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SUN -
the star around which the Earth orbits. An ordinary main-sequence star of spectral class G2, the Sun is a self-luminous
gaseous mass, comprising some 71 per cent hydrogen and 26 per cent helium by mass, having an absolute visual magnitude of
+4.83 and an effective surface temperature of 5,770 K. At the solar centre, the temperature rises to 1.5 x 107 K to provide
the pressure necessary to support the overlying mass against gravity which, at the Sun's radiating surface (the photosphere),
is twenty-seven times that of the Earth. This high temperature also sustains, by the nuclear fusion of hydrogen into helium,
a solar radiation output which totals 3.8 x 1026 watts emanating from the photosphere.
For descriptive purposes the Sun may be subdivided into a series of roughly spherical shells. The core, in which the
energy for the solar radiation is released by nuclear fusion, occupies less than one-thousandth of the Sun's volume, but is
160 times more dense than water, while the photosphere is 10-7 times less dense than water. Because of the enormous mass of
the Sun and the opacity of its material, radiation leaks from the core to the photosphere extremely slowly, taking longer
than about 10 million years to do so. During this leakage, X-rays are weakened and emerge as visible light. However, neutrinos
generated in the fusion process can escape freely through the Sun and provide, in principle, a means of direct observation
of the core. The failure to observe the predicted number of solar neutrinos has generated much debate about the solar structure.
In the final 15 per cent of the way to the solar surface from the centre, convection plays a major role in transporting energy
outwards and the circulating motions from this convection zone appear in the fine structure of the visible solar surface,
in particular its granulation. Convective motions also play a part in the transport of magnetic fields, generated by currents
in the rotating solar interior, to the surface where they appear in complex forms, known as solar activity, on both large
and small scales, the most intense fields appearing in sunspots. Outward from the photosphere lies the solar atmosphere in
which the temperature declines to a minimum of 4,200 K and thereafter rises, owing to the dissipation of shock waves coming
from subphotospheric convection, through the chromosphere and the transition layer, where it jumps to a value of 2 x 106 K
characterizing the corona. The high coronal temperature results in a continuous escape of matter into interplanetary space
as the solar wind. Both the chromosphere and the corona exhibit a wide range of variable or energetic phenomena, which are
further examples of solar activity. These are associated with magnetically active regions, of which some of the most important
are solar flares, prominences, and holes in the corona.
The mass of the Sun is 1.99 x 1030 kg and the mean radius of the approximately spherical photosphere is 7.0 x 105 km.
These figures are respectively 3.3 x 105 times the Earth's mass and 110 times the Earth's radius; the solar volume would thus
contain 1.3 million Earths. Solar rotation is observed in the motions of surface features, such as sunspots, in the photosphere
and overlying layers. The mean rotation period is 25.4 days about an axis inclined at 7.25 degrees to the ecliptic but ranges
from 25 days at the solar equator to 41 days near the poles. This rotation is also related to the oblateness of the Sun's
disc, amounting to a 0.005 per cent deviation from a sphere. The effect of this flattening on the Sun's gravitational field
contributes to the precession of Mercury's perihelion and so complicates the use of this as a test of the general theory of
relativity.
The importance of the Sun, both to astronomers and to mankind, lies in its proximity to the Earth, the mean distance
being about 150 million kilometres, or 1 astronomical unit. In consequence, the solar radiation maintains terrestrial conditions,
particularly the Earth's surface temperature, within habitable limits, as well as providing daytime illumination directly
and nocturnal illumination by reflection from the Moon. Solar energy is the ultimate source of all the energy we utilize,
except radioactivity. To the astronomer, the Sun is unique for three reasons. Firstly, it is the only star close enough to
the Earth to be readily observable as an extended object, that is, as other than a point of light: the photosphere has an
apparent angular diameter of 0.53 degrees. This enables astronomers to observe the structure of the Sun's atmosphere in great
detail. By contrast, the angular diameter of a similar star at the distance of Proxima Centauri (1.7 parsecs) would be only
0.007 arc seconds. Secondly, it is the only normal star close enough to detect its very weak radiation in the radio and X-ray
bands, although other stars are visible at these wavelengths during periods of strong activity such as flares or nova explosions.
Thirdly, the Earth orbits within the solar wind, which forms the outer layers of the solar atmosphere, permitting direct sampling
of solar material by spacecraft in the vicinity of the Earth and direct observation of the three-dimensional structure of
the solar atmosphere by spacecraft in solar orbit. Furthermore, the comparatively short distance to the Sun means that the
paths of charged particles (the solar cosmic rays) may be traced back to their sources on the Sun, whilst neutrons travelling
at nearly the speed of light from the Sun do not have time to decay before reaching the Earth.
-Mercury (in astronomy)
, the innermost planet of the solar system. It is visible as a magnitude 0 star at approximately two-monthly intervals
alternating between the evening and morning twilight. The planet follows a remarkably elliptical orbit moving from 46 to 70
million kilometres from the Sun and completes one orbit every 88 days. In a small telescope it shows phases like those of
the Moon, due to its varying position relative to the Earth and Sun. Very occasionally, the Earth, Mercury, and the Sun line
up precisely and the planet is then seen to transit the Sun as a small dark spot crossing the solar disc. This last occurred
on 12 November 1986 and will occur again on 14 November 1999. Observations of faint dusky markings seen on Mercury through
large telescopes around 1900 led astronomers to conclude that the planet rotated on its axis and revolved round the Sun at
the same 88-day rate, keeping the same face pointing sunwards. After being accepted for eighty years, the 88-day rotation
period was finally replaced by the correct 58.5-day period deduced from radar observations.
Mercury is 4,878 km in diameter, midway in size between Mars and the Moon and its daytime surface is baked by the proximity
of the Sun. The midday equatorial temperature varies, due to the elliptical orbit, from 415 oC at the closest point to the
Sun to 285 oC at the farthest. In contrast, the lack of an atmosphere allows the temperature to plummet to -175 oC at night.
The spacecraft Mariner 10, which passed close to Mercury on three occasions in 1974 and 1975, revealed the planet as a barren
airless world covered with impact craters similar to those on the Moon. The largest structural feature is the Caloris Basin,
a ring basin 1,300 km in diameter, whose floor has been intensely disrupted by fractures and ridges. Caloris was probably
caused by the impact of a body some tens of kilometres in diameter. Some craters show ray systems.
The perihelion advance of Mercury has been used to confirm predictions made by the general theory of relativity.
-Venus (in astrology),
the second major planet in order of distance from the Sun. Also known as Hesperus, the evening star, or Phosphorus, the
morning star, Venus can be the brightest object in the sky after the Sun and Moon and follows a near-circular path some 106
million kilometres from the Sun, taking 225 days to complete one orbit. It shows phases in the same way as Mercury and can
also pass in front of the Sun during a transit. The last transits were in 1874 and 1882 and the next are not until 7 June
2004 and 5 June 2012. In visible light the planet shows an almost featureless cloud-shrouded globe. However, in photographs
taken with ultra-violet light, it shows dark patches and streaks, which are temporary breaks in the dense lower cloud levels.
These race round the planet in only four days, carried by a high-altitude 350 km/h gale which never abates. The clouds closely
resemble natural terrestrial fogs or industrial smogs and are composed of sulphuric, hydrochloric, and hydrofluoric acid droplets.
The atmosphere is extremely dense and hot, made up of over 90 per cent carbon dioxide. The surface pressure is 91 times that
of the Earth and the global surface temperature is kept permanently at about 475 oC by the greenhouse effect. The solid globe,
with a mean density of 5,240 kg/m3, is 12,104 km in diameter and rotates slowly from east to west, that is, in a retrograde
manner, with a period of 243 days so that its day is longer than its year. The Soviet spacecraft Venera 9 and 10 gave the
first glimpses of the surface in 1975. Both spacecraft soft-landed and returned panoramic photographs showing hot stony desert
landscapes scattered with rocks of all sizes. Recent radar studies suggest that this is typical of the whole planet and show
that Venus has a cratered surface. Radar maps reveal two high continent-sized areas, Ishtar Terra and Aphrodite Terra, and
a very high mountain feature, Maxwell Montes. There is also evidence for the presence of long canyons and volcanoes, though
these seem to be inactive at present. Venus has no natural satellites
-Earth,
the planet third in distance from the Sun and the only one in the solar system on which life is known to exist. The Earth
is 12,756 km (7,926 miles) in diameter at the equator and has a mean density of 5,520 kg/m3. It rotates in 23 hours 56 minutes
about an axis tilted at 23 degrees 26 minutes to a line perpendicular to the Earth's orbit about the Sun. This pronounced
axial tilt produces the seasons, governed at each place by the annual variation in the amount of heat received from the Sun.
The Earth moves in an almost circular path averaging 149.6 million kilometres (93.5 million miles) from the Sun, taking 365.25
days to complete one orbit. It has one natural satellite, the Moon.
The Earth is composed of three parts: the crust, the mantle, and the core. The crust is a thin layer of rock with an
average thickness of about 40 km (25 miles) in the continental areas but only about 6 km (4 miles) under the oceans. The continental
crust, which varies from about 10 to 70 km (6 to 43 miles) in thickness, is not only thicker but is less dense than that below
the oceans. It is also more permanent: the oldest parts are about 3,800 million years old, whereas the oceanic crust is nowhere
more than 200 million years old. Chemically, the continental crust is richer in such elements as silicon, potassium, sodium,
uranium, and thorium. Below the crust is the mantle, the boundary between the two being marked by the Mohorovivicic discontinuity
or moho. The mantle extends to a depth of about 2,900 km (1,800 miles) and constitutes the bulk of the Earth--about 84 per
cent by volume. The material of which it is composed is believed to have a composition close to that of peridotite, a basic
igneous rock consisting largely of magnesium-rich silicates. The oceanic and continental crust, together with the upper and
stiffer part of the mantle immediately below, constitute the lithosphere, which extends up to a depth of 100 km (62 miles).
This is divided into a number of rigid plates that move very slowly in relation to one another and in relation to the Earth's
poles, most probably as a result of large-scale convection currents in the mantle. These movements, their cause, and their
consequences are the study of plate tectonics. Below the mantle is the central region of the Earth, the core, with a radius
of about 3,500 km (2,200 miles). It is generally agreed that iron constitutes about 90 per cent of the core. The other constituents
are uncertain; nickel, sulphur, oxygen, and silicon are possibilities. The pressure within the core is extreme; it is estimated
at between 1.3 and 3.5 million atmospheres. The temperature is also very high: 4,000 to 5,000 oC (7,200 to 9,000 oF). Evidence
from earthquakes shows that the inner core, with a radius of about 1,200 km (750 miles), is solid, but that the outer core
is molten.
From space the Earth appears as a bluish globe with dense cloud formations and brilliant polar ice caps. The rapid rotation
of the planet produces complex weather patterns in the atmosphere, seen from space as cloud bands and huge rotating cloud
patterns. Seventy per cent of the surface is covered with vast oceans of water, the continental land masses making up the
other 30 per cent. The surface of the Earth is not covered with the ancient craters which all the inner planets once had,
attesting to the relative youth of the surface and the severe weathering effects acting on it. The Earth's atmosphere consists
of 78 per cent nitrogen, 21 per cent oxygen, and with trace gases making up the other 1 per cent. This life-supporting atmosphere
is quite a recent development. Extensive ancient volcanic activity produced vast amounts of steam, which condensed to form
the oceans, and carbon dioxide, later incorporated into rocks. This allowed an atmosphere of nitrogen and oxygen to develop.
The Earth's surface is slowly being blanketed by dust from the disintegration of interplanetary debris which collides with
the Earth's atmosphere to produce meteors. A few bodies reach the ground as meteorites. The fluid metallic core of the Earth,
carried round by the daily rotation, forms a magnetic field which interacts with the solar wind to form the Earth's magnetosphere.
This envelope round the Earth forms a tail away from the Sun and shields the planet from much harmful cosmic radiation. Atomic
particles entering the tail are accelerated and dumped into the Earth's atmosphere near the poles to produce the aurora. In
space around the Earth are the two Van Allen radiation belts, two giant doughnut-shaped envelopes of trapped charged particles
at heights of about 3,000 and 22,000 km (2,000 and 14,000 miles).
It is the vast concentration of surface water at just the right temperature which makes the Earth such a haven for life.
It is unlikely that these conditions are unique to Earth. There should be thousands if not millions of similar life-sustaining
planets in the universe.
-Mars (in astronomy),
the fourth planet from the Sun. Mars moves in a markedly elliptic orbit, its mean distance from the Sun being 1.52 times
that of the Earth's. Its diameter is approximately 6,800 km, a little more than half that of the Earth. Its period of revolution
is 1.88 years, giving it a synodic period of 2.14 years. At opposition its distance from the Earth can vary from 100 million
km at aphelion to about 50 million km at perihelion, very close oppositions occurring every fifteen or seventeen years on
average. At such times Mars is a bright, reddish, star-like object and the subject of extensive telescopic study. By study
of the orbits of its two moons, Phobos and Deimos, discovered by the US astronomer Asaph Hall in 1877, the mass of Mars can
be measured to be one-tenth that of the Earth. The pictures from the Mariner and Viking spacecraft show a solid surface with
a wide variety of features: thousands of impact craters; many huge volcanic craters, none of which are now active; a great
equatorial valley running eastwards for 3,000 km before turning northwards; and many smaller valleys and chasms with branching
canyons. There also exist eroded plateaux, collapsed regions, and two white polar caps which change size as the Martian seasons
come and go. However, it has no canals; the network of waterways thought by Lowell and others in the late 19th century to
cover the surface does not exist. Bombardment by meteors, and erosion, weathering, and volcanic activity have created complex
surface conditions on Mars. It is possible that orbital and precessional changes over long periods of time may release water
in large quantities, creating torrents which erode out valleys. Among the prominent surface features are Argyre Planitia,
Chryse Planitia, Elysium Planitia, Hellas Planitia, Nix Olympica, Pavonis Mons, Syrtis Major, Tharsis Ridge, Utopia Planitia,
and Valles Marineris.
The planet's thin atmosphere (at the surface it is only 1 per cent of the density of Earth's atmosphere) is mainly composed
of carbon dioxide. The polar caps are made of snow, ice, and carbon dioxide and it is possible that much water may be permanently
locked up as ice in permafrost conditions. Clouds and sandstorms are often seen. At the equator the temperature can vary from
10 oC to below -75 oC. The automatic laboratories of the Viking landers detected no definite signs of life. However, in 1996
it was claimed that fossil remains of primitive microbes had been found in a meteorite originating on Mars, and this renewed
interest in the planet. Mars remains the planet least inhospitable to life as we know it and future missions to it will undoubtedly
put a high priority on the search for traces of life.
-Jupiter (in astronomy),
the fifth planet from the Sun and the largest planet of the solar system, easily visible with the unaided eye. Its mass
is about twice as large as the sum of the masses of all the other planets, and is about one thousandth of that of the Sun.
Like all the other giant outer planets, it emits more energy than it receives from the Sun, thus implying the existence of
an internal energy source. Although Jupiter is essentially a gaseous body, it has an inner core composed of rocky material,
with a mass of perhaps ten to twenty times that of the Earth, surrounded by a liquid envelope of lighter elements, and a massive
atmosphere. The satellite system comprises four large satellites, discovered by Galileo at the beginning of the 17th century;
they orbit about the planet in nearly circular, equatorial orbits. There are also a number of smaller satellites, divided
into three groups. The first is composed of small satellites in orbits very close to the planet; the second and the third
are on larger and more eccentric and inclined orbits; those in the case of the outermost group are retrograde, that is, move
in the opposite direction to Jupiter's motion in its orbit. The abundance of light elements such as hydrogen and helium in
Jupiter, compared with their relative deficiency in the inner planets, suggests that when the planet was formed the core was
able to attract and trap the gas that fell on the forming planet and now constitutes most of its mass. The atmosphere is characterized
by parallel bands that move with respect to each other, driven by Jupiter's internal energy source; darker bands are called
belts, and lighter ones zones. At the boundaries between belts and zones, vortices are present. One of these is the long-known
Great Red Spot, a gigantic storm system larger than the Earth, in which clouds rise several kilometres above the clouds of
surrounding regions. Jupiter is a strong emitter of radio signals. The magnetic field is much stronger than that of the Earth,
with a correspondingly much larger magnetosphere. The line joining the magnetic poles is tilted at about 10 degrees to the
rotation axis, and the equivalent of the terrestrial Van Allen radiation belts, in which electrically charged particles are
trapped, extend out to include some of the major satellites. Observations from the Voyager spacecraft led to the discovery
of a tenuous ring system about the planet, much less spectacular and massive than that of Saturn. It is mostly composed of
particles about 10-6 m in size. Of limited lifetime because they escape into the surrounding space from the ring, they are
probably replenished by the debris resulting from meteoroids impacting on larger bodies. Of the two small innermost satellites
of Jupiter, one happens to be within the ring and the other at its edge and it is possible that they are shepherding satellites
which play a role in maintaining the ring system.
-Saturn (in astronomy),
the sixth planet from the Sun and the outermost planet clearly visible with the unaided eye. Saturn is the second largest
planet in the solar system. Some of its main physical characteristics like size, composition, internal structure, and atmosphere
suggest an overall resemblance to Jupiter but the results of spacecraft investigations have shown that there are also important
differences between the two planets. Saturn, like Jupiter, is mostly composed of light elements such as hydrogen and helium
and has an overall density less than that of water. It has an inner rocky core of about ten to twenty times the mass of the
Earth, roughly the same size as the core of Jupiter. However, the percentage of helium in the envelope surrounding the core
is lower than that of Jupiter. This may be due to helium sinking into the interior, which could also be the explanation of
the internal energy source that Saturn, like all other giant planets, has. This energy source drives mass movements in the
atmosphere that show up as parallel bands, composed of darker belts and lighter zones, in motion with respect to each other.
A large white spot appears every 27-30 years. It is probably a giant storm containing frozen ammonia. The large size of Saturn's
core with respect to the envelope of light elements suggests that when it formed there was less gas available than for Jupiter,
thus leading to the smaller mass of Saturn. A peculiarity of the magnetic field of this planet is the very small angle between
the line joining the magnetic poles and the planetary rotation axis. Saturn's equator has an inclination to its orbit slightly
larger than that of the Earth, thus inducing seasonal effects in the behaviour of the atmosphere. Saturn's rings are well
known and were explored in great detail by the Voyager missions. The visibility of the ring system changes in a 29.5-year
cycle. When presented edge-on to the Earth, the rings cannot be seen at all well. The satellite system is also very interesting.
It is composed of seventeen bodies, of which the innermost ones orbit at the outskirts of the ring system, having strong gravitational
interactions with the rings themselves. Proceeding outwards there is a group of icy satellites of small to intermediate size,
some of which strongly influence each other's orbits. Titan is the second-largest satellite of the solar system and has a
substantial atmosphere. There are three more satellites beyond Titan, one of which is on a retrograde orbit, that is, it moves
about Saturn in the opposite direction to Saturn's orbital motion about the Sun.
-Uranus (in astronomy),
the seventh planet from the Sun and the fourth most massive planet of the solar system. Uranus is generally not visible
to the naked eye. It was discovered in 1781 by William Herschel who initially thought it was a comet. Because of its slow
motion, it became apparent that its orbit was nearly circular, and it was then clear that it was actually a planet. Uranus
had, in fact, appeared in contemporary star catalogues but because it moves so slowly, it was thought to be a star. This previous
information quickly led to a good knowledge of its orbit. Moreover, its computed distance from the Sun of 19.2 astronomical
units agreed well with that expected on the basis of Bode's Law, and this renewed interest in the attempt to fill the empty
place in the sequence at 2.8 astronomical units, which resulted in the eventual discovery of the asteroid Ceres. In the first
part of the 19th century, the observed motion of Uranus departed from that computed on the basis of the then-known planets
and the analysis of the required perturbations led to the discovery of Neptune. If the formation of Uranus, like that of Neptune,
took much longer than Jupiter and Saturn, allowing the light elements such as hydrogen and helium in their vicinity to be
removed by the already formed Sun, this might explain the smaller mass and higher density with respect to Jupiter and Saturn.
Among the outer planets, Uranus is characterized by several unique properties. Its equator is inclined at 97 degrees to the
plane of its orbit, its satellites are all small to medium in size and are in nearly circular equatorial orbits, and it is
surrounded by nine very narrow dark rings, some of which are elliptical. Uranus apparently lacks an internal energy source
comparable to those of the other outer planets and the line joining its magnetic poles is inclined at about 60 degrees to
the rotation axis, a value that is larger than for any other planet.
-Neptune (in astronomy),
the eighth planet from the Sun, and the third most massive in the solar system. Neptune was discovered in 1846 by the
German astronomers Johann Galle and Heinrich d'Arrest whilst trying to verify the computations of Le Verrier who predicted
that an undiscovered outer planet was perturbing the motion of Uranus. Adams had independently arrived at the same result.
Following the discovery, many observations of Neptune were found in the records of the preceding centuries, but the planet
moves so slowly that it was previously thought to be a star. It was therefore possible to obtain a good knowledge of its
orbit quite quickly. However, deviations of the observed motion of the planet from the computed one started to appear, and
this led astronomers to search for another more remote planet. This planet, Pluto, was found in 1930 not far from the predicted
position but it does not fully account for the variations in Neptune's orbit. Observations by Voyager 2 have greatly expanded
our knowledge of Neptune, which until recently was somewhat uncertain. Neptune emits 2.7 times more energy than it receives
from the Sun. Its atmosphere is composed mainly of hydrogen and helium with a mixture of methane, water, and ammonia. The
colour is thought to result from the presence of methane. The line joining the magnetic poles of Neptune is inclined at
about 47 degrees to the rotation axis of the planet. Spacecraft observations have led to the discovery of several rings,
one of which is characterized by an irregular distribution of material: three arcs in this ring appear to be thicker than
the rest of the ring as if the particles there were more densely packed than elsewhere. The satellite system is peculiar.
As well as the large satellite Triton and an outer satellite, Nereid, there are also six more satellites of small to medium
size, all very close to the planet, which were discovered by Voyager. Triton orbits Neptune in the opposite direction to
the planet's orbital motion and was probably captured by Neptune after its formation. Nereid has a very eccentric orbit.
-Pluto,
the outermost planet of the solar system, discovered by the US astronomer Clyde Tombaugh in 1930. Pluto's perihelion
lies inside the orbit of Neptune and this, together with its small size and mass, suggests that Pluto may have originally
been a satellite of Neptune. The problem with this theory is that the orbit of Pluto avoids any close encounter with Neptune
so it is difficult to see how they could have been associated in the past. There are two reasons for this avoidance. Firstly,
Pluto's orbital period is about 1.5 times that of Neptune so that when Pluto is near perihelion Neptune is always in a remote
part of its orbit. Secondly, Pluto always keeps far above or below Neptune's orbital plane whenever it is closer to the Sun.
Pluto could thus be the innermost or largest object of an outer belt of small bodies. Also remarkable is the fact that Pluto
has a comparatively large satellite Charon, which might have split from it following a shattering impact. Pluto and Charon
always keep the same face turned towards each other. Pluto's mean density of about 2,000 kg/m3 suggests that its interior
is a mixture of rocks, water ice, and methane ice; in fact, methane has been identified on the surface by photometry and spectrophotometry.
In 1989, a stellar occultation confirmed that the planet has a thin atmosphere, presumably composed mostly of methane, whose
density probably varies significantly between aphelion and perihelion as it is heated and cooled. A haze layer may surround
the solid surface. Pluto's brightness changes with its rotation rate of 6.4 days due to bright patches on the surface.
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