Silver City's Sidewalk Solar System starts on Swan Street next to Fox
Field. Go to Swan and 32nd and walk South on the sidewalk to the first
power pole (50 yd line).  You will see an 18 inch plaque of the Sun in 
the sidewalk.  This is the start of your journey.

Look above you and you will see a second "Sun" on the power pole.  This 
scale model has the same scale (3 billion to 1) for both the distance 
and the size dimensions.  As you walk south to encounter each planet 
plaque, look back at the "Sun".  It will appear the same size, as if 
you were on the real Planet looking at the real Sun.

Normal walking speed equals ten times the speed of light in this scale
model.  If you were on the 'Starship Enterprise' in 'Startrek', you 
would be traveling at Warp 2.

This project was created by Bill Neely of NF/ Observatory, Ltd.  NFO is an 
Educational Non-Profit Corporation, founded to further astronomical
education and research.  Gina Moore, a student at WNMU, did the layout and
the first draft of the plaque design in her semester in Physical Science
485, taught by Neely.  Michael Metcalf, an Art Professor at WNMU, made 
several artistic suggestions, and helped Neely cast the project in bronze
in 'Sculpture 2'. Tom Turner, the San Man, cemented the plaques into the
sidewalk.  Funding came from the Phelps Dodge Corporation and "Project Astro"
(a program of the Astronomical society of the Pacific, funded by NASA 
- pairing astronomers and teachers in the classroom).

This guide was copied, with some editing, from Bill Arnett's solar system
pages on the World Wide Web.  There are links to the Arnett's original 
pages (which have lots of extra information) from our Web Site.  Go to to find this guide, pictures of the plaques, and
a map of the project.



Project Astro (


Bill Arnett's Pages(

You may want to print this guide to help learn more about our solar system
as you walk along.

Our Calculations


Scale = 3 billion to 1

 Planet  A.U.'s from Sun  Kilometers from Sun  Miles from Sun Scaled distance in Meters   Scaled distance in Miles;  Scaled distance in Feet
 Mercury  0.387  57.9 million  36.0 million  19.3  0.012  63.36
 Venus  0.723  108 million  67.2 million  36  0.022  116.2
 Earth  1  150 million  93.0 million  50  0.031  163.7
 Mars  1.52  227 million  141 million  75.7  0.047  248
 Jupiter  5.2  778 million  483 million  259  0.161  850
 Saturn  9.54  1.43 billion  887 million  477  0.296  1563
 Uranus  19.2  2.87 billion  1.78 billion  957  0.595  3142
 Neptune  30.1  4.50 billion  2.8 billion  1500  0.932  4921
 Pluto  39.4  5.89 billion  3.66 billion  1963  1.220  6442


Scale = 3 billion to 1

 Object  Diameter in Kilometers  Diameter in Miles  Scaled size in milimeters  Scaled size in inches
 Sun  1,390,000 864,990  464  18.4
 Mercury  4,880  3032  1.6  0.06
 Venus  12,104  7521  4.0  0.16
 Earth  12,756  7927  4.3  0.17
 Mars  6794  4221  2.3  0.09
 Jupiter  143,000  88,850  48  1.9
 Saturn  121,000  74,901  40  1.6
 Uranus  51,118  31,765 17  0.68
 Neptune  49,532  30,779  17  0.66
 Pluto  2274  1413  0.8  0.03


The Sun is an ordinary G2 star, one of more than 100 billion stars in
our galaxy.

diameter: 1,390,000 km.
mass: 1.989e30 kg
temperature: 5800 K (surface)
15,600,000 K (core)

The Sun is by far the largest object in the solar system. It contains
more than 99.8% of the total mass of the Solar System (Jupiter contains most
of the rest).

The Sun is personified in many mythologies: the Greeks called it Helios
and the Romans called it Sol.

The Sun is, at present, about 75% hydrogen and 25% helium by mass (92.1%
hydrogen and 7.8% helium by number of atoms); everything else ("metals")
amounts to only 0.1%. This changes slowly over time as the Sun converts
hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator
the surface rotates once every 25.4 days; near the poles it's as much as 36
days. This odd behavior is due to the fact that the Sun is not a solid body
like the Earth. Similar effects are seen in the gas planets.

Conditions at the Sun's core are extreme. The temperature is 15.6 million
Kelvin and the pressure is 250 billion atmospheres. The core's gases are
compressed to a density 150 times that of water.

The Sun's energy output (3.86e33 ergs/second or 386 billion billion
megawatts) is produced by nuclear fusion reactions. Each second about
700,000,000 tons of hydrogen are converted to about 695,000,000 tons of
helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma
rays. As it travels out toward the surface, the energy is continuously
absorbed and re-emitted at lower and lower temperatures so that by the time
it reaches the surface, it is primarily visible light. For the last 20% of
the way to the surface the energy is carried more by convection than by

The surface of the Sun, called the photosphere, is at a temperature of
about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark
only by comparison with the surrounding regions). Sunspots can be very
large, as much as 50,000 km in diameter. Sunspots are caused by complicated
and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona,
extends millions of kilometers into space but is visible only during
eclipses (left). Temperatures in the corona are over 1,000,000 K.

In addition to heat and light, the Sun also emits a low density stream
of charged particles (mostly electrons and protons) known as the solar
wind which propagates throughout the solar system at about 450 km/sec. The
solar wind and the much higher energy particles ejected by solar flares can
have dramatic effects on the Earth ranging from power line surges to radio
interference to the beautiful aurora borealis.

The solar wind has large effects on the tails of comets and even has
measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb.

The Sun is about 4.5 billion years old. Since its birth it has used up
about half of the hydrogen in its core. It will continue to radiate
"peacefully" for another 5 billion years or so (although its luminosity will
approximately double in that time). But eventually it will run out of
hydrogen fuel. It will then be forced into radical changes which, though
commonplace by stellar standards, will result in the total destruction of
the Earth (and probably the creation of a planetary nebula).

There are nine planets and a large number of smaller objects orbiting the
Sun. (Exactly which bodies should be classified as planets and which as
"smaller objects" has been the source of some controversy, but in the end it
is really only a matter of definition.)

                                                      The Winged Messenger

Mercury is the closest planet to the Sun and the eighth largest.
Mercury is smaller in diameter than Ganymede and Titan but more massive.

orbit: 57,910,000 km (0.38 AU) from Sun
diameter: 4,880 km
mass: 3.30e23 kg

In Roman mythology Mercury is the god of commerce, travel and thievery,
the Roman counterpart of the Greek god Hermes, the messenger of the Gods.
The planet probably received this name because it moves so quickly across
the sky.

Mercury has been known since at least the time of the Sumerians (3rd
millennium BC). It was given two names by the Greeks: Apollo for its
apparition as a morning star and Hermes as an evening star. Greek
astronomers knew, however, that the two names referred to the same body.
Heraclitus even believed that Mercury and Venus orbit the Sun, not the

Mercury has been visited by only one spacecraft, Mariner 10. It flew by
three times in 1973 and 1974. 

Mercury's orbit is highly eccentric; at perihelion it is only 46 million
km from the Sun but at aphelion it is 70 million. The perihelion of its
orbit precesses around the Sun at a very slow rate. 19th century astronomers
made very careful observations of Mercury's orbital parameters but could not
adequately explain them using Newtonian mechanics. The tiny differences
between the observed and predicted values were a minor but nagging problem
for many decades. It was thought that another planet (sometimes called
Vulcan) might exist in an orbit near Mercury's to account for the
discrepancy. The real answer turned out to be much more dramatic: Einstein's
General Theory of Relativity! Its correct prediction of the motions of
Mercury was an important factor in the early acceptance of the theory.

Temperature variations on Mercury are the most extreme in the solar
system ranging from 90 K to 700 K. The temperature on Venus is slightly
hotter but very stable.

Mercury is in many ways similar to the Moon: its surface is heavily
cratered and very old; it has no plate tectonics. On the other hand,
Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the
second densest major body in the solar system, after Earth. Actually Earth's
density is due in part to gravitational compression; if not for this,
Mercury would be denser than Earth. This indicates that Mercury's dense iron
core is relatively larger than Earth's, probably comprising the majority of
the planet. Mercury therefore has only a relatively thin silicate mantle and

Mercury's interior is dominated by a large iron core whose radius is 1800
to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust)
is only 500 to 600 km thick. At least some of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms blasted
off its surface by the solar wind. Because Mercury is so hot, these atoms
quickly escape into space. Thus in contrast to the Earth and Venus whose
atmospheres are stable, Mercury's atmosphere is constantly being

The surface of Mercury exhibits enormous escarpments, some up to
hundreds of kilometers in length and as much as three kilometers high.
Some cut thru the rings of craters and other features in such a way as to
indicate that they were formed by compression. It is estimated that the
surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in
the planet's radius).

In addition to the heavily cratered terrain, Mercury also has regions of
relatively smooth plains. Some may be the result of ancient volcanic
activity but some may be the result of the deposition of ejecta from
cratering impacts.

Amazingly, radar observations of Mercury's north pole (a region not
mapped by Mariner 10) show evidence of water ice in the protected shadows of
some craters.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the naked eye, but it is
always very near the Sun and difficult to see in the twilight sky.

  False Color Radar Map of Venus' Surface (beneath the clouds)
                                                     The Bringer of Peace

Venus is the second planet from the Sun and the sixth largest.

orbit: 108,200,000 km (0.72 AU) from Sun
diameter: 12,103.6 km
mass: 4.869e24 kg

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and
beauty. The planet is so named probably because it is the brightest of the
planets known to the ancients. (With a few exceptions, the surface features
on Venus are named for female figures.)

Venus has been known since prehistoric times. It is the brightest object
in the sky except for the Sun and the Moon. Like Mercury, it was popularly
thought to be two separate bodies: Eosphorus as the morning star and
Hesperus as the evening star, but the Greek astronomers knew better.

Since Venus is an inferior planet, it shows phases when viewed with a
telescope from the perspective of Earth. Galileo's observation of this
phenomenon was important evidence in favor of Copernicus's heliocentric
theory of the solar system.

The first spacecraft to visit Venus was Mariner 2 in 1962. It was
subsequently visited by many others (more than 20 in all so far),
including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land
on another planet, and Venera 9 which returned the first photographs of the
surface (left). Most recently, the orbiting US spacecraft Magellan produced
detailed maps of Venus' surface using radar.

Venus is sometimes regarded as Earth's sister planet. In some ways they
are very similar:
-- Venus is only slightly smaller than Earth (95% of Earth's
diameter, 80% of Earth's mass).
-- Both have few craters indicating relatively young surfaces.
-- Their densities and chemical compositions are similar.
Because of these similarities, it was thought that below its dense clouds
Venus might be very Earthlike and might even have life. But, unfortunately,
more detailed study of Venus reveals that in many important ways it is
radically different from Earth.

The pressure of Venus' atmosphere at the surface is 90 atmospheres
(about the same as the pressure at a depth of 1 km in Earth's oceans). It
is composed mostly of carbon dioxide. There are several layers of clouds
many kilometers thick composed of sulfuric acid. These clouds completely
obscure our view surface. This dense atmosphere produces a run-away
greenhouse effect that raises Venus' surface temperature by about 400
degrees to over 740 K (hot enough to melt lead). Venus' surface is actually
hotter than Mercury's despite being nearly twice as far from the Sun.

There are strong (350 kph) winds at the cloud tops but winds at the
surface are very slow, no more than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all
boiled away. Venus is now quite dry. Earth would have suffered the same fate
had it been just a little closer to the Sun. We may learn a lot about Earth
by learning why the basically similar Venus turned out so differently.

Most of Venus' surface consists of gently rolling plains with little
relief. Data from Magellan's imaging radar shows that much of the surface of
Venus is covered by lava flows. There are several large shield volcanoes
(similar to Hawaii or Olympus Mons) such as Sif Mons (right). Recently
announced findings indicate that Venus is still volcanically active, but
only in a few hot spots; for the most part it has been geologically rather
quiet for the past few hundred million years.

There are no small craters on Venus. It seems that small meteoroids burn
up in Venus' dense atmosphere before reaching the surface. Craters on Venus
seem to come in bunches indicating that large meteoroids that do reach the
surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be about 800 million years old.
Extensive volcanism at that time wiped out the earlier surface including any
large craters from early in Venus' history.

The interior of Venus is probably very similar to that of Earth: an iron
core about 3000 km in radius, a molten rocky mantle comprising the majority
of the planet. 

Venus is usually visible with the naked eye. Sometimes (inaccurately)
refered to as the "morning star" or the "evening star", it is by far the
brightest "star" in the sky.


Earth is the third planet from the Sun and the fifth largest:

orbit: 149,600,000 km (1.00 AU) from Sun
diameter: 12,756.3 km
mass: 5.9736e24 kg

Earth is the only planet whose English name does not derive from
Greek/Roman mythology. The name derives from Old English and Germanic. There
are, of course, hundreds of other names for the planet in other languages.
In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil
(Greek: Gaia, terra mater - Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it
was understood that the Earth is just another planet.

The Earth is divided into several layers which have distinct chemical and
seismic properties (depths in km):

0- 40 Crust
40- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer
2890-5150 Outer core
5150-6378 Inner core

The core is probably composed mostly of iron (or nickel/iron) though it
is possible that some lighter elements may be present, too. Temperatures at
the center of the core may be as high as 7500 K, hotter than the surface of
the Sun. 

The Earth is the densest major body in the solar system.

The Earth's surface is very young. In the relatively short (by
astronomical standards) period of 500,000,000 years or so erosion and
tectonic processes destroy and recreate most of the Earth's surface and
thereby eliminate almost all traces of earlier geologic surface history
(such as impact craters). Thus the very early history of the Earth has
mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the
oldest known rocks are about 4 billion years old and rocks older than 3
billion years are rare. The oldest fossils of living organisms are less than
3.9 billion years old. There is no record of the critical period when life
was first getting started.

71 Percent of the Earth's surface is covered with water. Earth is the
only planet on which water can exist in liquid form on the surface
(though there may be liquid ethane or methane on Titan's surface and liquid
water beneath the surface of Europa). Liquid water is, of course, essential
for life as we know it. The heat capacity of the oceans is also very
important in keeping the Earth's temperature relatively stable. Liquid water
is also responsible for most of the erosion and weathering of the Earth's
continents, a process unique in the solar system today (though it may have
occurred on Mars in the past).

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of
argon, carbon dioxide and water. There was probably a very much larger
amount of carbon dioxide in the Earth's atmosphere when the Earth was first
formed, but it has since been almost all incorporated into carbonate rocks
and to a lesser extent dissolved into the oceans and consumed by living
plants. Plate tectonics and biological processes now maintain a continual
flow of carbon dioxide from the atmosphere to these various "sinks" and back
again. The tiny amount of carbon dioxide resident in the atmosphere at any
time is extremely important to the maintenance of the Earth's surface
temperature via the greenhouse effect. The greenhouse effect raises the
average surface temperature about 35 degrees C above what it would otherwise
be (from a frigid -21 C to a comfortable +14 C); without it the oceans would
freeze and life as we know it would be impossible.

The presence of free oxygen is quite remarkable from a chemical point
of view. Oxygen is a very reactive gas and under "normal" circumstances
would quickly combine with other elements. The oxygen in Earth's atmosphere
is produced and maintained by biological processes. Without life there would
be no free oxygen.

The interaction of the Earth and the Moon slows the Earth's rotation by
about 2 milliseconds per century. Current research indicates that about 900
million years ago there were 481 18-hour days in a year.

Earth's Satellite

Earth has only one natural satellite, the Moon.


                                                   The Bringer of War

Mars is the fourth planet from the Sun and the seventh largest:

orbit: 227,940,000 km (1.52 AU) from Sun
diameter: 6,794 km
mass: 6.4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name
due to its red color; Mars is sometimes referred to as the Red Planet. (An
interesting side note: the Roman god Mars was a god of agriculture before
becoming associated with the Greek Ares; those in favor of colonizing and
terra forming Mars may prefer this symbolism.) The name of the month March
derives from Mars.

Mars has been known since prehistoric times. It is still a favorite of
science fiction writers as the most favorable place in the Solar System
(other than Earth!) for human habitation. But the famous "canals" "seen" by
Lowell and others were imaginary.

The first spacecraft to visit Mars was Mariner 4 in 1965. Several
others followed including the two Viking landers in 1976.
Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on
1997 July 4.

Though Mars is much smaller than Earth, its surface area is about the
same as the land surface area of Earth.

Except for Earth, Mars has the most highly varied and interesting terrain
of any of the terrestrial planets, some of it quite spectacular:
- Olympus Mons: the largest mountain in the Solar System rising 24
km (78,000 ft.) above the surrounding plain. Its base is more than 
500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high 
- Tharsis: a huge bulge on the Martian surface that is about 4000 km
across and 10 km high.
- Valles Marineris: a system of canyons 4000 km long and from 2 to 7
km deep (top of page);
- Hellas Planitia: an impact crater in the southern hemisphere over 6
km deep and 2000 km in diameter.
Much of the Martian surface is very old and cratered, but there are also
much younger rift valleys, ridges, hills and plains.

There is very clear evidence of erosion in many places on Mars
including large floods and small river systems (right). At some time in
the past there was clearly water on the surface There may have been large
lakes or even oceans. But it seems that this occurred only briefly and very
long ago; the age of the erosion channels is estimated at about nearly 4
billion years. 

Early in its history, Mars was much more like Earth. As with Earth almost
all of its carbon dioxide was used up to form carbonate rocks. But lacking
the Earth's plate tectonics, Mars is unable to recycle any of this carbon
dioxide back into its atmosphere and so cannot sustain a significant
greenhouse effect. The surface of Mars is therefore much colder than the
Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of
remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and
traces of oxygen (0.15%) and water (0.03%). It is thick enough to support 
very strong winds and vast dust storms that on occasion engulf the entire 
planet for months. Mars' thin atmosphere produces a greenhouse effect but it 
is only enough to raise the surface temperature by 5 degrees (K); much less 
than what we see on Venus and Earth.

Mars has permanent ice caps at both poles composed mostly of solid
carbon dioxide ("dry ice"). 

Recent observations with the Hubble Space Telescope have revealed 
that the conditions during the Viking missions may not have been
typical. Mars' atmosphere now seems to be both colder and dryer than
measured by the Viking landers.

The Viking landers performed experiments to determine the existence of
life on Mars. The results were somewhat ambiguous but most scientists now
believe that they show no evidence for life on Mars (there is still some
controversy, however). Optimists point out that only two tiny samples were
measured and not from the most favorable locations. More experiments will be
done by future missions to Mars.

On 1996 Aug. 6, David McKay et al announced the first identification of
organic compounds in a Martian meteorite. The authors further suggest
that these compounds, in conjunction with a number of other mineralogical
features observed in the rock, may be evidence of ancient Martian

Exciting as this is, it is important to note while this evidence is
strong it by no means establishes the fact of extraterrestrial life. There
have also bee several contradictory studies published since the McKay paper.
Remember, "extraordinary claims require extraordinary evidence." Much work
remains to be done before we can be confident of this most extraordinary

When it is in the nighttime sky, Mars is easily visible with the naked
eye. Its apparent brightness varies greatly according to its relative
position to the Earth. 

Mars' Satellites

Mars has two tiny satellites which orbit very close to the surface,
Deimos and Phobos.

                                                The Bringer of Jollity

Jupiter is the fifth planet from the Sun and by far the largest.
Jupiter is more than twice as massive as all the other planets combined
(318 times Earth).

orbit: 778,330,000 km (5.20 AU) from Sun
diameter: 142,984 km (equatorial)
mass: 1.900e27 kg

Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods, the ruler of
Olympus and the patron of the Roman state. Zeus was the son of Cronus

Jupiter is the fourth brightest object in the sky (after the Sun, the
Moon and Venus; at some times Mars is also brighter). It has been known
since prehistoric times. Galileo's discovery, in 1610, of Jupiter's four
large moons Io, Europa, Ganymede and Callisto (now known as the Galilean
moons) was the first discovery of a center of motion not apparently centered
on the Earth. It was a major point in favor of Copernicus's heliocentric
theory of the motions of the planets; Galileo's outspoken support of the
Copernican theory got him arrested by the Inquisition. He was forced to
recant his beliefs and was imprisoned for the rest of his life.

Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11,
Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo is currently in
orbit around Jupiter and will be sending back data for at least the next two

The gas planets do not have solid surfaces, their gaseous material
simply gets denser with depth (the radii and diameters quoted for the
planets are for levels corresponding to a pressure of 1 atmosphere). What we
see when looking at these planets is the tops of clouds high in their
atmospheres (slightly above the 1 atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25%
by mass) with traces of methane, water, ammonia and "rock". This is very
close to the composition of the primordial Solar Nebula from which the
entire solar system was formed. Saturn has a similar composition, but Uranus
and Neptune have much less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas planets) is
highly indirect and likely to remain so for some time. (The data from
Galileo's atmospheric probe goes down only about 150 km below the cloud

Jupiter probably has a core of rocky material amounting to something like
10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in the form of liquid
metallic hydrogen. This exotic form of the most common of elements is
possible only at pressures exceeding 4 million bars, as is the case in the
interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of
ionized protons and electrons (like the interior of the Sun but at a far
lower temperature). At the temperature and pressure of Jupiter's interior
hydrogen is a liquid, not a gas. It is an electrical conductor and the
source of Jupiter's magnetic field. This layer probably also contains some
helium and traces of various "ices".

The outermost layer is composed primarily of ordinary molecular hydrogen
and helium which is liquid in the interior and gaseous further out. The
atmosphere we see is just the very top of this deep layer. Water, carbon
dioxide, methane and other simple molecules are also present in tiny

Jupiter and the other gas planets have high velocity winds which are
confined in wide bands of latitude. The winds blow in opposite directions
in adjacent bands. 

The vivid colors seen in Jupiter's clouds are probably the result of
subtle chemical reactions of the trace elements in Jupiter's atmosphere,
perhaps involving sulfur whose compounds take on a wide variety of colors,
but the details are unknown.

The Great Red Spot has been seen by Earthly observers for more
than 300 years. The GRS is an oval about 12,000 by 25,000 km,
big enough to hold two Earths. Similar structures have been seen on Saturn 
and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun.
The interior of Jupiter is hot: the core is probably about 20,000 K. 
The heat is generated by the slow gravitational compression of the planet. 
(Jupiter does NOT produce energy by nuclear fusion as in the Sun.

Jupiter is just about as large in diameter as a gas planet can be. If
more material were to be added, it would be compressed by gravity such that
the overall radius would increase only slightly. A star can be larger only
because of its internal (nuclear) heat source. (But Jupiter would have to be
at least 80 times more massive to become a star.)

When it is in the nighttime sky, Jupiter is often the brightest "star" in
the sky (it is second only to Venus, which is seldom visible in a dark sky).
The four Galilean moons are easily visible with binoculars; a few bands and
the Great Red Spot can be seen with a small astronomical telescope. 

Jupiter has 16 known satellites, the four large Galilean moons and 12
small ones.

                                                 The Bringer of Old Age

Saturn is the sixth planet from the Sun and the second largest:

orbit: 1,429,400,000 km (9.54 AU) from Sun
diameter: 120,536 km (equatorial)
mass: 5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated
Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus
(Jupiter). Saturn is the root of the English word "Saturday".

Saturn has been known since prehistoric times. Galileo was the first to
observe it with a telescope in 1610; he noted its odd appearance but was
confused by it. Early observations of Saturn were complicated by the fact
that the Earth passes through the plane of Saturn's rings every few years as
Saturn moves in its orbit. A low resolution image of Saturn therefore
changes drastically. 

Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1
and Voyager 2. Cassini, now on its way, will arrive in 2004.

Saturn is the least dense of the planets; its specific gravity (0.7) is
less than that of water.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of
water, methane, ammonia and "rock", similar to the composition of the
primordial Solar Nebula from which the solar system was formed.

Saturn's interior is hot (12000 K at the core) and Saturn radiates more
energy into space than it receives from the Sun. 

Two prominent rings (A and B) and one faint ring (C) can be seen from the
Earth. The gap between the A and B rings is known as the Cassini division.

Though they look continuous from the Earth, the rings are actually
composed of innumerable small particles each in an independent orbit. They
range in size from a centimeter or so to several meters. A few
kilometer-sized objects are also likely. The ring particles seem to be composed primarily of water
ice, but they
may also include rocky particles with icy coatings. The origin of the rings of 
Saturn (and the other jovian planets) is unknown. Though they may have had 
rings since their formation, the ring systems are not stable and must be
regenerated by ongoing processes,probably the breakup of larger satellites.

When it is in the nighttime sky, Saturn is easily visible to the naked
eye. Though it is not nearly as bright as Jupiter, it is easy to identify as
a planet because it doesn't "twinkle" like the stars do. The rings and the
larger satellites are visible with a small astronomical telescope. 

Saturn has 18 named satellites, more than any other planet. There may
very well also be several small ones yet to be discovered.

                                                         The Magician

Uranus is the seventh planet from the Sun and the third largest (by
diameter). Uranus is larger in diameter but smaller in mass than Neptune.

orbit: 2,870,990,000 km (19.218 AU) from Sun
diameter: 51,118 km (equatorial)
mass: 8.683e25 kg

Careful pronunciation may be necessary to avoid embarrassment; say
"YOOR a nus" , not "your anus" or "urine us".

Uranus is the ancient Greek deity of the Heavens, the earliest supreme
god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and
of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by
William Herschel while systematically searching the sky with his telescope on
March 13, 1781. It had actually been seen many times before but ignored as
simply another star (the earliest recorded sighting was in 1690 when John
Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus"
(the Georgian Planet) in honor of his patron, the infamous (to Americans)
King George III of England; others called it "Herschel". The name "Uranus"
was first proposed by Bode in conformity with the other planetary names from
classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane
of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At
the time of Voyager 2's passage, Uranus' south pole was pointed almost
directly at the Sun. This results in the odd fact that Uranus' polar regions
receive more energy input from the Sun than do its equatorial regions.
Uranus is nevertheless hotter at its equator than at its poles. The
mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its
north pole! Either its axial inclination is a bit over 90 degrees and its
rotation is direct, or it's a bit less than 90 degrees and the rotation is
retrograde. The problem is that you need to draw a dividing line
*somewhere*, because in a case like Venus there is little dispute that the
rotation is indeed retrograde (not a direct rotation with an inclination of
nearly 180).

Uranus is composed primarily of rock and various ices, with only about
15% hydrogen and a little helium (in contrast to Jupiter and Saturn which
are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the
cores of Jupiter and Saturn minus the massive liquid metallic hydrogen
envelope. It appears that Uranus does not have a rocky core like Jupiter and
Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Uranus' blue color is the result of absorption of red light by methane in
the upper atmosphere. There may be colored bands like Jupiter's but they are
hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter's, they are
very dark but like Saturn's composed of fairly large particles ranging up
to 10 meters in diameter in addition to fine dust. There are 11 known rings,
all very faint; the brightest is known as the Epsilon ring. The Uranian
rings were the first after Saturn's to be discovered. This was of
considerable importance since we now know that rings are a common feature of
planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones
already known. It is likely that there are several more tiny satellites
within the rings.

Uranus is sometimes just barely visible with the naked eye on a very
clear night; it is fairly easy to spot with binoculars (if you know exactly
where to look). A small astronomical telescope will show a small disk.

Uranus has 15 named moons plus 2 recently discovered ones which as yet have
not been given names.

                                                             The Mystic

Neptune is the eighth planet from the Sun and the fourth largest (by
diameter). Neptune is smaller in diameter but larger in mass than Uranus.

orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
mass: 1.0247e26 kg

In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.

After the discovery of Uranus, it was noticed that its orbit was not as
it should be in accordance with Newton's laws. It was therefore predicted
that another more distant planet must be perturbing Uranus' orbit.
Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very
near to the locations independently predicted by Adams and Le Verrier from
calculations based on the observed positions of Jupiter, Saturn and Uranus.
An international dispute arose between the English and French (though not,
apparently between Adams and Le Verrier personally) over priority and the
right to name the new planet; they are now jointly credited with Neptune's
discovery. Subsequent observations have shown that the orbits calculated by
Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had
the search for the planet taken place a few years earlier or later it would
not have been found anywhere near the predicted location.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug. 25
1989. Almost everything we know about Neptune comes from this encounter.

Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of
Neptune. Since 1979 Neptune has actually been the most distant planet from
the Sun; Pluto will again be the most distant in 1999.

Neptune's composition is probably similar to Uranus': various "ices" and
rock with about 15% hydrogen and a little helium. Like Uranus, but unlike
Jupiter and Saturn, it may not have a distinct internal layering but rather
to be more or less uniform in composition. But there is most likely a small
core (about the mass of the Earth) of rocky material. Its atmosphere is
mostly hydrogen and helium with a small amount of methane.

Neptune's blue color is the result of absorption of red light by methane
in the atmosphere.

Like a typical gas planet, Neptune has rapid winds confined to bands of
latitude and large storms or vortices. Neptune's winds are the fastest in
the solar system, reaching 2000 km/hour.

Like Jupiter and Saturn, Neptune has an internal heat source -- it
radiates more than twice as much energy as it receives from the Sun.

At the time of the Voyager encounter, Neptune's most prominent feature
was the Great Dark Spot in the southern hemisphere. It was about
half the size as Jupiter's Great Red Spot (about the same diameter as

Neptune also has rings. Earth-based observations showed only faint
arcs instead of complete rings, but Voyager 2's images showed them to be
complete rings with bright clumps. One of the rings appears to have a
curious twisted structure.

Like Uranus and Jupiter, Neptune's rings are very dark but their
composition is unknown.

Neptune's rings have been given names: the outermost is Adams (which
contains three prominent arcs now named Liberty, Equality and Fraternity),
next is an unnamed ring coorbital with Galatea, then Leverrier (whose outer
extensions are called Lassell and Arago), and finally the faint but broad

Neptune can be seen with binoculars (if you know exactly where to look)
but a large telescope is needed to see anything other than a tiny disk.

Neptune has 8 known moons; 7 small ones and Triton.


Pluto is the farthest planet from the Sun (usually) and by far the
smallest. Pluto is smaller than seven of the solar system's moons (the
Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld.
The planet received this name (after many other suggestions) perhaps
because it's so far from the Sun that it is in perpetual darkness and
perhaps because "PL" are the initials of Percival Lowell.

Pluto was discovered in 1930 by a fortunate accident. Calculations which
later turned out to be in error had predicted a planet beyond Neptune, based
on the motions of Uranus and Neptune. Not knowing of the error, Clyde W.
Tombaugh at Lowell Observatory in Arizona did a very careful sky survey
which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was
too small to account for the discrepancies in the orbits of the other
planets. The search for Planet X continued but nothing was found. Nor is it
likely that it ever will be: the discrepancies vanish if the mass of Neptune
determined from the Voyager 2 encounter with Neptune is used. There is no
tenth planet.

Pluto is the only planet that has not been visited by a spacecraft.
Even the Hubble Space Telescope can resolve only the largest features on
its surface.

Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was
discovered (in 1978) just before its orbital plane moved edge-on toward the
inner solar system. It was therefore possible to observe many transits of
Pluto over Charon and vice versa. By carefully calculating which portions of
which body would be covered at what times, and watching brightness curves,
astronomers were able to construct a rough map of light and dark areas on
both bodies.

Pluto's radius is not well known. JPL's value of 1137 is given with an
error of +/-8, almost one percent.

Though the sum of the masses of Pluto and Charon is known pretty well
-- they're so small and far away that even HST has difficulty resolving them.
The ratio of their masses is probably somewhere between 0.084 and 0.157; 
more observations are underway but we won't get really accurate data until a 
spacecraft is sent.

There are some who think Pluto would be better classified as a large
asteroid or comet rather than as a planet. Some consider it to be the
largest of the Kuiper Belt objects. There is considerable merit to the later
position, but historically Pluto has been classified as a planet and it is
likely to remain so.

Pluto's orbit is highly eccentric. At times it is closer to the Sun than
Neptune (it has been so since January 1979 and will continue until February
1999). Pluto rotates in the opposite direction from most of the other

Like Uranus, the plane of Pluto's equator is at almost right angles to
the plane of its orbit.

The surface temperature on Pluto is not well known but is probably
between 35 and 45 Kelvins (-228 to -238 C).

Pluto's composition is unknown, but its density (about 2 gm/cm3)
indicates that it is probably a mixture of 70% rock and 30% water ice much
like Triton. 

Little is known about Pluto's atmosphere, but it probably consists
primarily of nitrogen with some carbon monoxide and methane. The Pluto 
Express mission planners want to arrive at Pluto while the atmosphere is 

Pluto can be seen with an amateur telescope but it is not easy. 

Charon is named for the mythological figure who ferried the dead across
the River Styx into Hades (the underworld).

Charon was discovered in 1978 by Jim Christy. Prior to that it was
thought that Pluto was much larger since the images of Charon and Pluto were
blurred together.

It has been proposed that Charon was formed by a giant impact similar to
the one that formed Earth's Moon.


We would like to thank Phelps Dodge and Project Astro for providing the funding for this project. The San Man provided the labor to place the plaques in the sidewalk. Texas NM Power Company put the sun model up on their power pole and adjusted the lighting. Michael Metcalf gave countless artistic suggestions and technical help during the casting process. I took his sculpture class at WNMU for two semesters. Silver Welding also lent us equipment. Regina Moore did an excellent job of the layout as part of her Physical Science 485 course at WNMU. She also did a great job on much of the NFO Web Site. Regina took the proposal to the Town of Silver City. I would like to also thank the original supporters of the proposal: Dr. Elna Stowe, Superintendent, Silver Schools, Dr. Harrell Holder, Superintendent, Cobre schools, Tom Schnalzer, Principal, Jose Barrios Elementary, Alan Ramirez, Principal, 6th St. Elementary, Travis Columbus, Principal, Harrison Schmitt Elementary, Tom Sweitzer, Principal, La Plata Middle School, Dr. William Soules, Principal, Snell Middle School, John Lathrop, Principal, G.W. Stout Elementary.

A. W. Neely NFO