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
C while the centre of its core experiences temperatures of 15,000,000 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
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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 to 30
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
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.
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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.
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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
457degrees 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 telescopes do 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.
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Just look down :-)
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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 100C and two weeks of darkness when the
temperature plummets to -170. 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
1500C 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. 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.
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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 or 46.
The most frequently seen seem to be the 22 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. The larger 46
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 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.
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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.
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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
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 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.
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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.
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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 molecular hydrogen. 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.
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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
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.
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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.
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* Nov 2010. The latest observations now indicate that Eris is ever so slightly smaller than Pluto.
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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
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
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
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