Our solar system formed from a giant cloud of
gas and dust like this one - the Orion Nebula.
Ago, the most remote planet, a- be little more than a giant
c-Its composition is similar to Ihacicomets, and iLs orbit is
-it different from that of the other planets Some rocky leftovers
remained
near the Sun. Many of them inhabit the asteroid belt, a zone
between the orbits of Mars and Jupiter The asteroids never formed
a planet because the gravity of nearby Jupiter kept pulling them
a~ Today, millions of asteroids
- some of them as big as moons
- probably inhabit the asteroid belt, with many more scattered
throughout the solar system.
The solar system has probably ~hanged little since the planets and
moons formed. The Sun
churns steadily along, converting hydrogen to helium in its core.
The planets have evolved some what as they radiated away the hear
left over from their birth, or as ring systems have come and
gone, but there have been no alterations in the solar system's
basic layout.
The most significant change since the solar system's birth is the
development of life. Earth is the only planet known to support
life. Our planet is at just the right dis tance from the Sun for
life; the temperature is warm enough for liquid water, and we
have a. thick, oxygen-rich atmosphere. Fossils ~mdicate that the
first one-celled organisms appeared on Earth at least 3.5 billion
years ago, and per- haps as early as 3.9 billion years ago. But
larget more complex life forms did not appear until about 540
million years ago.
Life might have evolved else where in the solar system.
Scientists have found evidence that rnkroscopic life might have
inhabited Mars about 3.6 billion years ago, although the finding
has not been confirmed. Liquid water once flowed across the
martian surface, and might still exist in underground reservoirs
there, raising a slight hope that Mars might yet be inhabited by
simple organisms.
Water might also exist on one of Jupiter's moons, Europa, beneath
a mantle of ice. Complex organic molecules abound on Saturn's
largest moon, ~tan, although temperatures there are so cold that
water would be frozen solid. And organic molecules exist in icy
comets, leadfng some to sug gest that comets brought the
"seeds" for life to Barth - and perhaps to other
planets and moons, too.
Eventuall~ our solar system - and whatever life still inhabits it
will undergo a traumatic change.
About five or six billion years from now, the Sun will deplete
the hydrogen in its core and begin a relatively quick march
toward its ulti mate fate as a white dwarf:
a stellar corpse no bigger than Barth no longer able to produce
nuclear ener~
Along the way, the Sun's core will first contract and grow hot
enough to convert helium into heavier ele ments, while its outer
layers expand and cool. The Sun will grow to many times its
current size, swallowing Mercury1 Venus, and perhaps
Earth in the process.
In other words, it will become a red giant - an old, bloated star
that is rapidly nearing the end of its life.
As the red-giant Sun expands, Its gravitational grip on its outer
layers will loosen. Pushed by the star's own radiation, gas will
stream into space at speeds of sev eral miles per second, forming
a hot, colorful "bubble," called a planetary nebula,
around the dying stan Bnergy from the Sun's core will illuminate
the bubble like a celestial light bulb, until the gas disperses
and the bubble fades from
view.
As the gas moves outward into space, it will sow the seeds of new
stars and planets throughout our celestial neighborhood. Most of
the carbon in the universe, for example, probably formed in the
hearts of red-giant stars, then was hurled into space as those
stars died. When new generations of stars form, they contain more
car bon (and nitrogen, oxygen, and other heavy elements) than
their stellar ancestors.
So when our Sun dies, it will not only put on a beautihil displa~
it will scatter the raw material for new stars, new planets - and
per haps new life - throughout our comer of the Milky Way
The Sun wilt end its tife with a colorful planetary nebula
- a glowing shell of gas that will recycle the raw
materials for new stars and planets into the Milky Way.
SUN
Our Sun is a star, just like the stars that twinkle in our
flitir sky. But because the Sun is much closer to Earth rhan any
other star, it appears very' large and
a}nght. Without the Sun's wlight and heat, there would
he no Life on Earth.The Sun is a huge ball of gases - moarly
hydrogen and hehum, the two simplest and li~resr chemical ele
ments- These gases are so hot that they make the Sun glow- The
Sun does not "burn" in the same way that a fire burns.
Instead, heat and pressure inside the Sun cause atoms of hydrogen
and helium to "fuse," or join together, producing
nuclear energy.The Sun and planets formed 4.5 billion years ago
from a cloud of interstellar gas- This cloud gradually conderesed
to form a "proto star" - a ball of gas that gets hotter
and hotter as it is squeezed by gravity. Finally, the temperature
at the heart of the young Sun reached IS million degrees
Fahrenheit (10 million degrees centi grade) - hot enough for
nuclear reactions to occur - and the Sun began to shine. Enough
hydrogen remains in the Sun's core to keep our star shining for
another five billion years or more.
The Sun consists of a series of layers- From outside to inside,
they are'
·Corona
The outer atmosphere
Although it is extremely hot, gas in the corona is spread
very thin, so the only time we see the corona is during a total
solar eclipse, when the Moon completely covers the Sun's disk.
·chromosphere The inner atmosphere
Huge flares and loops of hot gas often shoot into the
chromosphere, extending tens of thousands of miles above the
Sun's surface. These flares shoot electrical ly charged particles
into the solar system; when they reach Earth, the particles can
disrupt radio and televi sion signals and cause color- fri
displays kno~ as the "atirome" - the northern
and southern lights.
·PhoIosphem
The Sun's visible surface
Although it is very hot (about 10,000 degrees Fahrenheit;
5,500 centi grade), the photosphere is much cooler than the Sun's
inner layers. Cool, dark blotches called sunspots sometimes
appear in the photosphere. Mosr of these giant magnetic storms
are larger than our Earth- The number of sunspots increases and
decreases every 11 years, although astronomers are not sure why
that is so.
·Convective zone
The Sun's "mixing pot"
The process of convection
- the same process that makes a pot of soup boil carries energ,
from the Sun's core to its surface. Detailed pictures of the
photosphere show large bubbles of hot gas wefling up from deep
inside the Sun.
·Radiative zone
The first step outward
Energ;,' from the core "mdi ates" outward
through this layer of hydrogen and heli urn gas into the
convective zone.
·Corc
The Sun's nuclear power plant
Hydrogen inside the core is packed so tightly that indi
vidual atoms ram into each other, forming heavier heli urn atoms
and releasing energ,,' in the process. But this energy takes many
thou sands of years to make its way to the photosphere and out
into space.
If you Wentl to the Sun
The Sun is much too hot for anything to land on it, or even
get near it. If you could stand or float on the Sun's shifting,
glowing gases, you probably would not be able to move, because
the Sun's strong gravity would make you weigh thousands of
pounds; one arm would weigh as much as your entire body weighs on
Earth. From the surface, you would see huge flares of hot gas
shoot mg thousands of miles into space, and the Sun's entire
surface would seem to boil like a pot of hot soup.
MERCURY
Mercury is the planet closest to the Sun. It is
slightly larger than Earth's Moon, and looks very much like the
Moon, with ancient impact craters scarring its rocky surfrtce.
Mercury flies along in its orbit at 29 miles per second - faster
than any other planet-
Because Mercury is so close to the Sun, noontime temperatures
soar to 700 degrees Fahrenheit (370 C). But because it has almost
no atmosphere to hold in the heat temperatu?es plummet to almost
300 degrees below ero (-185 C) at night. This torturous cycle of
heat and cold cracks the rocks on Mercury's surface, and even
cracks the surface itself.
Despite the heat, astronomers found in 1991 that Mercury might
have sriall ice caps at its north and south poles. They might
have formed when icy comets slammed into deep craters at the
poles. The floors of these craters remain in perpetual shadow, so
the Sun cannot melt the cometary remains.
Mercury is a dense, rocky planer. Its core probably consists of
iron and nickle. As Mercury spins on its axis, the iron in its
core generates a weak magnetic field that surrounds the planet.
Por many decades, scien tists thought the same side of Mercury
always faced the Sun. But in 1965,astronomers discovered that
Mercury completes three full turns on its axis for every two
orbits around the Sun. Because of this motion, we see the same
side of Mercury each time the planet comes closest to Earth.
Of the nine known plan ets in our solar system, only Pluto is
smaller than Mercury. To compare the sizes of the planets,
imagine that Earth is the size of a baseball. On that scale,
Mercury would be the size of a golf ball.
Only one spacecraft, Mariner 10, has ever visited Mercury- It
made three pass- es near the planet in 1974 and '75-It photographed
half of the planet's surface, measured temperatures, and
discovered a weak magnetic field. American and European space
agencies are drafting plans for future mis sions to Mercury, but
none will take place before the next century.
Because Mercury orbits so close to the Sun, it never strays far
from the Sun in Earth's sky. Mercury is visi ble only for a
brief time before sunrise or after sunset. It looks like a bright
star shining just above the hori zon. It moves quickly from
morning sky to evening sky. It is named for the Roman messenger
god, who had wings on his heels to cart" him swiftlv from
Earth to the heavens.
If You Went To Mercury
Mercury's atmosphere has long since boiled away into space
because of the heat of the nearby Sun. There is no air, no water,
and nothing can gro~ on the planet's sur face. Astronauts will
need strong shielding to protect them from the heat and the Sun's
harsh radiation. But bulky spacesuits will feel light because
Mercury's surface gravity is only about one-third Earth1s.
The Sun would look almost three times as large as it does from
Earth. A single "day" on Mercury - the
interval from sunrise to sunset - lasts 88 Earth days, and the
night lasts 88 days more. When Mercury is closest to the Sun, it
moves so quickly that for a time the Sun moves backward in the
sk~
Mercury's surface is hard and rocky, pitted with craters, and
covered with a thin layer of fine dust. Huge cliffs tower as much
as a mile above the surrounding land.
Venus
Venus, the second planet from the Sun, is the hottest world in
the solar system. It is blanketed by a thick layer of clouds that
heat its surface like a greenhouse garden.
But with a surface tempera
·ure of almost 900 degrees Fahrenheit (480 C), this
place is no garden! Orh& than its tempera rure, Venus is so
similar to Earth that it is sometimes called Earth's ('sister
planet. Its diameter and mass are almost identical to Earth's.
Wh~ them is it so hot?
The "greenhouse effect" on Venus is caused by its
atmosphere. Energy from the Sun can get through the atmosphere to
the planet's suri~ice, but it cannot escape again into space. The
trapped heat builds up, mak ~ng the planer grow hotter
~ind hotter. The same thing happens in an autornobite on a hot
day. Sunlight comes through the windows and warnis the inside of
the car, but the heat is trapped. The inside of a car can get dan
gerously hot in just a few minutes. And Venus has been heating up
for billions of years.
Even though Venus' orbit brings it closer to Earth than
other planet, its blanket clouds kept much of Venus a mystery.
But space probes sent by the Soviet Union and the United States,
as well as studies with ground-based radar, have allowed
astronomers to tee the surface of Venus for the first time- The
first explo ration of Venus by radar was in 1962. It revealed
that Venus spins backward on its axis. If you could see the Sun
through Venus' cloud cover, it would rise in the west and set in
the east.
Space probes have revealed that the atmospher ic pressure at the
surface of Venus is 90 times that of Earth's - four times higher
than the highest pressure ever endured by humans. This atmosphere
consists mainly of carbon dioxide - the same gas thar puts the
fizz in soft drinks. It is not breathable. In addition, the
clouds of Venus produce drops of sulfuric acid, a poi sonous
chemical.
Daytime on Venus is about as bright as a cloudy day on Earth, and
the winds on the ground are gentle. The ground on Venus is flat
and lifeless, as you would expect from the great weight of the
planet's atmosphere. Gravel and flattened boul ders are scattered
over the plains. Because of the heat and pressure on Venus, no
probe landing there has ever survived for more than an hour
More recent radar obser vations by the Magellan spacecraft have
allowed sci entists to map almost the entire surface of the
planet. Magellan discovered moun tains on Venus that are higher
than any on Earth, as
well as a valley that is longer and deeper than the Grand Canyon.
It also revealed that the surface of Venus may contain active
volca noes, which occasionally spew great geysers of molten rock
and hot gas into the already steamy atmosphere.
If You Went to Venus
Because of Venus' heavy atmosphere, the planet's air pressure
is very high. Air pressure is defined as the weight of the air in
the atmosphere pressing down on you. On Earth, we don't notice
the air pressure at all. The thick atmosphere on Venus would make
it diffi cult to see objects very far away from you.
Since carbon dioxide is poisonous to humans, you would not be
able to breathe on Venus. Thick, high clouds would make the Sun
just a yellow-orange smear of light.
Although the gravity on Venus is about the same as on Earth, the
weight of the atmosphere would crush you. There are always
thunder- storms somewhere on the planet, and lightning flashes
about 25 times every second.
Although there are mountains and valleys on Venus, most of the
planet is fairly flat, with very low hills. There is some dust
and gravel, but no soil - only flat, broken rocks.
Earth
Of all the planers in our solar system,
Earth is the
only one that can support human life- No other planer has oceans
of water, an oxy gen-rich atmosphere, and intelligent life-
Earth is special in manv less dramatic respects, too. It is the
largest body ui the solar system with a solid zurSice and the
only planet with ongoing plate tectonics
- the slow but constant shifting of large, fairly rigid
plates on the planet's surface.
These two facts are linked, since Earth's solid surface locks in
the heat necessary to force the crust to move.
In other ways, however, Barr" is just an average planet. It
is bigger than four planets in the solar system and smaller than
the other iouL Two planets, Venus and Mercury, are closer to the
Sun, and the other six planets are lirther aw~
Earth formed about 4.5 billion years ago, and it is still
evolving. Earthquakes and volcanoes are violent proof of Earth's
shifting crust. Beneath this thin crust of rock - which aver-
ages about four miles (7 km) thick under the oceans and about 25
mdes (40km) beneath the continents - lies a 1,700-mile (2,700-km)
layer of denser rock called the mantle.
And beneath the mantle is Earth's core, made mostly of nickel and
iron. The inner part of the core is solid, but the outer part is
liquid. From its edge to its center! the core aicasures about
2,000 miles (3,220km).
Oceans cover more than two-thin- of Earth's surface. The first
signs of life on Earth appeared in the oceans about 3.5 billion
to 3.9 billion years ago. The first tiny organisms were similiar
to forms of bacteria still living today. The first life on land,
simple plants, developed around 400 mil lion years ago. The age
of the dinosaurs followed 200 million years later, and human
beings evolved less than one million years ago.
Above Earth's solid sur face lies its atmosphere. The atmosphere
has more oxygen today than it did when Earth formed. Microscopic
life in Earth's early oceans converted atmospheric gases into the
air we breathe toda~ While the early atmosphere was mostly car
bon dioxide, more than three-quarters of Earth's atmosphere is
now nitrogen, and most of the rest of it is oxygen.
Technology created by humans is another thing that sets Earth
apart from other planets. Thousands of artificial satellites
orbit our planet, enabling humans to communicate with one
another, monitor the weath er, and, sometimes, keep an eye on
other people.
While You Are on Earth
Gravity holds you "down" on the planet's sufface
and makes you feel heav>. The oxygen in the atmosphere allows
you to breathe com fortably, and the Sun keeps you warm in the
daytime - sometimes too warm! Even so, Earth is still by far the
most comfortable planet for humans.
A huge variety of living things populates Earth - plants, birds,
fish, mammals
- while not even one living organism has been found on any other
planet or moon in our solar syrem.
By watching the sky, you can see changes in Earth's atmosphere,
as clouds form and move, rain falls, and lightning strikes. Water
droplets in the atmosphere produce rainbows when sun light passes
through them at the proper angle. People near the north and south
poles can see fantastic dis plays of light called aurorac, caused
by energetic particles from the Sun trapped in Earth's magnetic
field.
In Earth's night sky, you can look out through the atmosphere and
see stars and galaxies many trillk)ns of miles away. You can see
the Moon, our own companion world. And you can look at all the
other planets in our solar system - interesting places to visit,
but not as nice to live on as Earth.
Moon
Earrit's Moon is unusual compared to the other moons in our
solar system. Except ~t tiny Pluto, all the other planers are
much larg & than their moons. But
Is less than four times than its moon.
Compared to huge planets like Jupiter and Saturn,
which are 40 or 50 times larger than their moons, our Moon and
Earth could Ialmost be considered twin
But Earth and Moon don't look like twins. The Moon has no
atmosphere, for example, and no water (althou~ instruments
Iaboard the Clementine spacecraft hinted that some ice - perhaps
the remains of comets that crashed into rile Moon - might hide
inside crriters at the Moon's poles)- And the lunar surface is
covered with craters - the scars from thousands of meteors that
struck it over btllions of years.
Earth's strace looks varied because Earth is geo logically
active. That means thar Earth's heat causes changes on its
surface - changes such as earthquakes and volcanoes. When a
meteor strikes Earth, it makes a crater, just as it would on the
Moon. But on Earth, wind, rain, and earth- quakes gradually erase
or fill in these craters. Since the Moon has no atmosphere, there
is no rain or wind to erode the craters. And because the Moon's
interior is not hot and active like Earth's, there are no active
volcanoes on the Moon. "Moonquakes" are millions of
times less powerful than earthquakes and have little or no effect
on the Moon's surface. So craters fonned by meteorites on the
Moon last a long time.
From Earth, we always see the same side of the Moon. This
"synchronous rotation" of the Moon is caused by the
same force that causes tides on Earth's oceans - gravity. The
Moon's gravity' pulls on Earth, and Earth's gravity pulls on the
Moon.
Although this gravita tional attraction is not enough to make
Earth and Moon crash into each other, it is enough to pull the
water in Earth's oceans slightly toward the Mo9n,
creating the tides. Earth's gravity pulls on the Moon so much
that it has slowed down the Moon's rotation on its axis. This
gravitation at pull "locks on" to the side of the Moon
we can see, making that side of the Moon face Earth all the time.
Although the same side of the Moon always faces Earth, this
doesn't mean that one side of the Moon is always dark. Remember
that the Moon, like Earth, gets its light from the Sun. Just as
the Sun always shines on half of Earth's surface, one- half
of the Moon receives light from the Sun at any given time.
Prom 1969 to 1972, six Apollo missions sent 12 astronauts to the
Moon's surface. The astronauts gath ered lunar rocks and soil and
brought them back to Earth. They also left behind four
"laser targets" - small reflective devices aimed toward
Earth. Astronomers at The University of Texas McDonald
Observ'atory and elsewhere use these reflec tors to determine the
exact distance between Moon and Earth. They do this by beaming
pulses of laser light to the Moon. The laser beam is so accurate
that it can hit its target, only about a foot across, on the
surface of the Moon, and bounce back to Earth.
If You Went to the Moon
Like the astronauts who went to the Moon, you would have to
carry your own air with you on the lunar surface. You would also
need a space suit to protect you from the Sun's intense
radiation, because the Moon' does not have an atmos phere to
block its hatmfrl rays.
Although your space suit might be heavy, you would not notice it
too much. You would feel lighter on the Moon, thanks to its weak
gravity.
Because there is no atmosphere on the Moon, the sky is black,
even during the day. Depending on where you landed, you might see
Earth shining in the sky.
MARS
Although Mars is smaller and colder than Earth, it is still
quite similar to our planet. It has a thin atmos phere and polar
ice caps, and dn riverbeds crisscross its sufface. And frozen
water might exist beneath the red marrian soil - perhaps pro
viding a home for singk celled hving organisms. But it is nor the
plariet described in science-fiction books and movies. There are
no signs of great civilizations - past or present - on its red
surface.
In August 1996,. though, scientists announced the dis covery of
possible microscop ic fossils in a meteotite from Mars. The
meteorite was blasted into space when a large boulder slammed
into Mars. The meteorite eventu ally landed in Antarctica. Trace
elements inside the meteorite prove that it came rwfrom Mars.
Evidence suggests that miSroscopic organisms - a thousand times
smalier than a human hair - hved on Mars 3.6 billion years ago,
when the planet was much warmer and wetter than it is today. But
the finding has not yet been confirmed by additional tests.
Early in the next century, a spacecraft may scoop up samples of
martian rock and soil and return them to Earth. Scientists will
try to select a landing site that would have been likely to
harbor Life in the past.
Mars looks red because iron in its surface soil long ago reacted
with the tinv amount of oxygen left on Mars, making rust.
The surface of Mars is marked by ancient volca noes and a
"grand canyon" that is as long as the United States is
wide.
The lagest volcano on Mars, Olympus Mans, may also be the largest
volcano in the solar system. It towers 17 miles (27km) above the
sutrounding desert - three times higher than Earth's highest
mountain, Mount Everest. The base of Olympus Mons would cover the
state of MLssouri.
The heat from the mart- ian core, which powered Mars' volcanoes,
is mostly gone now, and most of its early atmosphere has boiled
away into space. A few thin clouds still punctuate the martian
sky, and scientists think. that some pools of frozen or liquid
water mav lurk underground. Although it is unlikely, the water
could harbor simple forms of life similar to those found in
Antarctic ice on Barth.
It is possible that Mars once had an atmosphere somewhat like
Earth's. And it is almost certain that water once covered part of
the martian surface, in rivers, lakes, and perhaps small seas.
Although no water flows over the martian surface today, frost
covers the north and south poles. These polar ice caps are made
mostly of frozen car bon dioxide (dry ice),
although the north polar cap probably contains a fair amount of
water
Two small moons, Phobos and Deimos, orbit the plan et. The larger
moon, Phobos, passes across the martian sky from west to east
twice a dav. It would look about half as big as the full Moon
does on Earth. Deimos is farther away and moves slowly from east
to west. Deimos would look like a small dot of light in the sky
Phobos is slowly moving closer and closer to Mars. In another 100
mil lion years or so, it will crash into Mars.
II You Went to Mars
Mars is more like Earth than any other planet in our solar
system, but it is still very different. You would have to wear a
space suit to provide air and to protect you from the Sun's rays,
because the planeCs thin atmosphere does not block harmfiil solar
radiation. Your spacesuit would also protect you from the
bitter.cold; temperatures on Mars rarely climb above freezing,
and they can plummet to 200 degrees below zero Fahrenheit (-143
C). You would need to bring water with you, although if you
brought the proper equip ment you could get water from the air or
the ground.
Huge sandstorms occa sionally sweep over the plains, darkening
the entire planet for days.
Jupiter
Jupiter is the largest plan et in the solar system. In fact.
it is far more massive than all the other planers combined.
Jupiter is so large that in some ways it acts like
a small sun that never qutte reached maturity. Its core
may be as hot as 54,000 degrees Fahrenheit (30,000 C)- hear,
produced bil lions years ago when Jupiter first formed, slowly
escapes into space.
Jupiter is a gas giant) which means that its surface is not solid
like the surface of Ea~ We can see only the top layer of
Jupiter's atmosphere. which is a s~'irling pattern of clouds.
Tile clouds form bands of different colors, making Jupiter look
like a striped beachball. A vast ocean of liquid hydrogen might
lie beneath the clouds, with a core of metallic hydrogen -
hydrogen squeezed to incred ible~ densities by the weight of the
material around it.
Jupiter's internal heat may drive the "weather" in its
upper atmosphere. On Earth, weather is driven by heat from the
Sun. But since Jupiter produces more heat than it receives from
the Sun, it may be responsible for its own weather. A probe
dropped into Jupiter's atmos phere by the Galileo space crafr
found that conditions were dryer, hotter, and windier than
expected. But the probe might have entered a region of the
atmosphere that is different from most of the planet.
Dark, narrow rings of rocks and boulders encircle Jupiter1 but
thev are much less impressive than the rings around Saturn.
Jupiter has at least 6 moons. Jo, Eun)pa, Ganymede, and
Callisto are the largest. They are called the Galilean moons
after Galileo Galilci, who discov ered them in 1610 using one of
the first telescopes.
Jo is the most famous satellite because active vol canoes dot its
surface. They erupt frequently, spewing yellow and red sulfur
dust up to 180 miles (300 km) high. When Galileo first scanned Jo
in early 1996, it found that the moon's surface had been
repaved by volcanic eruptions since the Voyager probes visited in
the late 1970s.
Europa, the next farthest moon from Jupiter, is cov ered by a
thick layer of ice. Galileo found that vast seas of "warm
ice" or liquid water might exist below the crust, warmed by
the moon's inter nal heat. Some scientists and fiction writers
have speculated that exotic organisms might live in these oceans.
The next satellite, Ganymede, is the solar sys tem's largest
moon. Grooves and ridges crisscross its sur face, and a thin
icecap cov ers one of its poles Callisto, the Galilean
satellite farthest from JupLter, is an icy, rocky moon coveted
with craters.
II You Went to JuDiter Because Jupiter's atmos phere
contains hydrogen, methane, and ammonia, it is poisonous ro
humans, so you would not be able to breathe there. And because
Jupiter is so massive, the gravity is ven' strono - vou would
have a hard time moving. Also, Jupiter is a gaseous planet, so
there probably isn't any solid ground under its clouds for humans
to stand on. Oceans of hydro gen lie below the storms in
Jupiter's atmosphere. Terrible lightning and winds roar across
the ocean with out stopping.
Daytime lasts only five hours, but you would not know day from
night because sunlight cannot penetrate the thick clouds. The
pressure ofjupiter's atmosphere will crush any human or machine
that falls through the clouds; the Galileo probe, for example,
survived for only about an hour as it parachuted through the
atmosphere.
The temperature on Jupiter vanes widely, but it ranges from much
hotter to much colder than any place on Earth. Humans will prob
ably never visit Jupiten But it is possible that some form of
life may exist there, although it would be very different from
life on Earth.
ASTEROIDS
Astronomers have discov ered thousands of asteroids in our
solar system, and mil lions mote probably await discovery. Still,
the massof all the asteroids combined is less than the mass of
the smallest planet, Flu to.
Most asteroids lie between Mars and Jupiter, in a vast ring
called the Asteroid Belt.
But others inhabit different regions of the solar system. The
Apollo asteroids, for example, cross or come near Earth's orbit.
One of the Apollo asteroids may have caused the extinction of?he
dinosaurs when it collided with Earth 65 million years ago
Asteroids are made mostly of different types of rock, often rich
in iron and other metals, and some ice. Asteroids farther from
the Sun may have more ice in ;1'em. Since many meteorites found
on Earth contain simi jar materials, astronomers believe that
most meteorites are broken bits and pieces of asteroids that land
on Earth.
The largest known aster cid, Ceres, is 623 miles (1,003 km) in
diameter, and orbits roughly 260 million miles (420 million km)
from the Sun. But most asteroids are less than a mile across.
Italian astronomer Giuseppe Piazzi discovered Ceres in 1801.
Since then, asrronomers have found more than 6,000 asteroids.
Many asteroids are visible through a telescope or even
binodulars, but only one of them, called Vesta, can be seen with
the unaided eye in a dark sky. So far only two asteroids have
been pho tographed close up: Gaspra and Ida. The Galileo space
probe studied them white it was heading foi Jupiter The pictures
show lump~ cratered, potato-shaped rocks. Galileo found that Ida
also has a tiny moon, called Dactyl. Gaspra and Ida look like
Phobos and Deimos, the moons of Mars. Phobos and Deimos may have
been asteroids that were trapped by Mars' gravity.
Billions of years ago, when the planets were form ing, only a few
very large asteroids might have inhab ited the asteroid belt. The
strong gravitational pull of Jupiter prevented these
"planetoids" from clumping together to form another
planer. Instead, over time, the asteroids crashed into one
another and split apart into smaller and smaller fragments. Such
collisions occur even now, leaving dust and smaller asteroids in
the asteroid belt.
Astronomers launched the Near Earth-Asteroid Rendezvous (NEAR)
space craft in 1.996 to study Eros, the largest year-Earth"
asteroid. Its lopsided orbit brings Eros near Earth. NEAR will
enter orbit around the asteroid in early 1999
If You We'!' to an Aseroids
Because asteroids are very small, their gravity is weak, so you
would not weigh very much when you stood on one. You would feel
more like you were floating in space than standing on something
solid.
An asteroid would not look flat like the surface of Earth does.
Earth is so big that when you stand on irs surface you cannot see
that it is curved. On a small asteroid., however, you would see
that the rock drops away from you on all sides. Asteroids are
irregularly shaped and bump~ not smooth and round like larger
planets. On a small asteroid, you could "walk" around
your whole world in a few minutes The surface would be hilly and
pockmarked with craters, and there would be no air to breathe, no
clouds, and no water
Someday astronauts may go to asteroids to study them or even to
mine the metals and ores they contain. Certain asteroids that
come near Earth would be easier for humans to reach than the
Moon. Still, humans are not likely to visit an asteroid any time
soon.
Saturn
Saturn is the famous "ringed planet." Only Jupiter
is larger than this gas giant. Although Saturn is the sixth
planet from the Sun, it is clearly visible to the unaided eye as
a bright golden star Its are visible through a small telescope.
Measured from edge to edge, Saturn's rings span abota 600,000
miles (one million km), or two-and-a- half times the distance
from Eartlito the Moon.
ISaturn's rings are made of ice and rock. They are not solid, but
consist of small chunks of frozen material in a layer just a few
thousand feet thick. They may have formed with Saturn itself, or
much later when some of its moons collided or were struck by a
comet or asteroid splitting into the frag ents we see today. In
iact, astronomers may have discovered several new rings in the
making in 1995, when the rings were turned edge-on to Earth.
Hubble Space Telescope found sever al "clumps" of
material near the outer edge of the rings. These clumps might be
for mer moons that were shat tered by recent impacts and are in
the process of spread ing out to form new rings.
Saturn is not a very dense planet; that is, the materials that
make it up are not as tightly crushed together as on other
planers. Saturn's densit~ in fact, is lower than that of water,
so the entireplanet would float if it could be dropped in waten
It is made mostly of hydrogen and helium, but, like Jupiter, its
atmosphere contains methane.
Although the cloud fea tures on Saturn ate not as striking as
those onjupiter, they do change. Winds caused by Saturn's
rotation force all the clouds into hor tzontal bands circling the
planet. At the equator, these winds reach speeds of 1,100 miles
(1,770km) per hour.
Saturn has more moons than any other planet in the solar system -
at least 20 have been discovered so fan Like the'moons of
Jupiter, Saturn's moons each have unique characteristics. For
example, ~tan - which is larger than the planet Mercury - has an
atmos phere of nitrogen, and may be covered by vast lakes or
oceans of liquid methane.
Another of Saturn's moons, lapetus, is bright and icv on one side
and dark and dusty on the other And one small moon, Mimas, was
once hit by an asteroid that almost broke the moon apart.
Enceladus, the moon next out after Mimas, has a bright icy
coating. Volcanoes may periodically belch ice and water from its
interion
If You Went to Saturn
Like all the gas giants, Saturn is not a very appeal ing
place for humans to vtsit. The hydrogen atmos phere is not
breathable, and the terrible winds that stir the atmosphere would
cer tainly tear apart any living thing.
If you could get through Sanim's clouds and suivive the crushing
atmospheric pressure, you would find a world without any solid
land. An ocean of liquid hydrogen, thousands of miles deep,
completely covers the solid core of Saturn. Liquid helium would
constantly splash into the ocean from the cloudy sky. You could
not see the Sun or Saturn's beautiful rings.
Saturn's moons, while very cold, would be a little more
hospitable. Though none has an atmosphere you could breathe, they
are solid, like Earth. Prom one of Saturn's moons, you could see
the Sun as a tin~ bright disc. And depending on where you stood,
Saturn itself might loom overhead a giant neighbor in the sky.
URANUS
William Herschel discovered Uranus in 1781.. While the planet
is sometimes barely visible to the unaided eye, no one realized it
was a member of our own solar system until Herschel plotted
its motion across the sky Many early skywatchers actually
recorded Uranus on star charts as a star! The planet was named
after the father of Saturn in Roman m',Tholo~.
Like Jupiter and Saturn, Uranus is a gas-giant world. Its clout-
aunosphere, which is made mostly of hydrogen and helium, appears
smooth.
Uranus is unusual in sev eral wa's. Unlike any other planet in
our solar system, it spir~ sideways. It may be that long ago
Uranus was hit by some large object that knocked the huge
planet on its side. Uranus also rotates
in the opposite direction from most of the other plan ets. Also,
Uranus' magnetic field has a strange wobble rather than being
lined up with its north and south poles.
Five large moons and many smaller ones orbit Uranus. Their
orbital paths are 'tsideways" compared to other
planets and moons because of Uranus' sidewavs spin-
The largest moon, Titania, measures 1,000 miles (1,610km)
in dianie ten One moon, Miranda, is so scarred with craters and
fractures that some scientists say it may have been smashed apart
at one time, then formed into a moon again afre~'ards. Cettainly
Miranda is one of the oddest moons in the solar system.
In addition to its moons, Uranus is also encircled by very thin
rings, similar to those around Jupiter However, the rings of
Uranus are dark and cannot be seen from Earth except indirectly
as the light from a distant star appears to fade in and out as
Uranus an4 its rings pass in front of the star Voyager 2
photographed the rings of Uranus and. con firmed a ring svstem
around Neptune, making it clear that all the gas giants have
rings around them - not just Saturn.
If You Went to Uranus
Day and night on Uranus would be very different from what we
experience on Earth. At the north or south pole, "winter"
would last for 42 years, and the Sun would not be visible at
all during thar rime. The Sun becomes visible again at the
"spring" equinox and remains in the sky for another 42
years.
Of course, you probably wouldn't see the Sun at all if you were
below Uranus' clouds. You could not breathe on Uranus, either,
because its atmosphere is poisonous. And you couldn't stand on
the surface, because there isn't one. Instead, the atmosphere
simply gets thicker and thicker, until it changes from gas to
liquid.
If there is a solid core to Uranus, it lies far below a deep
"ocean" of ammonia and methane. While the sur face graviry
on Uranus is not too different from that on Earth, the weight of
its thick atmosphere would make it very hard to move - in fact,
you could not survive in it.
NEPTUNE
Neptune is the fourth and outermost gas giant in our solar
system. It was discovered in 1846 after astron omen reafized that
a heavy, unknown object was affect mg the orbit of the planet
Uranus. They began looking for another planet and found Neptune.
Neptune and Uranus are so similar that they are somerimes called
twins.
Neptune moves so slowly in irs orbit around the Sun that it has
not completed a single orbit since it was dis covered.
Voyager 2, an American space probe, confirmed the presence of
several thin rings around Neptune when it flew past the planet in
1989.
Neptune has at least eight moons: Naiad, Thalassa,Despina,
Galatea, Larissa,Proteus,Triton, and Nereid.
Triton, the targest, is one- third as massive as Earth's Moon.
Triton is also the most unustial moon, hecause it orbits Neptune
in the oppo site direction from all the other satellites. Frozen
methane and nitrogen on Triton's surfi~ce reflect much of the
sunlight that reaches the satellite. This fact, com bined
with Triton's great dis tance from the Sun, makes the moon very
cold. In fact, with a surface temperature of about 435 degrees
below zero Fahrenheit (-260 C), Triton is even colder than Pluto.
There is also evidence that water and methane vol canoes erupt on
Triton's sur face. Because of its strange orbit, astronomers
think Triton may have been cap tured by Neptune instead of
forming at the same time as the planet itself Many mil lions of
years from~now, Triton's orbit will earn it too close to Neptune,
and the planet's gravity will pull the moon apart. Triton's
remains will scatter along its orbital path, forming rings as
bright as Saturn's.
Neptune's atmosphere is mostly hydrogen. Methane gas in the upper
part of the planet's atmosphere gives Neptune a beautiful blue
color. Streaks of bright white clouds cross the blue atmos phere.
Several large storm systems have been spotted on Neptune. One
notable storm, called the Great Dark Spot, vanished from the
southern hemisphere at about the same time that a similar storm
appeared in the north.
Neptune's thick atmos phere may conceal a deep ocean of water and
other liquids. A rocky core may lie beneath the ocean.
II You Went In Neptune
If you could survive on Neptune's liquid surface, under the
weight of its heavy atmosphere, you would not find it a very
friendly place. There is little or no oxygen to breathe, but
there is a great deal of methane and ammonia, both of which are
poisonous to humans. Winds rip through the atmosphere at hundreds
of miles per houn
Any solid ground would lie 5,000 miles (3,000 km) below you,
beneath the ocean. You could not see the Sun, stars, or any of
Neptune's moons through the thick cloud coven
Neptune's moons arc even colder than the planet itself. Triton's
surface is cold er than any other planet or moon in the solar
system. Through Triton's thin atmosphere, the Sun would look more
like a bright star than the large disk we see from Earth. Giant
blue Neptune would loom above you much of the time.
Pluto
Although it was the Ian of the nine planets in our
solar system to be discov ered, tiny Pluto is not the farthest
planer from the Sun right now. Because of Pluto's unusual orbit,
Neptune will be farther from the Sun until 1999. Pluto1s
orbit is more elliptical, or stretched our, than the orbit of any
other planet. On average it lies 39 times farther from the Sun
than Earth does.
Pluto is the smallest planet - smaller even than our own Moon.
Astronomers searched for a ninth planet during the 192Os
because they thought something was tugging at the giant planer
Neptune. In 1930, Clyde Tombaugh, a young astronomer at the
Lowell Observatory in Arizona, found Pluto. But astronomers soon
realized yhat Pluto is far too small to affect Neptune.
Recent cal culations, in fact, show that nothing is pulling at
Neptune at all; astronomers had simply miscalculated its
mass, so its orbit is slightly different from what they expected.
Because iris so unlike the other planets, some people wonder if
Pluto should be considered a planet at.all. Its orbit crosses
Neptune's path and is tilted compared to the other planets. But
it proba bly formed by processes simi lar to the other planets
and was pushed into its peculiar orbit by Neptune.
Pluto consists of ice and rock, although scientists are not sure
which is more abundant. There is a small amount of frozen methane
as well. Astronomers have also detected a very thin atmos phere
of primarily methane gas. The atmosphere forms when Plutots
lopsided orbit carries it closest to the Sun, and solar heat
vaporizes some of the ice on its sur face. As Pluto
retreats from the Sun, the gas in its atmosphere will freeze
again, adding a layer of fresh frost to the frozen planet.
Like Earth, Pluto has only one moon. But ins moon, Charon, is haW
the size of Pluto.
Scientists may launch a small,. fast probe toward Pluto
early in the next century.
The search for new plan ets has not ended with the discovery of
Pluto. Although a tenth planet at least as massive as Uranus or
Neptune would probably have been found by now, some astronomers
believe several planets the size of Pluto or Charon may be hid mg
on the outer fringe of the solar system. Recent dis coveries
indicate there are millions of icy, rocky objects orbiting in the
Kuiper Belt, a vast region that extends beyond Neptune. Perhaps
other worMs lie undiscov ered in the darkness beyond Pluto and
Charon.
If You Went In PLUT0
You could see Charon from only one side of the planer,
because the moons orbit around Pluto takes just as long as one
day on the planet. Thus, the moon always remains "over"
the same spot on Pluto's surface, and the same side of the moon
alwavs faces Pluto.
Surface temperatures right now hover at abt)ut 400 degrees below
zero Fahrenheit, although the planet will get colder as it moves
farther from the Sun. The Sun is so far away it would look
like a bright dot in the sky - the hrightest star visible.
Pluto3 atmosphere is much thinner than Earth's, and it contains
little or no oxygen. You couldn't breathe there. But you would
feel very light, since Pluto's gravitational pull is less than
one-tenth that of Earth's. The ground around you might look like
a dirty ice-skating rink - icy and hard.
Comets
Comets are solar-system leftovers that spend most of their
lives far beyond the realm of the planets. We know about them
because a few periodically visit the inner solar system,
wherethey can decorate our night skies in spectacular
fashion.Comet Hyakutake put on a beautiftil display in early
1996, and Comet Hale-Boppmay repeat the show in 1997.Comets often
are dc scribed as "dirty snowballs" lumps
of frozen gas and rc)ck. Their orbits are long and narrow, taking
them far from the Sun most of the time.As a comet nears the Sun, it
warms up. Ice evaporates from irs surface and forms a
larg, tenuous cloud called a coma. Solar energy and a steady
flow of electrically charged particles called the solar wind
blow material from the coma away from the Sun, forming a long,
glowing tail.
Comets probably formed at the same rime as
the Sun and planets, 4.5 billion years ago. But most
of them were Iticked far from the Sun by the powerful gravity of
the outer planets. Astronomers suspect that as many as one
trillion of these objects reside in a vast shell, called the Oort
Cloud, that extends as much as a tight-year from the Sun.
Because comets are so tiny, though, no one has ever seen a
cometary body inside the Qort Cloud.
Billions 'note of these icy bodies (which technically are called
comets only when they come close enough to the Sun to sprout
tails) orbit the Sun in the Kuiper Belt, which begins just beyond
the orbit ofNepnine. Astronomers have discov ered more than a
dozen giant comets beyond Neptune, and perhaps a cou ple of dozen
smaller ones.
English astronomer Edmund Halley was the first person to suggest
that comets are members of our solar system. Halley
thought that several of the bright comets recorded long before he
was born might really be the same comet approaching the Sun at
irregular inter vals. He realized that a par ticularly bright
comet was seen once every 76 years or so. It was recorded in
1531,1607, and 1682.
Halley predicred the comet would appear again in 1758. When it
did, Halley's theory was proved correct. Comer Halley was named
in his honor. It last approached the Sun in 1986, and will return
again in 2061.
Several countries sent probes to study Comet Halley in 1986. In
particu lar, the European Giotto spacecraft photographed the
comet from close range. It found that Halley is about 10 miles
(16km) tong and five miles (8 km) wide, and is coated with
organic mole cules that make Halley's sur face darker than
charcoal.
Giotto's images showed 'lets" of gas boiling off the comet's
surface. These jets can be powerful enough to change a comet's
orbit.
Many comets have slammed into Earth's surface during our planet's
history, causing global destruction. in 1994, Comet Shoe-
maker/Levy 9 rammed into Jupiten Jupiter's powerftil gravity
pulled the comet apart, so the planet was pelt ed by almost two
dozen impacts, which created huge "scars" in Jupiter's
cloudtops.
Astronomers hope to learn more about comets through two
spacecraft. An American probe, Stardust 1, will pass through the
tail of Comet Wild 2 in 2004. It will catch comet dust on a
sticky paddle and bring the material back to Earth. Later, a
European spacecraft, Rosetta, might land on the surface of a
comet and return with a chunk of its icy crust.
It You WOU to a Comet
If you stood on a comet as it approached the Sun, it
might vanish beneath your feet! Comets are giant snow balls, but
as they approach the Sun, some of their ice vaporizes. This gas
is expel led from the comet in high- speed 'lets" that also
carry' tiny bits of rock. A bit of Halley crippled the Giotto
spacecraft when it flew through the comet's tail in
1986.
METEORS
Meteoroids are small pieces of dust and rock - most the size of
peas or smaller - that litter the entire solar system. Some of
them end up as stars," or meteors, burning up with a flash
as they rush through Earth's atmosphere
Most of the meteors we see in the night sky come from comets,
which leave trails of dust and debris behind as they orbit
the Sun. Meteoroids also can be bits of rock or dust
broken off of asteroids, or fragments of material flung off from
plan cr5 and moons that are bit by large objects.
A few large meteoroids make it all the way to the ground;
they are called mete orites- Hundreds of tons of meteorites reach
Earth's sur face even, year, but most of them are extremely small
bits of rock. Far more mete ors burn up in Earth's atmos phere.
Most meteorites contain nickel and iron, are attracted by a magnet,
and are very heavy for their size. They formed around the same
time as Earth - 4.5 billion yeats ago. Because
meteorites have survived almost unchanged since the begin ning of
the solar system, they ate useful in understanding what
conditions were like at that time.
Large meteorites can leave craters on Earth's surface, like the
craters on the Moon, but few are visi ble today. Barringet Meteor
Crater in Arizona is proba bly Eaith's most famous craten though
not its largest-
In ~908 a large object vaponzed just above the ground in Siberia,
devastat mg the forest for 25 miles (40 km) around and knock mg
people to the ground 50 miles (80 km) away. Bun even larger
objects have struck Earth in the past.
For example, evidence of a huge meteor impact in the Gulf of
Mexico may explain the extinction of the dinosaurs 65 million
years ago. Some scientists believe the explosion caused when this
object struck the surface changed the climate of the entire
planet- A massive cloud of dust and soot would have blocked the
Sun, plunging much of Earth's surface into perpetual night, The
impact also could have created a global acid rain. These factors
might have wiped out many plant and animal species, eventually
leading to the extinction of the dinosaurs.
Meteor showers occur when many meteors appear in the sky in a
short time- They are caused by dust and small particles left
behind by comets. When Earth passes through the trail of dust
left by a comet, many bits of material enter our atmos
phere and burn up. Meteor showers occur at the same time each
year, as Earth reaches those places in its orbit where comet
debris is concentrated. When a comet has recently retl]rned to
the inner solar system and left a fresh trail of mete oroids
behind it, we might see a meteor 'storm" - thousands of
shooting stars brightening the sky in just a few minutes.
To view a meteor Shower
The best time to look for meteors is after midnight, when the
night sky is turn mg into the path of Earth's orbit. Although
meteors in a meteor shower will seem to "radiate" from
a specific part of the sky, they can actually appear anywhere.
Give your eyes about 20 minutes to adjust to the darkness, and
bring a chair or blanket outside with you so you can get
comfortable. Keep your eyes on the sky, but don't look too hard
or too long in any one direc tion - you have to be ready to turn
and look if you see a meteor out of the corner of your
eye.
GETTING TO KNOW THE SOLAR SYSTEM
BECOME A SKYWATCHER
Stargazing often intimidates begin ners because the sky itself
is so complex- It's divided into 88 constellations, most
of which are visible from the United States at different times of
the year- At any given moment on a clear, moonless night, a
thousand or more tars are visible. Throw in five planets,
a few star clusters, a spiral gaIax~ and the ocld bright comet,
and the catalog of objects visible to the unaided eye is
truly impressive - and overwhelming.To bring this complexity down
to Complexity down to earth, veteran stargazers suggest that you
begin your skywatching adventures indoors with a good skywatching
guide or starchart.
The only instrument that most beginnas need is their eyes. Our
eyes can detect subtle variations in bright- ness, determine the
colors and relative temperatures of stars, and track the motions
of the Moon and planets.
Finally you need a good viewing spot. Start with your back yard.
Then, as you gain experience, escape from
city lights into the darkness of the countryside. Find a state
park or other
safe, accessible spot. And try a night without moonlight, so you
can see meteors and the Milky Way.
As a next step, try a pair of binocu lars, which provide a clear
view of the moons of Jupiter and several star clus ters. They
allow you to see double stars that blur together when viewed with
the unaided eye. They offer sharp views of the mountains and
craters on the Moon-
You can find good binoculars at most department or discount
stores. Start wtth a pair of 7x35s or 7x50s (the numbers rate
their size and rnagnifica non). A good pair will cost from about
$50 to $3O0 or more.
A Skywatcher's Checklist
Get permission from an adult. Better yet, take one along
Wait for a night that is clear and dark. You can see more stars
when the Moon is not shining brightly.
Get away from streetlights. The farther you are from lights, the
more stars you can see.
< Give your eyes about 20 minutes to adjust to the dark.
You can get even more out of sky- watching by taking along a sky-
watching kit:
A blanket
A jacket
Binoculars
A simple star chart
A flashlight to read the star chart. Cover the lighted end
of the flash-
light with red'paper because red light does not disrupt your
night vision as much as white light does.
An observing log to help you keep track of the objects you see in
the pight sky. Keeping a record will help you see how the night
sky changes over time.You can make an observing log like this:
What to look for
What you see in the night sky depends on your location, the
sea son, and the time. A star chart can tell you what stars are
visible from your location.
Planets orbit the Sun, so they change position when compared to
the stars. A stat chart will show you where to look for planets
that are visible with the naked eye. The planet Venus, sometimes
called the "evening star" or "morning star, [S the
second-brightest object in the night sky - only the Moon is
brighten Jupiter, Mars, and Saturn often are visible later at
night than Venus. Planets reflect a steady light, unlike the
"twinkling" stars, which are much farther from Earth.
The Moon is the most prominent object in the night sky. One-half of
the Moon is always lit by the Sun, but we can't always see
the entire lighted part, which is what gives the Moon its phases.
Most calendars show Moon phases, and many daily newspapers tell
you when the Moon will rise and set.
MODEL THE SOLAR SYSTEM
True experiments in astronomv are difficult to perform because
the objects you want to study are mil lions of miles awan
Astronomer'; learn about the universe bv obsen mg stars and
planets, which is diffi cult to do in daytime and without
expensive equipment- However, you can perform demonstrations to
help you learn more about how the uni verse works-
materials:
A bright lamp, to represent the Sun
A globe, to represent Earth A baseball or orange, to represent
the Moon
What happens 'luring an eclipse?
There are two kinds of eclipses:
solar and lunat. A solar eclipse hap- pens when the Sun,
as seen from Earth, is "covered up" by the Moon. A
lunar eclipse is when the Moon passes through Earth's shadow.
This demonstration will show you how both kinds of eclipses
happen.
Put the larrip in the middle of the floor at eye level, and turn
it on- Turn out the other lights in the room. Set the globe on
the floor where it can receive light from the "Sun."
To create a solar eclipse
Hold the "Moon" so that its shad- 6w falls on
Earth. If you were stand ing on the parr of the globe where the
shadow falls, you would not be able to see the Sun, because the
Moon would be in the way.
To create a lunar eclipse
Move the Moon "behind" the globe, into its shadow.
If you were on the dark side of the globe, you would see the Moon
become very dark as it passed into Earth1s shadow.
Lunar eclipses occur about two times a year.
To create phases of the moon
The Moon looks a little different every night. Its shape
seems to change because of the way the Sun
lights the Moon's surface as the Moon orbits Earth. We call these
changes the phases of the Moon. This demonstration shows you why
they happen.
Put the lamp in the middle of the room and turn off all the other
lights. You will be Earth, and the baseball will be the Moon.
Face the "Sun" and hold the "Moon" in front
of you. The lamp will light up the side of the ball you can't
see. You will see only the dark half of the ball. This is what
hap pens when the Moon is between Earth and the Sun. We call this
phase of the Moon a new Moon. Wlien the Moon is new we cannot see
it in the night sk~ But the Moon is not all dark. One half of it
is lit up by the Sun - we just can't see it from Earth.
About a week after the new Moon, the Moon has moved one- quarter
of the way around Earth. Keep facing the Sun and hold the ball in
your left hand. Hold your Left arm straight out by your side.
You will be able to see half of the lighted surface of the
Moon. This is called a first-quarter Moon, because the Moon has
traveled one-quarter of the way around Earth.
Now turn your back to the Sun and hold the Moon in front of you -
Make sure you hold the Moon up high enough so that it receives
light from the Sun. This is a full Moon when we see all of the
lighted half of the Moon. The Moon has now gone half way around
Earth.
What if you hold the Moon lower, so that it is in your shadow and
does not receive any sunlight? That is what happens during a
lunar eclipse, when the Moon passes through Earth's shadow.
Finall~ turn to face the Sun again. Hold the Moon in your right
hand this time, and hold your right arm straight our to your
side. This is a third-quarter Moon, because the Moon has traveled
three-quarters of the way around Earth and is about to complete
one full orbit. Then it will be a new Moon again.
CREATE YOUR
OWN UNIVERSE
Throughout history, people have created stories to explain
what they see in the sky Here are tales about our solar system
from different parts of the world. Study these legends for clues
about the cultures that told them. You can learn how earlier
cultures viewed the world by reading the stories they told.If you
lived in a world without tele scopes and science books, how would
you explain the solar system? Create a sky legend of your own
describing Moon phases, or other aspects of the solar system.
Meteors, or "shooting stars," are tiny pieces of dust
and rock that burn up as they fall through Earth's atmosphere. An
American Indian tribe in the Pacific Northwest Great ed this
legend to explain them:
Coyote loved to dance beneath the stars from dusk until da'wn.
One nitit he said to the stars, "You are so beautiflil and
dance so divinely. May I come up and dance with you?" The
stars agreed, and Coyote made a ladder by firing arrows into the
sky so he could climb up to the heavens.
Coyote's friends were jealous, many of them climbed up the ladder
afrer him. The coyotes dance amo?1g the stars all night long, but
whenever a coyote gets tired, he falls back to Earth, creating a
bright, glowing streak in the sky.
The Rabbit in the Moon
(As told by different cultures)
The dark areas that scar the sur face of the Moon are large
"seas" of volcanic rock. They formed more than three
billion years ago, when giant asteroids punched holes in the
Moon's cr'ust, allowing hot lava to bubble to the surface.Almost
every culture has seen a "picture" in these dark
markings. For Irnan~ it was the "man in the Moon others,
including the Aztecs, it was a rabbit. According to one Aztec
tale, Sun and Moon were born when the gods gathered at
Teotihuacan to create a new cosmic age. Four previous ages had
ended in destruction.
Crie god sacrificed hirnself upon a flineral pyre, and was
tnansformed into the Sun. A second god followed and became the
Moon. After a long wait, Sun and Moon both rose in the east, but
they hovered near the hori zon. And the Moon shined as bright
lyas the Sun.
To set the cosmos right, the other gods sacrificed themselves,
too. But just before their deaths, one of them flung a rabbit
upon the Moon to darken its face. You can still see the rabbit
today as a dark silhouette on the face of the Moon.
The Venus Gold
(From Mesoamerican myths)
Because it's so bright, and because it moves back and
forth from evening sky to morning sky, Venus has played an
important role in the religion and skyl ore of many different
cultures.
To the Toltecs, Maya, Aztecs, and
other cultures of Mexico and Central America, Venus represented
the most important of all the gods. He was known by several
names, includ ing Quetzalcoatl. According to mythology, he
brought civilization and culture to his people.
The legend of Quetzalcoatl includes death and rebirth, so it's
not surprising that he was associated with Venus. After a lengthy
appear ance in the evening sky, Venus drops from sight entirely
for a few days. It's as though the planet died and passed through
the underworld. But about eight days later, it's
"reborn" when it reappears in the morning sky.
Következõ lap!!