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Jupiter (planet), fifth planet from the sun and the largest planet in the earth's solar system. With the exception of the sun, the moon, and Venus, Jupiter is the brightest object in Earth's sky-more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the ancient Romans named Jupiter for Jove, the chief god of Roman mythology.
Jupiter orbits the sun at an average distance of 780 million km (484 million mi), which is about five times the distance from Earth to the sun. Jupiter's year, or the time it takes to complete an orbit about the sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.
Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a dense ball of gas. It has a relatively small core of molten rock and iron, but Jupiter has no solid surfaces. Jupiter's mass is about 318 times the mass of Earth and its diameter is about 11.2 times the diameter of Earth. The force of gravity at the level of the highest clouds in Jupiter's atmosphere is about 2.5 times the force of gravity on Earth.
Because Jupiter has such a large diameter and high rate of rotation, material at the surface must travel quickly to circle the planet. This speed gives the material a great deal of momentum, or a strong tendency to fly away from the planet and continue traveling in a straight line through space. Material at the equator has the highest speed because, in a Jovian day, it must travel the greatest distance to circle the planet. Hence, material at the equator has the greatest momentum, and the strongest tendency to fly away from the planet. Because of Jupiter's weak, gaseous structure, the planet can not hold this material in as well as a more solid planet could, which results in Jupiter having the distorted shape of a flattened ball. The diameter of its equator is 143,000 km (89,000 mi), yet the diameter through its axis of rotation is only 133,700 km (83,000 mi).
Observations from Earth
Jupiter was first viewed through a telescope in 1610 by Italian philosopher and scientist Galileo Galilei. Until that time, the dominant world view, which was developed by 2nd-century Alexandrian astronomer Ptolemy, held that all of the stars and planets move in orbits around the earth. Galileo, however, observed four satellites, or moons, in orbit around Jupiter. This simple observation of astronomical objects in orbit about another astronomical object other than the earth touched off what is known as the Copernican revolution, named after Polish astronomer Nicolaus Copernicus. Copernicus had earlier developed a cosmology in which the earth orbits the sun, which is now known as the Copernican System. The Copernican revolution was one of the key elements of the Renaissance and the Age of Enlightenment that continues to influence thinking to the present day. The moons that Galileo saw were collectively named the Galilean moons in honor of their discoverer.
When viewed through a modern telescope, the oblate (flattened) disk of Jupiter has a pearly color with bands of pastel browns and blues. Earth-based observers can best observe Jupiter when it is near solar opposition-that is, when Jupiter is on the side of Earth opposite the sun, or when both planets are aligned with the sun on the same side of the sun. At opposition, the distance from Earth to Jupiter is at its annual minimum, and Jupiter appears as much as one and one-half times larger than it does at other times. Also at opposition, Jupiter rises at sunset and sets at sunrise, which means that it is visible all night long. Because Jupiter orbits the sun in the same direction as Earth, Earth has to travel a little more than a full year to catch up to Jupiter from one opposition to the next. The time interval between exact oppositions is about 399 days.
In the mid-1950s radio astronomers discovered that Jupiter emits strong radio waves at many frequencies. The radio data indicates that Jupiter has a magnetic field similar to Earth's, but much stronger: At its upper atmosphere Jupiter's magnetic field is about ten times more intense than Earth's field at Earth's surface. Also like Earth's field, Jupiter's field is tipped about 10° relative to its axis of rotation. The interaction of Jupiter's magnetic field with charged particles ejected from the sun creates radio noise near the poles and auroras similar to Earth's aurora borealis, or northern lights.
As Jupiter rotates its north and south magnetic poles are obscured to different extents, which makes the intensity of radio noise vary in a regular pattern. The pattern repeats at intervals of 9 hours 55.5 minutes, indicating that this is the rate of rotation of Jupiter's interior. With this rapid rotation, Jupiter's entire surface can be observed in two days during the long observing periods of opposition. |
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Saturn (planet), sixth planet in order of distance from the sun, and the second largest in the solar system. Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes. He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae).The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, "It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic." The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet. |
