Information about Our Solar System
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Sun 
Lying at
a distance of 30,000 light
years from the centre of the Milky Way
galaxy the Sun takes over 200 million years to complete one orbit. It
is the
central body of the Solar System,
containing 99.8% of its mass, and is the nearest star to the earth
lying at an
average distance of 149,600,000 km.
Without its energy life on earth would
not be possible. With a diameter of
1,392,000 km the Sun is classed as a
yellow dwarf star that is 4.6 billion
years old and is about half way through its
life cycle. It will eventually expand
into a red giant star and finally
collapse to a white dwarf. The effective surface
temperature of the Sun is around 5,800
degrees C while the centre of its core experiences temperatures of
15,000,000
degrees C. This core is extremely dense
and although it contains only 1.5% of the volume of the Sun it
contains over half of the Sun's mass.
The Sun is composed mainly of hydrogen
(70%) and helium (28%) with about 2% of
heavier elements. It generates its energy by the nuclear
fusion process and is losing mass at a
rate of 4 million tonnes every second. It can take as long as 20
million years
for the energy created in the core to
reach the Sun's surface. This energy is given off as heat and light and
a low
density stream of charged particles
known as the solar wind which blows throughout the solar system at
speeds of
450km/second.
Because the Sun is virtually a ball of gas it rotates at different speeds at different latitudes. At the equator the rotational period is around 25.4 days and near the poles the period is closer to 36 days. It has a period of energy fluctuations from high to low and back to high again. This is known as the sunspot cycle and is roughly 11 years long. The Sun's magnetic field reverses after each sunspot cycle and so takes 22 years to get back to where it was before. At the moment the solar cycle is at one of its low points and there are only a few small sunspots to be seen.
Warning! Under NO circumstances try to observe the Sun without the proper solar filters.
Mercury
Since the
reclassification of Pluto
from the "Planet" to the "Dwarf Planet"
category, Mercury has now become the smallest of the main Solar System
planets. The closest planet to the Sun,
Mercury is a rocky planet with a
diameter of 4,878 km orbiting the Sun in 87.97
days at an average speed of 47.89
km/second. Its distance from the Sun varies from 46,375,318 km to
70,310,966
km and has the largest known difference
in daytime-nighttime temperatures in the Solar System. The
sunlight striking Mercury when it is at
its closest point to the Sun is 2.3
times as intense as it is at the most distant
point in its orbit. Because it has
almost no permanent atmosphere to shield
it from the sunlight during the day and
hold the heat during the night its
daytime temperature reaches 447 degrees
C and plummets to minus 153 degrees C at
night. Strangely Mercury's day is longer
than its year. The solar rotation period, noontime to noontime or one
Mercurian day is 175.84 Earth days which
equals two Mercurian years.
Photography of half of Mercury's surface
by Mariner 10 in 1974 and 1975 showed it
to resemble the Moon in being heavily cratered with intervening areas
of lava flooded plains with the largest
impact feature detected being 1,300 km in diameter. Whereas the
features on
the Moon are named after scientists and
astronomers, the features on Mercury are named after world cultural
figures such as architects, composers,
writers and artists. As Mercury orbits so close to the Sun it can only
be observed from the Earth in either
morning or evening twilight and at times can be quite elusive as at its
best only
ventures 28 degrees in angular distance
from the Sun. Transits of Mercury across the Sun's disc as seen from
the
Earth are reasonably common but can only
occur in the months of May or November with the intervals at which
they occur being 3, 7, 10 or 13 years.
The next will occur on May 9th 2016.
Venus
The
second planet from the Sun, Venus
is the brightest object in the night sky
apart from the Moon. One of the reasons it is so bright is the very
high
reflectivity of its upper atmosphere
which reflects 76 percent of the light
it receives from the Sun. It is also the
planet which comes the closest to the
Earth, at times being closer than 42,000,000 km. Depending on its
position
in its orbit around the sun it can be
seen as either the "Morning Star" or the "Evening Star" sometimes
rising
before the Sun in the morning sky and
sometimes setting after the Sun in the evening sky. Ancient astronomers
thought
that the "Morning Star" and the "Evening
Star" were two different objects and named them Phosphorus and
Hesperus.
It was Pythagoras, a Greek mathematician
and philosopher, who realized that they were in fact the same
object. With an equatorial diameter of
12,104 km it was envisaged that Venus was the Earth's sister planet and
was very similar to the Earth in its
makeup. This could not be further from the truth as space probes have
revealed
extremely hostile atmospheric and
surface conditions. The atmospheric pressure on Venus is 91 times
greater
than the atmospheric pressure here on
Earth with the atmosphere saturated with clouds of sulfuric acid
aerosols.
Venus orbits the Sun at a speed of 35
km/second and takes 224.7 days to
complete the journey. It is the only
planet apart from Uranus which rotates backward
on its axis and so the Sun rises in the
west and sets in the east with one Venetian day, noon to noon, being
equal
to 116.67 Earth days long. The orbit of
Venus is the most circular of all the planets and because the tilt of
its axis
of rotation is less than 3 degrees from
the line of its orbit around the Sun it has no weather patterns on its
surface.
Its surface temperature remains at a
constant 457 degrees C, plus or minus 5 degrees C, all year round even
through
the long Venetian night. Venus is the
hottest of all the planets and the high constant temperature is due
mainly
to the huge amounts of carbon dioxide in
its atmosphere. This thick carbon dioxide atmosphere allows small
amounts
of visible light to penetrate which is
reflected off the surface as infrared radiation and trapped by the
atmosphere.
Because of this constant surface
temperature and because of its slow rotation the winds blowing across
the
surface of the planet reach no more than
10 km/hour but at altitudes higher than 50 km there have been winds up
to
360 km/ hour detected.
Because Venus orbits the Sun inside the
orbit of the Earth it can never be
observed overhead in the middle of the night as it only reaches an
angular
distance of 48 degrees from the Sun as
seen from the Earth. Although telescopesdo not reveal any surface
features on the planet it is interesting
to observe its changing phases from gibbous to crescent as the angle at
which the Sun's light strikes the planet
changes from our earthly perspective. On rare occasions Venus can be
seen to
cross the Sun's disc. These transits can
only occur in the months of June or December with the last one
occurring on June 8th 2004 and the next
one due on June 6th 2012. Make the most of the 2012 transit as the next
one after that will not occur until
December 11th 2117.
The Zodiacal
Light 
The
Zodiacal light is often confused
with the brightness of the sky due to a
lingering twilight as it occurs in
roughly the same area of the sky. It is produced
by sunlight reflecting off cosmic dust
particles in orbit around the Sun. Most of the particles that can be
seen have
sizes in the range from 10-100
micrometers and are produced by decaying comets and colliding asteroids
and are
spiraling slowly in towards the solar
surface.
The light is seen as a cone of light
about 20 degrees to 30 degrees wide at
the horizon and on a clear night can extend halfway to the zenith. It
is most
noticeable when the Sun is more than 18
degrees below the horizon and astronomical twilight has ended. The
best months to observe the Zodiacal
light from southern latitudes are from September into November in the
evening
sky and February and March in the
morning sky. These are the months when the ecliptic, the imaginary line
plotted in the sky by the Sun's path, is
at its largest angle to the horizon. A
clear moonless night away from light pollution
is a must to observe this delicate
phenomenon.
Earth
Just look down :-)
Earth is our home. It is a small globe of rock 12,756 kilometers across with a bit of water splashing around. It orbits around the Sun every 365.25 days at an average distance of 149,597,870 kilometers. It rotates every 23 hours and 56 minutes about a tilted axis of 23.45 degrees which allows seasonal variation. There is a thin atmosphere, about 100 kilometers thick, of mainly Nitrogen, Oxygen and Carbon Dioxide. It is the only place known to support life as we know it, though some other candidates await confirmation in the solar system and some recently discovered Earth-like bodies around other stars.
Moon
The only natural satellite of the Earth and the fifth largest natural satellite in the Solar System, the Moon is visible by virtue of reflected sunlight. It has a diameter of 3,476 km and lies at an average distance of 384,400 km from the Earth taking 27.322 days to complete one orbit at a speed of just over 1 km/second. This is the same amount of time it takes to spin once on its own axis and so always shows the same face to the Earth. Because the Moon wobbles slightly as it orbits we get to see about 59% of its surface. Since the Earth is moving in its orbit around the Sun as the Moon orbits the Earth it takes a little longer to go through its cycle of phases as The only natural satellite of the Earth and the fifth largest natural satellite in the Solar System, the Moon is visible by virtue of reflected sunlight. It has a diameter of 3,476 km and lies at an average distance of 384,400 km from the Earth taking 27.322 days to complete one orbit at a speed of just over 1 km/second. This is the same amount of time it takes to spin once on its own axis and so always shows the same face to the Earth. Because the Moon wobbles slightly as it orbits we get to see about 59% of its surface. Since the Earth is moving in its orbit around the Sun as the Moon orbits the Earth it takes a little longer to go through its cycle of phases as seen from the Earth. From one full Moon to the next a period of 29.5 days elapses. Each spot on the Moon is subjected to two weeks of daylight during which surface temperatures reach 100 degrees C and two weeks of darkness when the temperature plummets to -170 degrees.
The daily rotation of the Earth and the slower eastward revolution of the moon in its orbit usually produce two high tides and two low tides in the Earth's oceans every 24 hours 50 minutes. Traces of argon, helium, oxygen, methane, nitrogen, carbon monoxide and carbon dioxide have been detected in the Moon's atmosphere but this atmosphere is so thin as to be almost negligible. These gasses are, in part, the result of micrometeorites striking the surface of the Moon but because of the Moon's low gravity, about 17% that of the Earth's; they are very quickly lost into space. It is because of this lack of atmosphere that the face of the Moon hasn't changed significantly for millions of years. The temperature at the center of the Moon is around 1500 degrees C and its core, consisting mainly of metallic iron alloyed with a small amount of sulfur and nickel, is around 1200 km in diameter. Directly above the core lies the mantle, 1100 km thick and composed mainly of iron, magnesium and silicates and this is covered by the crust with a depth of 60 km which consists primarily of calcium, aluminium and silicates. The dark lowland plains are known as Maria, Latin for seas, because the first observers imagined them to be oceans of water. The name persists today even though we know that the Moon is waterless. The lunar highlands are named after mountain ranges on the Earth and so we have the lunar Alps and the Apennines.
Because of the lack of atmosphere on the Moon there can be no twilight and the line, known as the terminator, dividing the night and day sides of the moon has bright daylight on one side and pitch blackness on the other. At times when viewed through a telescope sunlit peaks can be seen protruding from the night side of the terminator. Most of the lunar craters are round, the few elongated ones being caused by impacting bodies that arrived at a sharper angle to the surface than 5 degrees. The origin of the Moon is still being debated with the Giant Impact Theory seeming to have taken favoritism. This proposes that a glancing high speed collision of a huge possibly Mars sized object with the primitive Earth, smashed what crust the Earth had and blasting it into space. The ejected material condensed to form a ring of orbiting debris and the Moon coalesced from this. It is our nearest celestial neighbor and so far the only other world upon which mankind has so far set foot.
Moon
Halos 
We tend
to take the Moon for granted
and most of us hardly give it a second
glance. However when atmospheric conditions are right the moon can
produce some interesting and rather
stunning effects. Perhaps the most
common of these effects are the Moon halos,
large rings which circle the moon on a
hazy cold night and have a radius of either 22 degrees or 46 degrees.
The
most frequently seen seem to be the 22
degrees halos. They are more intense when the Moon is at its
brightest, around full Moon but can be
witnessed to a lesser degree at other
bright phases of the Moon. For the rings to
appear there needs to be a layer of thin
cloud containing millions of tiny ice crystals covering much of the sky
high
in the upper atmosphere. Each of these
ice crystals behaves like a small lens and because most of these
crystals
have the same elongated hexagonal shape
the moonlight entering one face of the crystal and exiting its
opposing face is bent at an angle of 22
degrees. The larger 46 degrees ring which is far less typical may be
caused
by the light exiting at different angles
from the crystals.
A much smaller ring, usually about 3
degrees in radius can often be seen
around the Moon and this is known as a Lunar Corona. They are also
caused by
high thin cloud but the moonlight is
being refracted by water droplets in the clouds and not by ice
crystals. On rare
occasions Moon Halos and Lunar Coronae
can be seen together. The Lunar Coronae have been known to swell or
shrink slightly depending on the size of
the water droplets that are present in the passing clouds. There can
also be
colours present on the coronae but these
colours are always very subtle and more likely to be obvious when the
Moon
is full.
So the next time you are walking home at
night or just sitting out under the
night sky check it out. If there is a
bright Moon up there at the time it is well
worth a look.
Mars
The
fourth planet from the Sun, Mars is
easily distinguishable by its sometimes
intense reddish-orange hue which is stronger than any of the stars and
the cause of its association with the
Roman God of War. This redness is the result of the large amounts of
iron
oxide in the surface rocks and at times
the sky appears pink from the fine particles of dust suspended in the
atmosphere. Mars orbits the Sun once
every 686.98 earth days in a markedly elliptical path that takes it
from 210
million km to 250 million km from the
Sun. If the Earth passes Mars when Mars is at its closest to the Sun,
only 57
million km separate the pair and Mars
will shine at a magnitude equal to that
of the planet Jupiter. These close
approaches occur at intervals of around
15 years and astronomers can sometimes take the opportunity to observe
Mars at
its best. Unfortunately it is at this
point in its orbit when occasionally massive dust storms can engulf the
whole
planet spoiling the view. When Mars is
its most distant from the Sun, nearly 100 million km separate it from
the Earth and it can appear unimpressive
even in large telescopes. Because it orbits outside the orbit of the
Earth
Mars can never appear as a crescent but
at times can display a gibbous phase, like the Moon when it is a few
days from full. The summer daytime
temperature never rises much above 20 degrees C and can plummet to -140
degrees C just before sunrise each
morning.
Mars has a diameter of 6,795 km, just
over half the Earth's diameter and
rotates once on its axis every 24 hours 37 minutes and 23 seconds. This
axis is
tilted at 29.19 degrees to its orbital
plane and these values help to make the Red Planet the most Earth like
of
all the other planets even though it has
only 28 percent of the Earth's surface area. Although there is no
liquid water
at present, the presence of ancient
water courses on its surface point to the existence of running water in
the
ancient past. White polar caps expand
and contract with the changing of the Martian seasons and the ancient
volcanoes would have gushed out enough
gas to make the earlier atmosphere much heavier than the 7 millibars
that
it is today. This is 0.007 percent of
the Earth's atmospheric pressure and consists mainly of carbon dioxide
(95
percent), nitrogen and argon. Olympus
Mons, 600 km wide and towering 26 km above the surrounding plain, is
the largest known volcano in the Solar
System. It dwarfs the largest volcano on Earth, Hawaii's Mauna Kea
which rises a mere 9.3 km from its 120
km diameter base on the floor of the Pacific Ocean. Another dramatic
feature is Mariner Valley, a massive
rift 75 km wide, up to 7 km deep and extending 4000 km across the
planet
and making the North American Grand
Canyon look like a small crack in the ground.
Mars has two moons, Phobos and Deimos
which are probably asteroids captured by
Mars early in its evolution. They were both discovered in 1877 by
astronomer Asaph Hall, the larger Phobos
measuring 27 X 22 X 19 km and Deimos measuring 15 X 12 X 11 km.
Phobos is remarkable in that it orbits
Mars three times a day. It is the only known moon in the Solar System
that
orbits more quickly than its parent
planet spins. It also lies closer than any other moon to its parent, a
mere
6,000 km above the surface of Mars. This
should make a spectacular sight for the first astronauts who might set
foot on this fascinating neighbor of
ours.
Asteroids
There seems to be two main theories as to how the asteroids came into existence. The first suggests that they are the remnants of a planet destroyed in a massive collision long ago and the second, and it seems the most popular, is that they are left over rocky matter that failed to coalesce into a planet during the formation of the Solar System. Most known asteroids (about 95%) orbit in the asteroid belt, or main belt, between the orbit of Mars and Jupiter at distances of 1.7 to 4.0 astronomical units from the Sun. (One astronomical unit is equivalent to the distance from the Earth to the Sun or 149,597,870 kilometers). These main belt asteroids orbit the Sun in periods between 3 and 6 years. Like the planets, most asteroids follow direct orbits within 10 degrees of the plane of the ecliptic (The average plane of the Earth's orbit around the Sun) although in a more pronounced ellipses than the orbits of the planets. Other asteroids have very different orbits from these main belt objects and indeed some, the Atens group even cross the orbit of the Earth and are the subject of close scrutiny by some of the world's major observatories. Another group, the Trojans, orbits the Sun 60 degrees ahead and behind Jupiter in its orbit. Asteroids vary in size from the largest, the 933 km diameter Ceres, to objects much less than 1km wide. Ceres however has now been taken from the list of asteroids and classified as a "dwarf planet" along with Pluto. It was discovered on the first day of January 1801 by Giuseppe Piazzi and at first was thought to be a comet. Asteroids shine by reflected sunlight and although there have been several hundred thousand discovered only one, Vesta, is sometimes bright enough to be seen with the naked eye. Even though it is not the largest of these objects it has a surface that reflects sunlight better than any of the others. Despite the great number of asteroids so far discovered their total mass is less that of the Earth's Moon.
Jupiter
By far
the largest of the Solar System
planets, Jupiter is in fact 2.5 times
more massive than all of the other Solar System planets combined. Named
after the
chief god in Roman mythology it is the
undisputed senior member of the planetary family. A line of 11
Earths would be needed to equal
Jupiter's equatorial width and it would
take 1,318 Earths to fill its great volume.
It has an equatorial diameter of 142,985
km and a polar diameter of 133,718 km giving it a distinctly flattened
appearance. This oblateness is caused by
its rather swift rotational period which is 9 hours 50 minutes and 30
seconds at
the equator and 9 hours 55 minutes and
41 seconds at higher latitudes. This difference in rotation rate is
possible because the planet is made up
largely of gas. A small telescope brings into view some of the details
of
Jupiter's disk, dark belts of cloud
parallel to the equator and an
eye-shaped marking in the southern hemisphere known as the
Great Red Spot which was first observed
in 1665. This feature is an anticyclone 14,000 km wide and 40,000 km
long, large enough to swallow three
Earths. The spot however is not always red: most often it is pinkish
and
can sometimes fade to a light grey. The
face of Jupiter never looks the same twice adding to the intrigue of
this giant world. The key to Jupiter's
weather is the fact that it gives off
twice as much heat as it receives from the
Sun. The planet was hot when it formed
and still retains some of that heat today. This internal store of heat
drives the complex cloud systems of the
planet, keeping the Great Red Spot and its smaller relatives alive far
longer
than any storms on Earth with massive
flashes of lightning being recorded on Jupiter's night side by the
Voyager
space craft. The winds in the upper
atmosphere have been recorded at over 640 km/hour.
Jupiter orbits the Sun every 11.86 years
at an average distance of 778 million
km, over five times the distance of the Earth from the Sun. Its average
orbital speed is 13.06 km/second and
since its rotation axis is only tilted
3 degrees in relation to the perpendicular
of its orbital plane, it has almost no
seasonal variation. It is accompanied in its journey around the Sun by
at
least 63 moons, most of which are small
rocky bodies. The larger moons, Io, Europa, Ganymede and Callisto are
all
bigger than the Earth's moon and are
easily visible in even the smallest telescope. With a diameter of 5,262
km,
Ganymede is the largest moon in the
Solar System, even larger than the planet Mercury. These four moons
were
first seen by the astronomer Galileo
Galilei in 1609 and are referred to as the Galilean Moons.
The atmosphere of Jupiter is made up of
hydrogen (86.4 percent) and helium (13.3
percent) with the other 0.3 percent a mixture of methane, ammonia,
ethane, acetylene phosphine and carbon
monoxide. Minute traces of water vapour have also been detected. At
a depth of 1,000 km the hydrogen becomes
liquid and at a depth of 25,000 to 30,000 km it becomes metallic.
It is thought that a solid rocky core,
twice the size of the Earth, lies at the centre of the planet. The
ratio of
hydrogen to helium in the atmosphere
makes it almost identical to the Sun's atmosphere and if Jupiter were
eighty
times more massive it would have enough
internal heat and pressure to start the nuclear fusion process
occurring and it would become a star.
The planet also has a magnetic field
which is 19,000 times stronger than the
Earth's magnetic field and in 1979 the
space craft Voyager 1 detected a series of rings made up of small rocky
material
orbiting the planet.
Jupiter is normally quite easy to spot
when it is in the night sky as it
outshines everything but the Moon, Venus and on rare occasions the
planet Mars.
It can be spotted even under light
polluted city skies so the next time it
is in your neighborhood make an effort
to catch a glimpse of this king of the
planets.
The "JUPITER 2" program is a free download and will give you the positions of the Galilean moons for any date you set it for.
Saturn
The
ringed planet orbits the Sun every
29.46 years at an average speed of 9.64
km/second and an average distance of 1,430 million km, 9.5 times
farther than the Earth is from the Sun.
With a equatorial diameter of 120,537 km it is second in size only to
Jupiter
and is the sixth planet from the Sun.
Its polar diameter however is 10 percent smaller than its equatorial
diameter and
is the most oblate or flattened of all
the planets; on Jupiter the difference
is only 6 percent. The rotation rate at its
equator is 10 hours 14 minutes and at
higher latitudes, closer to the poles,
is around 10 hours 40 minutes. Saturn is
the least dense of all the planets and
is the only one to have a density less than water, about 70 percent in
fact. It
is often stated that if a large enough
cosmic ocean could be found, Saturn would float on it.
Its rocky core is embedded in a layer of
liquid metallic hydrogen 21,000 km deep over which lies a layer of
molecularhydrogen. Saturn radiates 2.5
times more heat than it receives from the Sun, a legacy of its birth.
The continuous release of interior heat
is responsible for generating convection
currents in the atmosphere and winds at Saturn's equator have been
recorded
at speeds of 1,800 km/hour. In 1990 the
Hubble Space Telescope recorded an enormous storm that erupted
near the planet's equator, an event
which happens roughly every thirty years during summer in the planet's
northern hemisphere. The atmosphere
consists of 96.7 percent hydrogen, 3 percent helium and traces of
methane,
ammonia, ethane, acetylene and water
vapor.
Through a telescope, Saturn appears as a
tranquil, ochre coloured disk, darker at
the poles and with some horizontal bands. It is surrounded by a
system of rings, orbiting in the plane
of its equator, so intricate and
dazzling that it is considered by many to be the
most beautiful sight in the Solar
System. The planet is tilted at an angle
of almost 27 degrees to the plane of its
orbit and at approximately 15 year
intervals the rings are seen edge on as viewed from the Earth and seem
to
disappear momentarily. Galileo first saw
the rings in 1610 but could not determine just what they were due to
the
primitive state of his equipment. It was
left to Christian Huygens in 1659 to discover their real identity. As
seen from the Earth, the rings look like
a continuous solid disk encircling the planet but are in truth made up
of
many smaller ringlets, each containing
millions of objects from tiny fragments of ice and dust to house sized
boulders. The rings are 63,000 km in
width but are thought to be less than 1 km thick. Material at the inner
edge of the rings orbits the planet
every 5.6 days while the material at the outer edge takes 14.2 days to
complete
the journey.
To date there have been 60 moons
discovered orbiting Saturn. The largest
of these is Titan, the second largest of all the Solar System moons.
With a
diameter of 5,150 km it is only slightly
smaller than Jupiter's largest moon Ganymede and larger than the planet
Mercury. Titan is unique among moons in
having a thick atmosphere mostly made up of nitrogen (90 percent) and
methane making up the majority of the
rest. Clouds of orange smog top the atmosphere which has a pressure on
Titan's surface 50 percent greater than
the Earth's atmosphere at sea level. The Earth and Titan are the only
two
Solar System bodies known to have liquid
bodies on their surface as recent spectral observations have confirmed
the
presence of liquid hydrocarbon lakes on
Titan.
Without a doubt, Saturn is a most
spectacular sight in any telescope of at
least medium size. It is well worth the effort to catch a glimpse of
this
marvelous object which in the past has
resulted in more than a few people
rushing off to purchase a telescope of their own
after a seeing Saturn in someone else's
instrument.
Uranus
Uranus is
an enigmatic blue-green world
which has only recently begun to give up
its secrets. It is the third largest of the gas giants and with a
diameter of
15,128 km is four times the size of the
Earth. The rotation rate of its axis has been determined at around
17.23
hours. It orbits the Sun every 84.1
years at a speed of 6.8km/second and at an average distance of 2.9
billion
km. With its rotation axis orientated
close to its orbital plane it, in effect
orbits on its side turning in reverse to its
orbital direction. This has very strange
consequences as during its 84 year orbital period each pole spends 42
years
in total darkness followed by 42 years
in sunlight. This odd tilt of its polar axis is a bit of a mystery but
is
thought to have been caused by the
impact of a large comet or asteroid sometime in the early part of its
history.
At its brightest Uranus should in theory
be detectable to the naked eye but only
appears as a faint star, so faint that it was never noticed by ancient
astronomers for whom the Solar System
stopped at Saturn. Discovered by William Herschel on March 13th 1781,
it
was the first of the planets to be
discovered telescopically. In 1977 astronomers watched as Uranus passed
in
front of a star. Unexpectedly they
noticed the star winked on and off a number of times both before and
after being obscured by the disc of the
planet. From this observation and others like it which followed
astronomers deduced that Uranus was
encircled by nine rings, so faint that
they are not visible through a telescope from the
Earth. Voyager 2 confirmed the discovery
in 1986 finding eleven rings and also taking the number of discovered
moons to twenty one. The planet is
thought to have a rocky core consisting mainly of iron and silicon
which is about the size of the Earth.
This core is enveloped in a thick layer of liquid ice and rock but
unlike
Jupiter and Saturn it lacks a layer of
metallic hydrogen. Its outer atmosphere
is made up of 84 percent hydrogen, 14
percent helium and the other 2 percent
contains methane, acetylene, hydrogen cyanide and carbon monoxide.
Telescopically Uranus shows a
featureless greenish disc which requires
at least a medium sized telescope to observe. It is however worth the
effort
to locate and observe this distant world
and wonder at the excitement that must have been felt by William
Herschel when he first began to realize
that he had discovered a new member of the Solar System.
Neptune
With the
reclassification of Pluto as a
dwarf planet, Neptune is now the
outermost of the Solar System's planetary family and the smallest of
the
gas giants. It was named after the
ancient Roman god of the sea, Poseidon to the ancient Greeks. With a
diameter of 49,492 km it is large
enough, if it were hollow, to hold thirty Earths. It orbits the Sun
every
164.79 years at an average speed of 5.45
km/second and an average distance of 4,504,300,000 km. This is
thirty times the distance the Earth is
from the Sun and is the most circular of all the planetary orbits.
Neptune's core is thought to rocky,
primarily consisting of iron and silicon surrounded by an atmosphere of
84
percent hydrogen and 14 percent helium.
The other 2 percent is methane with minute traces of carbon monoxide,
hydrogen cyanide, acetylene and ethane.
The planet radiates twice as much heat as it receives from the Sun
resulting in wind speeds of up to 2,000
km/hour being recorded in its upper atmosphere, the strongest
winds of any of the planets.
Of Neptune's eight satellites, Triton is
the largest and with a diameter of 2,700
km it is two thirds the size of the Earth's moon. It has a surface
temperature of -240 degrees C and in
places nitrogen gas shoots upward from vents for 8 km into the thin
atmosphere of nitrogen vapor before
drifting downwind to form dark plumes on the ground. Triton orbits
Neptune in
a retrograde manner, that is, in the
opposite direction to the planet's rotation. This probably suggests
that it
is a body that originated somewhere else
in the Solar System that has been captured by Neptune's gravity.
Because of this retrograde orbit,
Neptune's tidal forces are causing
Triton to move slowly closer to the planet. In
around 250 million years it will be
close enough to be torn apart by these tidal forces and will add a new
ring to Neptune's four existing rings,
discovered by Voyager 2 in 1989.
Unlike the accidental discovery of
Uranus, the existence of Neptune was
predicted in advance of its discovery. Observations showed that Uranus
was not
keeping to its predicted course and one
likely reason was that it was being affected by the gravity of a
yet unseen planet. In 1845 English
mathematician John Couch Adams calculated the new planet's position and
the following year a Frenchman, Urbain
Le Verrier also came up with the same prediction. Neptune was found
on September 23rd 1846 by astronomer
Johann Gall and astronomy student Louis d'Arrest at the Berlin
observatory using the calculations of Le
Verrier. It was less than 1 degree from its predicted position. It is
believed that Galileo may have observed
Neptune in December 1612 and January 1613 but did not recognize it
for what it was. His journals show that
he observed an object very close to Jupiter on both these dates,
recording that he noticed the movement
of this object relative to a nearby
star. It is entirely possible that he did
indeed spot the elusive Neptune as
modern astronomy computer programs set to these dates show the two
planets
in the constellation Virgo, superimposed
upon each other.
The Voyager 2 flyby of Neptune in August
1989 showed Neptune as a dynamic blue
world with white cloud features and a dark blue spot called the
Great Dark Spot, similar to the Great
Red Spot on Jupiter. Images from the Hubble Space Telescope show
that this dark spot has disappeared
since then but others have formed in the meantime. In earthbound
telescopes we see only a featureless
blue disc floating eerily against the background sky and although it
lacks
the impact on the senses that the
planets Jupiter and Saturn can provoke it is still worth the effort to
locate this distant world.
Pluto and the Dwarf
Planets 
Like Neptune, the existence of Pluto was predicted by the wandering positions of Uranus and Neptune. Its discovery in 1930 by young astronomer Clyde Taumbah lauded the precision of the astronomical calculations. This later turned out to be incorrect, as Pluto is far too small a body to effect the orbits of the 2 gas giant planets.
For the next 7 decades it was thought to be alone on the edge of the solar system. In 1995 Dan Brown discovered another similar body orbiting in the outer solar system. Since then many more similar objects have been discovered. So in 2000 the IAU International Astronomical Union decided to re-classify Pluto and all the larger of the new discoveries as Dwarf Planets. Public sentimentality protested against the demotion of 'poor old Pluto' but logic has prevailed and the IAU decision stands. This does not take anything away from Pluto at all. In 2015 the New Horizons spacecraft flew by Pluto after a 9.5 year journey and discovered that Pluto and its moon system are surprisingly varied little worlds.
Astronomers
continue to
find sizable objects in the Kuiper Belt.
The illustration shows some of the
largest bodies, including Neptune's
captured moon Triton.
The Earth's Moon has been included to
give an indication of relative sizes.
Data on the largest Dwarf Planets
134340 Pluto *, Diam 2,320 klms, Period 248.5 years, Mean Dist to Sun 29.7 to 49.3 AU, Mag 13.6, 5 moons.
136199 Eris, Diam 2,300 klms **, Period 557 years, Mean Dist to Sun 37.8 to 97.6 AU, Mag 18.8, 1 moon.
90377 Sedna, Diam 1,600 klms, Period 10 - 12 thousand years, Mean Dist to Sun 76.1 to 975.5 AU, Mag 20.4.
136472 Makemake, Diam 1,500 klms, Period 310 years, Mean Dist to Sun 38.5 to 53.1 AU, Mag 16.7, 1 moon.
136108 Haumea, Diam 1,436 klms, Period 283 years, Mean Dist to Sun 34.7 to 51.5 AU, Mag 17.3, 2 moons.
50000 Quaoar, Diam 1,250 klms, Period 288 years, Mean Dist to Sun 41.9 to 45.2 AU, Mag 18.5, 1 moon.
1 Ceres, 950 klms, 4.6 years, Mean Dist to Sun 2.7 AU, Mag 6.9.
90482 Orcus, Diam 950 klms, Period 245 years, Mean Dist to Sun 30.3 to 48 AU, Mag 19.1 1 moon.
20000 Varuna, Diam 800 klms, Period 283 years, Mean Dist to Sun 40.9 to 45.3 AU, Mag 19.9.
An interesting Fact... there are 7 moons in the Solar System, including our Moon, that are larger than Pluto.
* From
2009, larger dwarf planets
such as Pluto, Eris, Haumea and
Makemake are classed as "Plutoids".
** November 2010, a stellar
occultation event indicated that Eris
is now just slightly smaller than
Pluto.
Comets
Perhaps
the most mysterious and awe
inspiring of all sky spectacles are the
comets that reach naked eye visibility. These visitors from the outer
reaches of
the solar system have been observed
since antiquity and were quite often looked upon as omens that foretold
of
impending disaster. The comets that
return within periods of less than two hundred years are thought to
come from
the Kuiper belt, a region of space just
past the orbit of Pluto. The most famous of these is Halley's Comet
which
returns to the inner solar system every
76 years and is due again in the year 2062. The first recorded sighting
of this comet was in 240 B.C. which
gives astronomers observational data for 30 apparitions. The longer
period
comets come from a more distant region
of space called the Oort cloud which lies at a distance of around
100,000
astronomical units from the Sun. (An
astronomical unit is the distance from the Earth to the Sun,
149,597,870
kilometers).
Comets are often referred to as dirty
snowballs and are comprised mainly of
dust and frozen gasses circling the Sun in highly elliptical orbits. As
they
approach the Sun they begin to
sublimate, the outer part of the nucleus
of the comet turning directly from a solid
to a gas. The gas and dust is forced
away from the head of the comet by radiation pressure and by the solar
wind
which usually results in two types of
tails being formed, the ion tail and the dust tail. The ion tail,
consisting
of ionized molecules, points directly
away from the Sun and is typically blue in colour. The dust tail
contains solid
particles which reflect sunlight and is
more curved than the ion tail and lies back more in the direction of
the comets
orbit. These tails can be millions of
kilometers long and are what makes some comets such a great spectacle.
Comets that barely reach naked eye
visibility are a reasonably rare and
occur on average about every three or four years whereas bright comets
with
spectacular long tails are observed
roughly every ten years or so. Scientist now have the technology to
land a
spacecraft on the nucleus of a comet and
the Rosetta space mission will do so in the near future helping to
unlock
some of the mysteries of these curious
objects.
