saturn also was noted to be the slowest-moving. At a distance from the Sun that is 9.5 times as far as Earth's, Saturn takes approximately 29.5 Earth years to make one solar revolution. The Italian astronomer Galileo in 1610 was the first to observe Saturn with a telescope. Although he saw a strangeness in Saturn's appearance, the low resolution of his instrument did not allow him to discern the true nature of the planet's rings.

Saturn occupies almost 60 percent of Jupiter's volume but has only about one-third of its mass and the lowest mean density-about 70 percent that of water-of any known object in the solar system. Hypothetically, Saturn would float in an ocean large enough to hold it. Both Saturn and Jupiter resemble stars in that their bulk chemical composition is dominated by hydrogen. Also, as is the case for Jupiter, the tremendous pressure in Saturn's deep interior maintains the hydrogen there in a fluid metallic state. Saturn's structure and evolutionary history, however, differ significantly from those of its larger counterpart. Like the other giant, or Jovian, planets-Jupiter, Uranus, and Neptune-Saturn has extensive systems of moons (natural satellites) and rings, which may provide clues to its origin and evolution as well as to those of the solar system. Saturn's moon Titan is distinguished from all other moons in the solar system by the presence of a significant atmosphere, one that is denser than that of any of the terrestrial planets except Venus.

The greatest advances in knowledge of Saturn, as well as of most of the other planets, have come from deep-space probes. Four spacecraft have visited the Saturnian system: Pioneer 11 in 1979, Voyagers 1 and 2 in the two years following, and, after almost a quarter-century, Cassini-Huygens, which arrived in 2004. The first three missions were short-term flybys, but Cassini went into orbit around Saturn for years of investigations, while its Huygens probe parachuted through the atmosphere of Titan and reached its surface, becoming the first spacecraft to land on a moon other than Earth's.

Basic astronomical data

Saturn orbits the Sun at a mean distance of 1,427,000,000 km (887 million miles). Its closest distance to Earth is about 1.2 billion km (746 million miles), and its phase angle-the angle that it makes with the Sun and Earth-never exceeds about 6°. Saturn seen from the vicinity of Earth thus always appears nearly fully illuminated. Only deep space probes can provide sidelit and backlit views.

Like Jupiter and most of the other planets, Saturn has a regular orbit-that is, its motion around the Sun is prograde (in the same direction that the Sun rotates) and has a small eccentricity (noncircularity) and inclination to the ecliptic, the plane of Earth's orbit. Unlike Jupiter, however, Saturn's rotational axis is tilted substantially-by 26.7°-to its orbital plane. The tilt gives Saturn seasons, as on Earth, but each season lasts more than seven years. Another result is that Saturn's rings, which lie in the plane of its equator, are presented to observers on Earth at opening angles ranging from 0° (edge on) to nearly 30°. The view of Saturn's rings cycles over a 30-year period. Earth-based observers can see the rings' sunlit northern side for about 15 years and then, in an analogous view, the sunlit southern side for the next 15 years. In the short intervals when Earth crosses the ring plane, the rings are all but invisible.

Saturn's rotation period was very difficult to determine. Cloud motions in its massive upper atmosphere trace out a variety of periods, which are as short as about 10 hours 10 minutes near the equator and increase with some oscillation to about 30 minutes longer at latitudes higher than 40°. Scientists attempted to determine the rotation period of Saturn's deep interior from that of its magnetic field, which is presumed to be rooted in the planet's metallic-hydrogen outer core. However, direct measurement of the field's rotation was difficult because the field is highly symmetrical around the rotational axis. At the time of the Voyager encounters, radio outbursts from Saturn, apparently related to small irregularities in the magnetic field, showed a period of 10 hours 39.4 minutes; this value was taken to be the magnetic field rotation period. Measurements made 25 years later by the Cassini spacecraft indicated that the field was rotating with a period 6-7 minutes longer. It was believed that the solar wind is responsible for some of the difference between these two measurements of the rotational period. Not until Cassini flew inside Saturn's rings on its final orbits was the rotation period accurately measured. By relating waves observed in the rings to slight variations in Saturn's gravitational field, the rotation period of the planet was determined to be 10 hours 33 minutes 38 seconds. The time differences between the rotation periods of Saturn's clouds and of its interior have been used to estimate wind velocities ( see below The atmosphere).

Because the four giant planets have no solid surface in their outer layers, by convention the values for the radius and gravity of these planets are calculated at the level at which one bar of atmospheric pressure is exerted. By this measure, Saturn's equatorial diameter is 120,536 km (74,898 miles). In comparison, its polar diameter is only 108,728 km (67,560 miles), or 10 percent smaller, which makes Saturn the most oblate (flattened at the poles) of all the planets in the solar system. Its oblate shape is apparent even in a small telescope. Even though Saturn rotates slightly slower than Jupiter, it is more oblate because its rotational acceleration cancels a larger fraction of the planet's gravity at the equator. The equatorial gravity of the planet, 896 cm (29.4 feet) per second per second, is only 74 percent of its polar gravity. Saturn is 95 times as massive as Earth but occupies a volume 766 times greater. Its mean density of 0.69 gram per cubic cm is thus only some 12 percent of Earth's. Saturn's equatorial escape velocity-the velocity needed for an object, which includes individual atoms and molecules, to escape the planet's gravitational attraction at the equator without having to be further accelerated-is nearly 36 km per second (80,000 miles per hour) at the one-bar level, compared with 11.2 km per second (25,000 miles per hour) for Earth. This high value indicates that there has been no significant loss of atmosphere from Saturn since its formation. For additional orbital and physical data, see the table.

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Saturn | Facts, Size, Temperature, Atmosphere, Color, Rings, & Moons

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Saturn, ringed planet that is the second largest planet in the solar system in mass and size and the sixth nearest planet in distance to the Sun. When viewed through even a small telescope, the planet encircled by its magnificent rings is arguably the most sublime object in the solar system.

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What is the period of revolution of Jupiter?

When was Jupiter's ring discovered?

Is Jupiter the largest planet in the solar system?

What is Jupiter made up of?

Jupiter, the most massive planet of the solar system and the fifth in distance from the Sun. It is one of the brightest objects in the night sky; only the Moon, Venus, and sometimes Mars are more brilliant. Jupiter is designated by the symbol ♃.

When ancient astronomers named the planet Jupiter for the Roman ruler of the gods and heavens (also known as Jove), they had no idea of the planet's true dimensions, but the name is appropriate, for Jupiter is larger than all the other planets combined. It takes nearly 12 Earth years to orbit the Sun, and it rotates once about every 10 hours, more than twice as fast as Earth; its colourful cloud bands can be seen with even a small telescope. It has a narrow system of rings and 92 known moons, one larger than the planet Mercury and three larger than Earth's Moon. Some astronomers speculate that Jupiter's moon Europa may be hiding an ocean of warm water-and possibly even some kind of life -beneath an icy crust.

Jupiter has an internal heat source; it emits more energy than it receives from the Sun. The pressure in its deep interior is so high that the hydrogen there exists in a fluid metallic state. This giant has the strongest magnetic field of any planet, with a magnetosphere so large that, if it could be seen from Earth, its apparent diameter would exceed that of the Moon. Jupiter's system is also the source of intense bursts of radio noise, at some frequencies occasionally radiating more energy than the Sun. Despite all its superlatives, however, Jupiter is made almost entirely of only two elements, hydrogen and helium, and its mean density is not much more than the density of water.

Knowledge about the Jovian system grew dramatically after the mid-1970s as a result of explorations by three spacecraft missions- Pioneers 10 and 11 in 1973-74, Voyager 1 and 2 in 1979, and the Galileo orbiter and probe, which arrived at Jupiter in December 1995. The Pioneer spacecraft served as scouts for the Voyagers, showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment. The greater number and increased sophistication of the Voyager instruments provided so much new information that it was still being analyzed when the Galileo mission began. The previous missions had all been flybys, but Galileo released a probe into Jupiter's atmosphere and then went into orbit about the planet for intensive investigations of the entire system until September 2003. In July 2016, the Juno orbiter arrived at Jupiter for a mission expected to last two years. Other looks at the Jovian system were provided in late 2000 and early 2001 by the flyby of the Cassini spacecraft on its way to Saturn and in 2007 by the flyby of the New Horizons spacecraft on its way to Pluto. Observations of the impacts of the fragmented nucleus of Comet Shoemaker-Levy 9 with Jupiter's atmosphere in 1994 also yielded information about its composition and structure.

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Jupiter | Facts, Moons, Rings, Temperature, Size, & Color

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Jupiter, the most massive planet in the solar system and the fifth in distance from the Sun. It is one of the brightest objects in the night sky; only the Moon, Venus, and sometimes Mars are more brilliant. Jupiter takes nearly 12 Earth years to orbit the Sun, and it rotates once about every 10 hours.

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Mars, fourth planet in the solar system in order of distance from the Sun and seventh in size and mass. It is a periodically conspicuous reddish object in the night sky. Mars is designated by the symbol ♂.

Sometimes called the Red Planet, Mars has long been associated with warfare and slaughter. It is named for the Roman god of war. As long as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The planet's two moons, Phobos (Greek: "Fear") and Deimos ("Terror"), were named for two of the sons of Ares and Aphrodite (the counterparts of Mars and Venus, respectively, in Greek mythology).

In recent times Mars has intrigued people for more-substantial reasons than its baleful appearance. The planet is the second closest to Earth, after Venus, and it is usually easy to observe in the night sky because its orbit lies outside Earth's. It is also the only planet whose solid surface and atmospheric phenomena can be seen in telescopes from Earth. Centuries of assiduous studies by earthbound observers, extended by spacecraft observations since the 1960s, have revealed that Mars is similar to Earth in many ways. Like Earth, Mars has clouds, winds, a roughly 24-hour day, seasonal weather patterns, polar ice caps, volcanoes, canyons, and other familiar features. There are intriguing clues that billions of years ago Mars was even more Earth-like than today, with a denser, warmer atmosphere and much more water- rivers, lakes, flood channels, and perhaps oceans. By all indications Mars is now a sterile frozen desert. However, close-up images of dark streaks on the slopes of some craters during Martian spring and summer suggest that at least small amounts of water may flow seasonally on the planet's surface, and radar reflections from a possible lake under the south polar cap suggest that water may still exist as a liquid in protected areas below the surface. The presence of water on Mars is considered a critical issue because life as it is presently understood cannot exist without water. If microscopic life-forms ever did originate on Mars, there remains a chance, albeit a remote one, that they may yet survive in these hidden watery niches. In 1996 a team of scientists reported what they concluded to be evidence for ancient microbial life in a piece of meteorite that had come from Mars, but most scientists have disputed their interpretation.

Since at least the end of the 19th century, Mars has been considered the most hospitable place in the solar system beyond Earth both for indigenous life and for human exploration and habitation. At that time, speculation was rife that the so-called canals of Mars-complex systems of long, straight surface lines that very few astronomers had claimed to see in telescopic observations-were the creations of intelligent beings. Seasonal changes in the planet's appearance, attributed to the spread and retreat of vegetation, added further to the purported evidence for biological activity. Although the canals later proved to be illusory and the seasonal changes geologic rather than biological, scientific and public interest in the possibility of Martian life and in exploration of the planet has not faded.

During the past century Mars has taken on a special place in popular culture. It has served as inspiration for generations of fiction writers from H.G. Wells and Edgar Rice Burroughs in the heyday of the Martian canals to Ray Bradbury in the 1950s and Kim Stanley Robinson in the '90s. Mars has also been a central theme in radio, television, and film, perhaps the most notorious case being Orson Welles's radio-play production of H.G. Wells's novel War of the Worlds, which convinced thousands of unwitting listeners on the evening of October 30, 1938, that beings from Mars were invading Earth. The planet's mystique and many real mysteries remain a stimulus to both scientific inquiry and human imagination to this day.

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Mars | Facts, Surface, Moons, Temperature, & Atmosphere

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Mars, fourth planet in the solar system in order of distance from the Sun and seventh in size and mass. It is a periodically conspicuous reddish object in the night sky. There are intriguing clues that billions of years ago Mars was even more Earth-like than today.

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Earth, third planet from the Sun and the fifth largest planet in the solar system in terms of size and mass. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life. It is designated by the symbol ♁. Earth's name in English, the international language of astronomy, derives from Old English and Germanic words for ground and earth, and it is the only name for a planet of the solar system that does not come from Greco-Roman mythology.

Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe ( see heliocentric system), enlightened thinkers have regarded Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of Earth's horizon, that the conception of Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation. Humans first witnessed Earth as a complete orb floating in the inky blackness of space in December 1968 when Apollo 8 carried astronauts around the Moon. Robotic space probes on their way to destinations beyond Earth, such as the Galileo and the Near Earth Asteroid Rendezvous (NEAR) spacecraft in the 1990s, also looked back with their cameras to provide other unique portraits of the planet.

Viewed from another planet in the solar system, Earth would appear bright and bluish in colour. Easiest to see through a large telescope would be its atmospheric features, chiefly the swirling white cloud patterns of midlatitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white, because of the clouds above and the snow and ice below. Beneath the changing patterns of clouds would appear the much darker blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space. Not only do they constitute a modest fraction of the land area, which itself is less than one-third of Earth's surface, but they are often obscured by clouds. Over the course of the seasons, some changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere.

Scientists have applied the full battery of modern instrumentation to studying Earth in ways that have not yet been possible for the other planets; thus, much more is known about its structure and composition. This detailed knowledge, in turn, provides deeper insight into the mechanisms by which planets in general cool down, by which their magnetic fields are generated, and by which the separation of lighter elements from heavier ones as planets develop their internal structure releases additional energy for geologic processes and alters crustal compositions.

Earth's surface is traditionally subdivided into seven continental masses: Africa, Antarctica, Asia, Australia, Europe, North America, and South America. These continents are surrounded by five major bodies of water: the Arctic, Atlantic, Indian, Pacific, and Southern oceans. However, it is convenient to consider separate parts of Earth in terms of concentric, roughly spherical layers. Extending from the interior outward, these are the core, the mantle, the crust (including the rocky surface), the hydrosphere (predominantly the oceans, which fill in low places in the crust), the atmosphere (itself divided into spherical zones such as the troposphere, where weather occurs, and the stratosphere, where lies the ozone layer that shields Earth's surface and its organisms against the Sun's ultraviolet rays), and the magnetosphere (an enormous region in space where Earth's magnetic field dominates the behaviour of electrically charged particles coming from the Sun).

Knowledge about these divisions is summarized in this astronomically oriented overview. The discussion complements other treatments oriented to the Earth sciences and life sciences. Earth's figure and dimensions are discussed in the article geodesy. Its magnetic field is treated in the article geomagnetic field. The early evolution of the solid Earth and its atmosphere and oceans is covered in geologic history of Earth. The geologic and biological development of Earth, including its surface features and the processes by which they are created and modified, are discussed in geochronology, continental landform, and plate tectonics. The behaviour of the atmosphere and of its tenuous, ionized outer reaches is treated in atmosphere, while the water cycle and major hydrologic features are described in hydrosphere, ocean, and river. The solid Earth as a field of study is covered in geologic sciences, the methods and instruments employed to investigate Earth's surface and interior are discussed in Earth exploration, and the history of the study of Earth from antiquity to modern times is surveyed in Earth sciences. The global ecosystem of living organisms and their life-supporting stratum are detailed in biosphere.

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Earth | Definition, Size, Composition, Temperature, Mass, & Facts

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Earth, third planet from the Sun and the fifth largest planet in the solar system in terms of size and mass. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life. It is designated by the symbol ♁.

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Venus, second planet from the Sun and sixth in the solar system in size and mass. No planet approaches closer to Earth than Venus; at its nearest it is the closest large body to Earth other than the Moon. Because Venus's orbit is nearer the Sun than Earth's, the planet is always roughly in the same direction in the sky as the Sun and can be seen only in the hours near sunrise or sunset. When it is visible, it is the most brilliant planet in the sky. Venus is designated by the symbol ♀.

Venus was one of the five planets-along with Mercury, Mars, Jupiter, and Saturn-known in ancient times, and its motions were observed and studied for centuries prior to the invention of advanced astronomical instruments. Its appearances were recorded by the Babylonians, who equated it with the goddess Ishtar, about 3000 bce, and it also is mentioned prominently in the astronomical records of other ancient civilizations, including those of China, Central America, Egypt, and Greece. Like the planet Mercury, Venus was known in ancient Greece by two different names-Phosphorus ( see Lucifer) when it appeared as a morning star and Hesperus when it appeared as an evening star. Its modern name comes from the Roman goddess of love and beauty (the Greek equivalent being Aphrodite), perhaps because of the planet's luminous jewel-like appearance.

Venus has been called Earth's twin because of the similarities in their masses, sizes, and densities and their similar relative locations in the solar system. Because they presumably formed in the solar nebula from the same kind of rocky planetary building blocks, they also likely have similar overall chemical compositions. Early telescopic observations of the planet revealed a perpetual veil of clouds, suggestive of a substantial atmosphere and leading to popular speculation that Venus was a warm, wet world, perhaps similar to Earth during its prehistoric age of swampy carboniferous forests and abundant life. Scientists now know, however, that Venus and Earth have evolved surface conditions that could hardly be more different. Venus is extremely hot, dry, and in other ways so forbidding that it is improbable that life as it is understood on Earth could have developed there. One of scientists' major goals in studying Venus is to understand how its harsh conditions came about, which may hold important lessons about the causes of environmental change on Earth.

Basic astronomical data

Viewed through a telescope, Venus presents a brilliant yellow-white, essentially featureless face to the observer. Its obscured appearance results from the surface of the planet being hidden from sight by a continuous and permanent cover of clouds. Features in the clouds are difficult to see in visible light. When observed at ultraviolet wavelengths, the clouds exhibit distinctive dark markings, with complex swirling patterns near the equator and global-scale bright and dark bands that are V-shaped and open toward the west. Because of the all-enveloping clouds, little was known about Venus's surface, atmosphere, and evolution before the early 1960s, when the first radar observations were undertaken and spacecraft made the first flybys of the planet.

Venus orbits the Sun at a mean distance of 108 million km (67 million miles), which is about 0.7 times Earth's distance from the Sun. It has the least eccentric orbit of any planet, with a deviation from a perfect circle of only about 1 part in 150. Consequently, its distances at perihelion and aphelion (i.e., when it is nearest and farthest from the Sun, respectively) vary little from the mean distance. The period of its orbit-that is, the length of the Venusian year-is 224.7 Earth days. As Venus and Earth revolve around the Sun, the distance between them varies from a minimum of about 42 million km (26 million miles) to a maximum of about 257 million km (160 million miles).

Because Venus's orbit lies within Earth's, the planet exhibits phases like those of the Moon when viewed from Earth. In fact, the discovery of these phases by the Italian scientist Galileo in 1610 was one of the most important in the history of astronomy. In Galileo's day the prevailing model of the universe was based on the assertion by the Greek astronomer Ptolemy almost 15 centuries earlier that all celestial objects revolve around Earth ( see Ptolemaic system). Observation of the phases of Venus was inconsistent with this view but was consistent with the Polish astronomer Nicolaus Copernicus's idea that the solar system is centred on the Sun. Galileo's observation of the phases of Venus provided the first direct observational evidence for Copernican theory.

The rotation of Venus on its axis is unusual in both its direction and its speed. The Sun and most of the planets in the solar system rotate in a counterclockwise direction when viewed from above their north poles; this direction is called direct, or prograde. Venus, however, rotates in the opposite, or retrograde, direction. Were it not for the planet's clouds, an observer on Venus's surface would see the Sun rise in the west and set in the east. Venus spins very slowly, taking about 243 Earth days to complete one rotation with respect to the stars-the length of its sidereal day. Venus's spin and orbital periods are very nearly synchronized with Earth's orbit such that, when the two planets are at their closest, Venus presents almost the same face toward Earth. The reasons for this are complex and have to do with the gravitational interactions of Venus, Earth, and the Sun, as well as the effects of Venus's massive rotating atmosphere. Because Venus's spin axis is tilted only about 3° toward the plane of its orbit, the planet does not have appreciable seasons. Also, Venus's rotational period slowed by 6.5 minutes between two sets of measurements taken in 1990-92 and 2006-08. Astronomers as yet have no satisfactory explanation for Venus's peculiar rotational characteristics. The idea cited most often is that, when Venus was forming from the accretion of planetary building blocks ( planetesimals), one of the largest of these bodies collided with the proto-Venus in such a way as to tip it over and possibly slow its spin as well.

Venus's mean radius is 6,051.8 km (3,760.4 miles), or about 95 percent of Earth's at the Equator, while its mass is 4.87 × 10 ²⁴ kg, or 81.5 percent that of Earth. The similarities to Earth in size and mass produce a similarity in density-5.24 grams per cubic centimetre for Venus, compared with 5.52 for Earth. They also result in a comparable surface gravity-humans standing on Venus would possess nearly 90 percent of their weight on Earth. Venus is more nearly spherical than most planets. A planet's rotation generally causes a bulging at the equator and a slight flattening at the poles, but Venus's very slow spin allows it to maintain its highly spherical shape.

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Venus | Facts, Color, Rotation, Temperature, Size, & Surface

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Venus, second planet from the Sun and the brightest planet in the sky.

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Mercury, the innermost planet of the solar system and the eighth in size and mass. Its closeness to the Sun and its smallness make it the most elusive of the planets visible to the unaided eye. Because its rising or setting is always within about two hours of the Sun's, it is never observable when the sky is fully dark. Mercury is designated by the symbol ☿.

The difficulty in seeing it notwithstanding, Mercury was known at least by Sumerian times, some 5,000 years ago. In Classical Greece it was called Apollo when it appeared as a morning star just before sunrise and Hermes, the Greek equivalent of the Roman god Mercury, when it appeared as an evening star just after sunset. Hermes was the swift messenger of the gods, and the planet's name is thus likely a reference to its rapid motions relative to other objects in the sky. Even in more recent eras, many sky observers passed their entire lifetimes without ever seeing Mercury. It is reputed that Nicolaus Copernicus, whose heliocentric model of the heavens in the 16th century explained why Mercury and Venus always appear in close proximity to the Sun, expressed a deathbed regret that he had never set eyes on the planet Mercury himself.

Until the last part of the 20th century, Mercury was one of the least-understood planets, and even now the shortage of information about it leaves many basic questions unsettled. Indeed, the length of its day was not determined until the 1960s, and Mercury's nearness to the Sun gave scientists bound to Earth many observational hurdles, which were overcome only by the Messenger ( Mercury Surface, S pace Environment, Geochemistry, and Ranging) probe. Messenger was launched in 2004, flew past the planet twice in 2008 and once in 2009, and settled into orbit in 2011. It mapped the entire surface of Mercury before crashing into the planet in 2015. Mercury's proximity to the Sun has also been exploited to confirm predictions made by relativity theory about the way gravity affects space and time.

At first glance the surface of the planet looks similar to the cratered terrain of the Moon, an impression reinforced by the roughly comparable size of the two bodies. Mercury is far denser, however, having a metallic core that takes up about 61 percent of its volume (compared with 4 percent for the Moon and 16 percent for Earth). Moreover, its surface shows significant differences from lunar terrain, including a lack of the massive dark-coloured lava flows known as maria and the presence of buckles and scarps that suggest Mercury is shrinking.

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Mercury, the innermost planet of the solar system and the eighth in size and mass. Its closeness to the Sun and its smallness make it the most elusive of the planets visible to the unaided eye. Because its rising or setting is always within about two hours of the Sun's, it is never observable when the sky is fully dark.