Mammals, as a kingdom, are curious creatures, always exploring and investigating the unknown. Humans, with their advanced intellect and technological innovations take this curiosity to new levels. We have an intense desire to know everything. We are always searching for the answers to how things work and why things are the way the are. One key we have found to answering the questions of today is to learn what happened in the past. It is easier to understand where you are going if you already know where you’ve been.

Cosmologists take our study of the past as far back as they can. There are limits to our knowledge however (more likely we just haven’t figured out how to pass those limits yet). We can only travel so far back in time before everything we have learned about our universe no longer applies. This is what we call the "beginning." Not because nothing existed before that beginning, but rather because we have no way of describing it. Before our universe was created, space and time did not exist as we know it. If we were somehow able to peer past our beginnings, none of our laws of physics would even apply. It is unlikely that we would even be able to comprehend what we saw. Perhaps we have the capability to understand what happened before the beginning and our knowledge just has not reached that point yet. Perhaps we shall never truly understand what happened before time was created. We can, however, travel back approximately 15 billion years (give or take five billion years or so), to where the universe was simply a point in space. This is where our journey begins, at a point in time now known as the Big Bang.

Fifteen billion years ago everything in our universe existed in singularity. A singularity is a point in space that is infinitely small but can have an extremely large amount of mass in it. It is unclear what happened next but something caused this singularity to explode, hurtling all the mass that was in it outward at an incredible rate. The energy from this explosion caused reactions in some of these particles, known as quarks, causing them to combine and form the first protons. Later, neutrons and electrons would form and start the basis for all of the elements that we know. For a more in depth discussion of the first few moments of our universe’s existence please visit this page.

It would seem that matter flung out into space in such a way would remain fairly uniform in size and shape. <DIAGRAM>Uniform Explosion<DIAGRAM> This is clearly not what our universe looks like today. Nowhere can there be seen galaxies or solar systems. Clearly the matter was not uniformly distributed. For the most part, the matter was uniform in density as it expanded in to space. It was not, however, perfectly uniform. The fluctuations in density around the creation of the universe were probably no larger than a very small fraction of a percent. In fact, nearly 500,000 years after the Big Bang, our own Milky Way galaxy was only estimated to be about ½% more dense than the surrounding regions. This does not seem to be a significant amount but in reality it is. You see, the more dense regions of the universe had stronger gravitational attraction to each other than their surrounding areas. This gravitational attraction would have slowed their expansion down ever so slightly. With the universe expanding at such an astounding rate, the slower expanding areas were basically left behind. Relative to the areas around them, the densities of the slower expanding areas became much larger. Again, returning to our own Milky Way galaxy, approximately 1.2 billion years after the Big Bang, the Milky Way’s relative density was closer to twice that of its surrounding areas. It was somewhere around this time, cosmologists believe, that the inner portions of the galaxy began to form. <DIAGRAM>Galaxies form from Big Bang<DIAGRAM>

<IMAGE>Galaxy with clouds very apparent<IMAGE> Galaxies consist mostly of cold clouds of gas and dust. These clouds are called nebula and are mostly made up of hydrogen. For the most part the cloud remains stable and floats along within the galaxy. It does not take much, however, to turn this inactive cloud into a glowing ball of fire known as a star. All the cloud needs is for some of the atoms that make it up to be disturbed or agitated. As the first few atoms begin moving they come closer to other atoms. Their proximity to these atoms increases the gravitational attraction between the two atoms. As more atoms become attracted to each other, their mass increases and their ability to attract even more atoms increases. Fairly soon, you have a giant ball of hydrogen forming in space. As the atoms come closer together, they collide more often, generating heat. The heat generated from these collisions causes the ball of hydrogen to begin to glow faintly. This is a protostar. For more information on stars and how they form please visit the section of this web site entitled A Star’s Life.

For reasons not yet clearly understood, all of the mass from the nebula does not always go into the formation of the star. Sometimes, other bodies are formed with the matter left unused by the forming star. These bodies are what we call the planets, of which our solar system has nine. Our sun takes up about 98% of all the mass in our solar system. The remaining 2% went into the formation of every other body in our solar system including all of the planets, moons, comets, and asteroids.

The planets, by far, make up the majority of the remaining 2% of mass. There are two distinct types of planets, the terrestrial planets (inner planets) and the jovian planets (outer planets.) The terrestrial planets (Mercury, Venus, Earth, and Mars) are relatively small, rocky, and composed of heavier elements. The jovian planets (Jupiter, Saturn, Uranus, and Neptune) are considerably larger than the terrestrial planets, gaseous, and composed of much lighter elements. Since the jovian planets are mostly made of gas they are also referred to as the "gas giants." It should be noted that Pluto, although considered a planet, is not classified with either type of planets. This is because Pluto is clearly an outer planet (Pluto is the farthest planet from the sun) yet it shares none of the features of the outer planets. Astronomers believe that Pluto was formerly a moon of Neptune that was somehow knocked out of orbit. This could explain why Pluto’s features are much more similar to a moon of one of the outer planets instead of one of the outer planets themselves.

The reason for the distinction between the two types of planets has much to do with their proximity to the sun. The energy released from the sun during its formation made the areas closer to the sun considerably warmer than those farther away. This meant that the lighter elements, like hydrogen and helium, would exist in a gaseous form. You’ll remember that gases are much more active than liquids or solids. Because of their more active state, it was harder for them to be caught by the forming planet’s gravitational fields. Only the heavier elements remained in a solid form where they could be captured by the forming planets. The planets closer to the sun would not have access to hydrogen during their formation. Where the outer planets formed, they were far enough away that hydrogen existed in a solid form and could be captured by the planet’s gravitational field. Without access to hydrogen, the most abundant element in our solar system, the inner planets could not become nearly as large as the outer planets. They did not have the resources to make themselves that large. You may wonder why we refer to the outer planets as the gas giants if hydrogen exists as a solid that far away from the sun. The planets are heated as they form, just like a star. The reason they do not produce energy like a star is because they do not have enough mass to reach the required temperature to perform nuclear fusion (this is discussed more fully in A Star’s Life.) As the planets warm, hydrogen can move from a solid state to a gaseous one. Now that it is caught so deeply within the planet’s gravitational field, however, it cannot escape.

One thing we have not mentioned at all yet is the end, be it of a planet, star, or even the universe itself. Besides the simple answer that this is a section on the beginnings there is good reason for this. The death of a star will be discussed in A Star’s Life. The universe, however, is a different story all together. When will the universe end? Will it ever end? The real answer is that we do not know. The answer is dependent upon the exact amount of mass that is in the universe, which is something we have yet to determine. If enough mass exists, then eventually the gravitational force of that mass will stop the expansion of the universe and cause it to contract and implode upon itself. However, if there is not enough mass, it is likely that the universe will continue expanding forever. We do not know what will happen. In any event, if the universe is coming to an end, it certainly won’t be any time soon.

We head now on a journey through the life of a star. Those of you who have already read the section on The Beginnings already know a little about how stars start their formation. For those who haven’t, don’t worry, we are about to repeat it here for you as well. If you’ve read this already in The Beginnings section, feel free to continue on to the next page or read it again for a review.

<IMAGE>Planetary Nebula<IMAGE> Planets form from cold clouds of gas and dust. These clouds are called nebula and are mostly made up of hydrogen. For the most part the cloud remains stable and floats along within the galaxy. It does not take much, however, to turn this inactive cloud into a glowing ball of fire known as a star. All the cloud needs is for some of the atoms that make it up to be disturbed or agitated. As the first few atoms begin moving they come closer to other atoms. Their proximity to these atoms increases the gravitational attraction between the two atoms. As more atoms become attracted to each other, their mass increases and their ability to attract even more atoms increases. Soon, you have a giant ball of hydrogen forming in space. As the atoms come closer together, they collide more often, generating heat. The heat generated from these collisions causes the ball of hydrogen to begin to glow faintly. This is a protostar. <IMAGE>Protostar<IMAGE>

Whether or not a protostar makes it to a regular star (called a main sequence star) or not has much to do with the amount of mass that exists in the protostar. In fact, much of a star’s life has to do with the mass of the star. Without enough mass, the collapsing protostar will not generate enough heat to start nuclear fusion (which is the signature of a star.) A star’s mass will also determine how long it will live and what form it will take when it dies. Protostars without at least 40% of the mass of our sun (also referred to as a solar mass or M) would not form into a star. They would become brown dwarfs. Astronomers are still trying to determine the point that separates a brown dwarf from a very large planet like Jupiter.

Star’s with at least 0.4M will go on to form main sequence stars. What characterizes a main sequence star is the nuclear reactions going on in its core. Once the core temperature of a star reaches 15 million degrees Kelvin, nuclear fusion can begin. Nuclear fusion in main sequence stars involves the fusing of hydrogen atoms into helium atoms. The amount of energy released during this reaction is given by Einstein’s equation E=MC². The force of these nuclear reactions is strong enough to withstand the force of gravity and the star is said to be in stable equilibrium.

Main sequence is the term used to describe the stage in which a star remains for most of its life. It can clearly be seen in the Hertzprung-Russel (HR) diagram. The HR diagram is a measure of a star’s luminosity vs. surface temperature. As you can see below, main sequence stars run from the upper-left to the lower-right corners of the diagram. In the lower-left you will find "white dwarfs," which are remnants of low-mass stars and in the upper-right are "red giants," which are stars in their dying stages.

<IMAGE>HR Diagram<IMAGE>

As mentioned earlier, the lifetime of a star is highly dependent on its mass. The reason a star is able to stay "alive" for any period of time is that the forces of the nuclear reactions in its core are powerful enough to overcome the force of gravity trying to crush the star. Eventually, the star will run out of fuel and it will gravitationally collapse. It might make sense then that a high-mass star would live longer than a low-mass star because it has more fuel. Actually, it is the opposite. A high-mass star does have access to more fuel, but it also needs to burn it much faster. The large amount of mass in a high-mass star also means larger gravitational forces. In order to remain in stable equilibrium, the star must burn its fuel faster to overcome those gravitational forces. Low-mass stars, like our own sun, can live for around 10 billion years. High-mass stars can have life spans as short as 3 million years.

Mass: Luminosity: Life Expectancy:

25M 200000L 3e6 years

15M 30000L 15e6 years

3M 65L 500e6 years

1.5M 5L 3e9 years

1M 1L 10e9 years

0.8M 0.4L 13e9 years

0.4M 0.03L 200e9 years

(Solar mass M = mass of our sun) (Luminosity L = luminosity of our sun)

A star spends 90% of its life on the main sequence, after this time it runs out of core hydrogen to burn and gravity begins to cause the star to collapse again. As the core collapses, it rises in temperature. According to Newton’s Third Law of Motion, however, for every action there is an equal and opposite reaction. As the core of the star begins to collapse and heat, the outer layers of the star expand and cool. As the star cools it turns red in color. This type of star is known as a red giant. Since red giants are very large stars they are also very luminous. Once the temperature of the core reaches 100 million Kelvin, it is hot enough to begin converting helium into carbon. This reaction is powerful enough to again counteract the force of gravity.

In low-mass stars (less than 8M), the volatile nature of the helium to carbon reactions causes the star to pulsate. This pulsation is strong enough that the outer layers of the star are ejected from the core, forming what is known as a planetary nebula. All that is left of the star now is its helium core. The core of the star has now become a white dwarf. Do to the very condensed nature of the atoms, white dwarfs are incredibly hot, giving them a white color. They are also very small (no more than a 1000 kilometers across) which is why there are not very luminous and were only just discovered recently. When the star eventually runs out of helium to burn it simply dies out and becomes a black dwarf, a body in space composed mostly of carbon.

High- mass stars go through a much more violent struggle during their death. The outer layers of a high-mass star do not eject to form a planetary nebula; rather they begin nuclear reactions of their own. As the core burns helium to carbon and oxygen, the outer layers begin converting hydrogen to helium. Again, the star will eventually run out of helium in its core. The core will begin collapsing again and rising in temperature. Then, at the next critical temperature, carbon and oxygen will begin fusing together. The next layer above will fuse helium to carbon and oxygen, the layer above that hydrogen to helium, etc. There is, however, a point at which these high-mass stars run out of fuel. The elements and temperature necessary to fuse those elements together are listed below.

Element à New Element: Necessary Temperature:

Hydrogen à Helium 10 million Kelvin

Helium à Carbon and Oxygen 100 million

Carbon and Oxygen à Neon, Magnesium, Silicon, and Sulfur 1 billion

Silicon and Sulfur à Iron, Nickel, + similar atomic weight 10 billion

<IMAGE>The Core Layers<IMAGE>

When a star is large enough (about 8M), the core can begin fusing silicon and sulfur into iron. Iron’s molecular structure does not allow it to be fused to create any new elements. Without any nuclear reactions counteracting the force of gravity, the star begins a rapid collapse. The force of gravity is so strong that it pushes the electrons out of their orbit and into the nucleus of the iron atoms. The electrons combine with the protons to form neutrons. The core of the star is now totally composed of neutrons. Gravity, however, is still attempting to crush the star further. The neutrons reach a point where they cannot be crushed anymore and recoil, like a spring, causing a massive explosion that sends a huge shock-wave through the outer layers of the star. The incredibly high temperatures that result from this explosion (in excess of 100 billion degrees Kelvin) cause reactions with all the elements discussed previously to make up most of the elements known in the universe. The entire core collapses in less than a second and the resulting explosion is referred to as a supernova. Supernovae can be brighter than galaxies and sometimes become so bright they can be seen during the day here on Earth. The dust from this explosion will later be used to form another star, perhaps with planets around it. All that is left of the star is its core of neutrons. This is a neutron star.

For the extremely high-mass stars (greater than 15M), even the nuclei of the iron atoms are not strong enough to resist gravity and the collapse of the core continues further. When the force of gravity is great enough to cause a neutron star to collapse, a black hole is formed. In a black hole, the force of gravity becomes so strong that even light cannot escape it. All the atoms continue collapsing into a small pinpoint in space called a singularity. A singularity is infinitely small but can contain an infinite amount of mass in it. Black holes consist of two parts; the singularity and the event horizon. The event horizon is a disc that encircles the singularity and is the point at which light cannot escape the gravitational affects of the black hole. The event horizon is what gives the black hole its name as it is literally a black circle sitting in space. The event horizon can be calculated by the equation R_s=2GM/C². Once something crosses the event horizon, it is lost forever. According to Einstein’s General Theory of Relativity, nothing can exceed the speed of light. Since the force of gravity in a black hole is strong enough to keep even light in it, there is no way anything can escape it.

Goddess of Love

Venus, the second planet out from the sun, is named after the roman god of love. Early photographs showed a planet filled with thick, swirling clouds. It was believed that these clouds hid a beautiful civilization beneath, and it was for this reason that Venus received her name.

Unfortunately, Venus does not hold a beautiful civilization beneath her clouds. In fact, it is quite the opposite. Venus is literally a ball of fire floating through space and serves as a stark reminder of what can happen to a planet overcome by the greenhouse affect. Venus’ atmosphere is made up of almost 96% carbon dioxide. The sun’s radiation heats the surface of the planet. When the heat tries to escape, it is trapped by the carbon dioxide in the planet’s thick atmosphere, making Venus the hottest of all the planets at 482 degrees Celsius (900 degrees Fahrenheit).

Venus’ landscape is a venerable see of fire. There are noticeable mountains and highland areas with valleys filled with lava. Lava is a result of the large volcanic activity across the surface of the planet. In fact, 85% of the planet’s surface is believed to be volcanic rock. Most of the impact craters are very large. Small asteroids or other objects would burn up while passing through Venus’ thick atmosphere. Only the larger asteroids could penetrate and leave lasting scars.

Unlike the other planets, Venus rotates from east to west. On Venus, the sun rises in the west and sets in the east. A day on Venus lasts approximately 243 Earth days, whereas a year lasts only 225 Earth days.

Mother Earth

Planet Earth, home sweet home. It is the third planet from the sun and the only known planet with life on it. The Earth’s atmosphere is key to life existing on our planet. It keeps the planet at a temperature where running water exists and also protects us from the harmful ultra-violet radiation of the sun. Our atmosphere is in trouble though. Since the industrial revolution, we have been pouring more greenhouse gases into our atmosphere than it can support. There are many indications that these gases may be causing a rise in the surface temperature of our planet. If we are not careful, our planet may become the next Venus. For more information on the greenhouse affect and global warming visit this site.

The surface of the planet is covered by about 75% water. There are seven major continents (N. America, S. America, Europe, Asia, Africa, Antarctica, and Australia) each with its own distinct cultures. Earth is still geographically active as evidenced by the Earth quakes and volcanic eruptions seen regularly.

A common misunderstanding about Earth is the cause of the seasons. Many people believe that the seasons are a result of the planets eccentric orbit, believing that when the planet gets closer to the sun, it gets warmer and vice verse. The real cause of the seasons is that Earth is tilted on its axis by about 23.5 degrees. As a result, the sun’s rays are most direct on different parts of the Earth at different times of the year. When the sun’s rays are most direct in the Northern Hemisphere, it experiences summer (which, incidentally is when the Earth is farthest from the sun). When the sun’s rays shine most directly on the Southern Hemisphere, it experiences summer and the Northern Hemisphere experiences winter. <DIAGRAM>Sun’s Rays on Earth<DIAGRAM>

When charged particles in the sun’s solar wind collide with the Earth’s magnetic field they become trapped and are drawn to the poles. There, they collide with particles in the air and cause them to glow. These are called the aurora or northern and southern lights. <IMAGE>Auroral Effects<IMAGE>

God of War

Mars receives its name for its deep red, war-like appearance in the sky. Until recent years, Mars was believed to harbor extraterrestrial life. Astronomers viewed straight lines on the planet’s surface and believed them to be irrigation canals. Another feature leading to hope that life existed on the planet was that Mars appeared to go through seasonal changes very similar to the Earth’s.

Recent images of the planet, however, show a barren, dust covered landscape with no signs of any irrigation canals. In fact, there are no signs of running water on the planet. Scientists believe that the lack of water, and Mars constant bombardment by ultra-violet radiation from the sun, leave little chance of life as we know it existing on the planet. There are, however, gorges and canyons that may have been carved by running water millions of years ago when the planet’s atmosphere was stronger. Even now it appears that there may still be water on the planet, frozen in the polar caps.

King of the Gods

Jupiter, king of the Gods, is by far the largest planet in our solar system, making the name quite fitting. The sun contains an estimated 98% of the mass of the entire solar system. Jupiter contains an estimated 98% of what remains. Similar in composition to the sun, Jupiter’s atmosphere is made up of mostly hydrogen and helium.

<IMAGE>Jupiter’s Banding<IMAGE> The banding that is seen on Jupiter is the result of a very complex weather system on the planet. Winds and storms buffet the planet constantly. The Great Red Spot that can be easily seen on Jupiter is a very complex storm raging on the planet. Lightning has also been observed on the planet, although much more powerful than the lightning experienced on Earth. Auroral effects, similar to those on Earth, have also been seen on the planet.

Jupiter does contain a ring system, like Saturn, although they are all but invisible here on Earth. Voyager 1 first discovered the rings in 1979 as it passed by the planet on its trip out of the solar system. They are not nearly as complex as the ring systems found around Saturn. They are made mostly of particles of dust from asteroids that smashed into the inner moons of Jupiter.

Unlike the terrestrial planets, the Jovian planets tend to attract a large number of satellites. This is, in part, because of their much stronger gravitational fields. Jupiter has 16 such satellites, four of which (Callisto, Europa, Ganymede, and Io) were large enough to be discovered by Galileo in 1610. The other 12 moons are relatively small and are believed to have been captured by Jupiter’s gravitational pull rather than have formed with the planet itself.

Father of Jupiter

Saturn is the sixth planet from the sun and the second largest. Saturn is the only planet less dense than water, meaning it could float (if there were some way to put it in an ocean.) Like Jupiter, Saturn also has a ring system. Saturn’s, however, is much more pronounced, being visible here on Earth. Also, like Jupiter, Saturn has a large number of moons, 18 to be exact. <IMAGE>Saturn’s Rings<IMAGE>

<IMAGE>Saturn Flattened at Poles<IMAGE> Saturn is visibly flattened at its poles. This is a result of the planet’s incredibly fast rotation. A day on Saturn only takes about 11 hours to complete.<IMAGE>Banding on Saturn<IMAGE> Banding also marks Saturn similar to that seen on Jupiter, resulting from strong winds on the planet, some reaching speed in excess of 1,100 miles per hour. Another similarity between the two planets is that Saturn has a similar auroral affect by its poles as well.<IMAGE>Saturn’s Auroral affects<IMAGE>

What makes Saturn special is its ring system. Saturn is one of the most spectacular objects to view in the night sky. Much, however, remains unknown about Saturn’s rings. We hope to discover more about the planet during the Cassini mission, scheduled to reach the planet in July, 2004. For now, we can only theorize about the origins of the rings. It would appear that the rings may be the remnants of former moons of the planet, shattered by impacts from asteroids. What is known is that the rings show a significant amount of water, all frozen of course. The rings themselves seem to be formed by many tiny droplets, ranging from a few centimeters to a few meters in diameter.<IMAGE>Close-up of Saturn’s Rings (False-Color)<IMAGE>

Father of Saturn

Uranus, discovered in 1781 by William Herschel, is the third largest planet. As with the other Jovian planets, Uranus has 15 moons orbiting it. The methane in Uranus’ atmosphere absorbs red light, giving Uranus its blue appearance. Uranus, like Jupiter, also has a ring system, although not nearly as pronounced as Saturn’s.

The strangest feature of Uranus is its axis of rotation. Uranus it tipped 90 degrees and sits on its side, appearing to roll around the sun. It is believed that this tilt is a result of a collision with a planet sized body many years ago.<DIAGRAM>Uranus Tilted by Collision<DIAGRAM> This has a very interesting affect on the planet’s magnetic field. Uranus’ magnetic field is tilted about 60 degrees off of the planet’s axis. This, coupled with the planets odd rotation, results in a magnetic field that spirals out behind the planet.

God of the Sea

Neptune, discovered in 1846 by Johann Gottfried Galle, is the outermost of the Jovian planets. Like Uranus, the methane in Neptune’s atmosphere absorbs red light and gives the planet its blue color. Like the other Jovian planets, Neptune has a faint ring system surrounding it similar to Jupiter’s and Uranus’.

Neptune, like Jupiter, has raging storms on the planet as well. The Great Dark Spot is one such storm easily visible on the planet. Another storm, discovered by
Voyager 2, is much smaller and circles the planet approximately every 16 hours.

God of the Underworld

God of the Underworld is certainly a proper name for this planet. The planet is almost six billion kilometers from the sun and receives little light or warmth from it. The planet is frozen and engulfed in near darkness. Discovered in 1930 by Clyde W. Tombaugh, very little is known about the planet and no spacecraft has visited yet.

Pluto is considered the farthest planet from the sun, but this is not always true. It has a very eccentric orbit and, at times, will loop inside the orbit of Neptune. It only lasts for about 20 years, a small portion of Pluto’s 249 year orbit about the sun. On February 11^th, 1999 Pluto crossed out of the orbit of Neptune to return to being the farthest planet from the sun and will remain that way until the year 2226. Pluto is slightly off of the ecliptic and will never collide with Neptune during its orbit.<IMAGE>Pluto’s Orbit<IMAGE>

Pluto, similar to Uranus, is tipped considerably, around 122 degrees. When the planet was first discovered, the South Pole was the only thing seen by astronomers. Forty years later, the view granted to astronomers is that of the planet’s equator.

Pluto and its only moon Charon share a synchronous rotation, meaning that the same side of the planet and the moon will always face each other. This is similar to our moons synchronous rotation, allowing us to only see one side of the moon. The difference is that on Pluto, only one side of the planet will ever get to see Charon.

Pluto is very different from its neighboring planets. It shares none of the characteristics of a Jovian planet. It is small, rocky, and composed of heavy elements. This leads some astronomers to theorize that it was formerly a moon of Neptune, knocked out of its orbit somehow.

An eclipse, in astronomy, is when one celestial object blocks all or part of another object from view. Usually when we talk about eclipses we are talking about those related to Earth: solar and lunar eclipses. In a solar eclipse, the moon moves between the Earth and the sun. The shadow cast by the moon falls over a portion of the Earth and blocks the majority of sunlight there.<DIAGRAM>Solar Eclipse<DIAGRAM> In a lunar eclipse, the Earth comes between the moon and the sun and the moon is enveloped in Earth’s shadow.<DIAGRAM>Lunar Eclipse<DIAGRAM>

The shadow cast by a celestial body is called the umbra. It is a cone-shaped shadow that falls behind that body relative to the sun. The Earth, being large compared to the moon, casts a considerably large umbra. The moon’s umbra, however, is much smaller. During a solar eclipse, the largest area to be covered by the moon’s shadow is around 150 miles or so in radius. This lasts, on average, for about three minutes before the moon moves out from between the sun and the Earth. Because of this, seeing total solar eclipses is not a common occurrence. For example, in the United States, the last total eclipse occurred in 1991 in Hawaii. The last total eclipse in the lower 48 states occurred in 1979 in Washington. The next total eclipse in the U.S. will not occur again until 2017.

If an eclipse occurs when the moon comes between the Earth and the sun, then why don’t we have an eclipse every time there is a new moon? The reason is that the moon’s orbit is not exactly on the ecliptic, but rather it is tilted slightly. We can only have an eclipse when the moon’s orbit falls on the ecliptic during the new moon or full moon phases.

For more information on eclipses (when the next ones will be and where they will be most visible) visit this site.

A common misunderstanding about the phases of the moon is that they are caused when the moon moves into the Earth’s shadow. This is a lunar eclipse. The dark portion of the moon we see is the moons own shadow. How can the moon be in it’s own shadow? Think of it this way. When the sun shines on the moon, half of the moon is lit and the other half is in darkness. What we see during the phases of the moon is parts of the moon that are lit and parts of the moon that are dark.<DIAGRAM>Phases of the moon<DIAGRAM>

The moon has many phases, but they can all be broken down into eight categories; four distinct categories and four transitional categories. The four distinct categories are; new moon, first quarter, full moon, and last quarter. The transitional categories describe the moon as it changes between those four and are; waxing crescent, waxing gibbous, waning gibbous, and waning crescent.<IMAGE>Phases of the Moon<IMAGE>

You will never see a new moon in the night sky. A new moon occurs when all that is visible from Earth is the dark side of the moon. Then, as the right side of the moon slowly gets brighter, we see a waxing crescent. The right side of the moon continues to get brighter as we move through first quarter and through waxing gibbous. Now, the moon has reached its full phase, where the entire moon is lit up. Next, the shadow begins encroaching from the right and we travel from waning gibbous, to last quarter, to waning crescent. Lastly, the entire moon is covered in shadow and we start again with another new moon.

The lunar cycle lasts approximately one month, with each major phase being about a week apart. Coincidence? Not at all. Our calendar is a mixture of a solar calendar and a lunar one. A month (or moonth) is a measurement of one lunar cycle. There are four weeks in a month because of the four phases of the moon. The reason a lunar cycle doesn’t work out to be exactly one month is because of the adjustments made when the lunar calendar was merged with the solar calendar.

Another interesting tidbit involves the number of days in a week. Ancient peoples relied heavily on astronomy for many things. The harvest time was determined by changes in weather and the phase of the moon. You may remember watching TV and hearing a stereotypical Indian character mention "I have not seen you in many moons." This is because they used the sky to tell time. They also noticed many things about the sky, including some of the planets. Not all the planets were visible to ancient peoples, but they were able to discover Mercury, Venus, Mars, Jupiter and Saturn. Besides the stars in the sky, there were seven celestial bodies they were aware of, the five planets, the sun, and the moon. There are seven days in our week as well. Sunday, Monday (Moonday), and Saturday (Saturnday) have the most obvious relationship to those seven celestial bodies. What about the other days of the week? Are they truly related? Well, in Spanish, you say Tuesday as Martes, which could possibly be a reference to Mars. Also in Spanish, Friday is Viernes, which may be a reference to Venus. Wednesday, Miercoles, might be Mercury and Thursday, Jueves, might be Jupiter.

Johannes Kepler is considered one of the pioneers of astronomy. He came up with what are now known as Kepler’s Three Laws of Planetary Motion. Kepler was born on December 27, 1571 in Weil der Stadt, Germany. He was one of seven children, three of which died at infancy. He was described as a very sickly child, and constantly suffered from one ailment or another. Kepler’s grandfather was believed to be a nobleman and the Mayor of Weil. Kepler’s father, however, became a mercenary and barely avoided being hanged. Kepler’s mother, raised by an aunt who was later burned for being a witch, nearly met the same fate as well.

Kepler graduated from the University of Tuebingen with a large focus on theology. It was here that Kepler was introduced to Copernicus’ heliocentric model of the universe. Kepler was fascinated by this, but believed Copernicus’ data was in error. Kepler made his own calculations and published his findings in 1596. He was still, however, going on the assumption that the planets had perfectly circular orbits.

After being pressured to leave the University of Graz because of his Lutheran faith, Kepler began working as Tycho Brahe’s assistant until Brahe’s death in 1601. Kepler used the data collected by Brahe to calculate the orbit of Mars. It was during these calculations that Kepler came up with his first two laws. While investigating the orbits of the other planets, Kepler came up with his third law. It was later determined that Kepler’s laws not only applied to the motion of the planets, but to that of comets as well. His three laws were as follows:

1) planets move in elliptical orbits with the sun at on of the focus points

2) planets sweep out equal areas in equal times

3) T_a²/T_b²=R_a³/R_b³

So what does all that mean? Kepler’s first law states that the planets do not move in perfect circles (despite what many people believed), but rather ellipses. His second law is a tad more complicated. If you mark a planet’s position at any given period of time, and then mark a second position sometime later the planet will have swept out a wedge.<IMAGE>Wedge Swept out by planet<IMAGE> As long as you use the same period of time between your marks you will find that no matter what wedge you choose to calculate the area for, it will always remain the same. What this boils down to is that the planets/comets move faster when they come closer to the sun, and slower as they move farther away. Kepler’s third law can be used to find out unknown information about another planet. It says that the ratio of one planet’s period squared to another planet’s period squared is proportional to the ratio of those same planets radius cubed. We usually use Earth as one of the planets because we know both its period and its radius. Earth’s period is one year and its radius (or average radius since this is an ellipse) is one astronomical unit. Kepler’s third law then simplifies to:

T²=R³

What this tells us is that if we square the period (measured in years) of a planet (which we can determine by its motion in the sky) and then take the cube root of that number, we will know the average distance from that planet to the sun (in astronomical units.)

To find out more about how stars are formed visit A Star’s Life. The sun is an average star, sitting at about the middle of the HR-Diagram. Its surface temperature is around 6000 degrees Kelvin, giving it a yellow appearance. It is a relatively low-mass star, expected to live for around ten billion years or so. Fortunately, for us, the sun has only been around for about four and a half billion years. So, we have plenty of time (probably about another four and a half billion years) before our sun starts dying on us. Even then, since it is a low-mass star, at least we won’t have to see what it is like to experience a supernova from close range as our sun will only dwindle down to a white dwarf.

Without the sun, there would not be life as we know if on our planet. Plants have found a wonderful way of taking the energy from the sun and turning it into energy our bodies can use. Through photosynthesis, plants convert Carbon Dioxide and water into sugar, with sunlight providing the energy for this reaction. Without plants producing sugar like this, the entire food chain would collapse. It is believed the dinosaurs suffered extinction when a large asteroid impacted the planet millions of years ago, the dust from the impact blocked out the sun for years. The plants were the first to die because they no longer had any energy to perform the chemical reactions they needed for food. Then, the animals that ate the plants died, and after them, the animals that ate those animals, etc.

The sun may be the primary source of energy for life on our planet but it is equally dangerous to that life. All of the sun’s energy is in the form of electromagnetic radiation. This includes radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Life as we know it cannot withstand any of the radiation above visible light (ultraviolet-gamma.) You can easily see this when you have been out in the sun too long and your skin burns. This is a result of overexposure to ultraviolet radiation. Fortunately, our atmosphere protects us from these forms of radiation.

The sun’s energy comes as a result of the nuclear reactions in its core. The energy travels from the core through the radiative zone. From there it enters the convective zone. The convective zone can best be thought of as a pot of boiling water. The energy from the sun "boils" up through the convective zone and is released through the outer layers of the sun. <IMAGE>Our Suns Layers<IMAGE>

Another interesting feature of our sun is sunspots. Sunspots are most likely the result of complex magnetic reactions on the surface of the sun. These spots can be seen on Earth as visibly darker regions on the sun’s surface. Galileo first noticed sunspots back in the early 1600’s. Since then, it has been determined that the sunspots follow a cycle and have a strong impact on the Earth. It is now known that sunspots occur in 11 year cycles. Every 11 years there is a flurry of sunspot activity followed by 11 years of relative quite. Sunspots are also linked closely with solar flares and coronal mass ejections, both of which have a profound impact on the Earth. The levels of electromagnetic radiation that reach the Earth can cause electrical equipment to go haywire. Even before the advent of electricity, the results of sunspots were noticed. It can be seen in the growth of trees as well as our weather. Every 11 years the space between a tree’s rings, a measurement of how healthy an environment the tree grew in that year, varies considerably. Between 1645 and 1715 it appears that sunspot simply stopped. No one is quite sure why. What is known, however, is that between those dates, Europe experienced what is known as the Little Ice Age. We are still trying to figure out exactly what affects sunspots may have on our planet.

Here’s a few simple tips to help you on your way to observing the stars:

1. It takes your eyes a while to get adjusted to the light (or lack thereof) when you are out looking at stars. After about 20-30 minutes you will start seeing stars that you could not see when you first went out.
2. Going hand-in-hand with item 1 is the use of a red light filter. If you need to use light to find something (or look at a star chart) use a red light filter. Red light will not shock your eyes as much. If you use regular white light then you will have to wait another 20-30 minutes for your eyes to adjust again.
3. Avoid light pollution if you can. Light pollution is most noticeable by any type of city. Light pollution is when all the lights from the city drown out the fainter stars in the sky. The ideal place to look for stars is as far away from any light source as possible. You can’t always avoid the city lights, however, so just remember that you will only be able to see some of the brighter stars in the sky.
4. Try to find an area without many buildings or trees around it. Sometimes the best constellations to view are close to the horizon. Trees and buildings will often block your view of these constellations.
5. To help you find those stars in the sky, remember this tip. Hold your arm straight out in front of you, parallel to the ground, and make a fist (palm facing left with the right hand or right with the left hand.) This is approximately ten degrees above the horizon. Now make a fist with your other hand and place it on top of your first fist. This is about 20 degrees above the horizon. Also, if you hold your arm straight out and raise your pinky up, your pinky covers about ½ of a degree of sky.<DIAGRAMS>Simple measuring tools<DIAGRAMS>
6. Make sure you get a star chart for the right latitude. Star charts are very different depending on where you live. When you purchase a star chart, make sure it is good for your location.

All of the planets and the moons lie on approximately the same plane in space. We call this plane the ecliptic. Even ancient peoples noticed the significance of this portion of the sky and picked out 12 constellations on it (Why 12? Most likely because there are 12 lunar cycles in one year and this made an easier way of telling time. You did not have to remember that this was the 11^th lunar cycle, you simply had to see that the constellation of Sagittarius was in the sky at night.) These are called the constellations of the zodiac. It was believed that the constellation that was "in the sun" when you were born, or the opposite of the one visible in the night sky, would affect your entire life. Even today you can pick up a newspaper and read your horoscope depending on your zodiac sign.<IMAGE>The Ecliptic<IMAGE>

Like the planets, comets orbit the sun as well. Comets, however, tend to have VERY eccentric orbits, approaching fairly close to the sun, then traveling well outside the orbit of Pluto. What makes comets such a spectacular view in the sky is their tail. This bright, cone-shaped cloud following the comet provides wonderful photographic opportunities.

Comets are made mostly from ice and dust particles. When a comet is outside the orbit of Jupiter it tends to simply be another object floating in space. Once it comes inside the orbit of Jupiter, however, the solar wind is strong enough to start blowing the dust from the comet. As the comet gets closer, the temperature rises, and the ice begins sublimating (going directly from a solid to a gas without passing through the liquid stage.) Many people believe that the tail of a comet is directly behind the comet and is caused by dust being left behind as the comet races through space. It is really the solar wind causes that causes a comet’s tail by blowing all of the debris off of the comet. Because of this, a comet’s tail always faces away from the sun and not necessarily behind the comet’s path.<DIAGRAM>Comet’s Tail always points away from sun<DIAGRAM>

Some of you may be wondering what happens to all of the debris left behind from a comet. The debris results in meteor showers on our planet. When you see what is called a "falling star" it is really a piece of stellar debris that is burning up in the Earth’s atmosphere. When we pass through the debris left by a comet the sky is filled with these "falling stars." Since comets follow an orbit, we are always able to know when we will next be passing through the debris of a comet. Below is a list of the month (watch the news, most likely the weather, to find out on what day the shower is supposed to peak that year), the meteor shower associated with it, the approximate number of "falling stars" per hour, and the comet associated with the debris:

<DIAGRAM>

Early January Quadrantids

Mid-Late April Lyrids

Early May Eta Aquarids 20-50 Halley

Late July Southern Delta Aquarids

Mid August Perseids

Mid Late October Orionids 20 Halley

Mid November Leonids Temple-Tuttle

Mid December Geminids

Hale-Bopp (1997): Hale-Bopp was an incredibly bright comet, very clearly visible even to the naked eye. In fact, when it was first discovered outside the orbit of Jupiter, it was 1000 times brighter than Haley’s comet was at the same distance. It is approximately 40 kilometers across which, for a comet, is rather large. Following the wave of fear from Hyakutake, Hale-Bopp was also supposed to bring the end of the world. Early tabloids claimed to spot a UFO following the comet. Although quickly shown false by scientist, 39 members of the Heaven’s Gate cult killed themselves as the comet approached, claiming the UFO behind the comet was their vehicle to a better place.

Hyakutake (1996): Tabloids claimed that this comet was on a path to impact Earth. This, coupled with the millenium only a few years away caused many people to start fearing the end of the Earth. Although the majority of fears were calmed, the spotting of Hale-Bopp in 1997 caused those fears to resurface.

Shoemaker-Levy 9 (1994): Although not a particularly bright comet, this comet’s claim to fame was its bombardment of Jupiter in July of 1994. In 1992 the comet came too close to Jupiter and was ripped apart by the planets gravitational field. In 1994, over 20 pieces of the former comet smashed into the planet with more force than the entire arsenal of nuclear weapons we have on our planet right now.

Haley’s (1986): Haley first discovered the orbital period of the comet in the early 1700’s. Later, searching through historical records showed the first recorded sighting of the comet to be in 240 BC. Mark Twain was born while Haley’s comet was in the sky. 76 years later he died with the comet in the sky as well. In 1986, when the comet returned, it was the most celebrated return of a comet ever. The next time Haley’s comet will pass by Earth will be in 2061.

As seen by the reactions to Hale-Bopp and Hyakutake, even people today fear comets. They were even more feared throughout history. Ancient peoples had identified the planets and stars and knew their positions in the sky. Comets, to them, were unpredictable and were associated with great events happening. Below are simply some of the events associated with comets in early times.

Comet of 44 BC: Appeared shortly after the death of Julius Caesar.

Comet of 66: Thought to signify the fall of Jerusalem in 70 AD.

With comets, we understand their orbital paths and know exactly where the comet will be and at what time. For the most part, asteroids remain the same. Most of the asteroids in our solar system exist in the asteroid belt, located between Mars and Jupiter. Asteroids, however, are much less predictable. Because asteroids are not as visible as comets are they can seem to appear out of nowhere. We know the orbits of many of the asteroids that come close to the Earth’s orbit, also called Apollo asteroids. The problem is that there are still thousands of them that we do not know about yet. This is why asteroids are one of the most feared objects in our solar system. That fear is well founded as well, for if a large enough asteroid were to impact the Earth it could destroy life as we know it.

The chances of such an impact are extremely small (about 1 every 250,000 years or so), but due to the catastrophic nature of such an impact we study the skies closely, searching for any such objects. Many programs, such as NASA’s Near Earth Object (NEO) Program actively search for asteroids that could impact Earth. After finding such asteroids they calculate the asteroids orbit to see if it will cross the path of our planet. It is estimated that we will need about 10 years warning to send spacecraft out to an asteroid to prevent its impact.

Any asteroid smaller than 50 meters will simply burn up when it enters our atmosphere. Asteroids between one and two kilometers in size will cause extensive damage to an area. Asteroids over two kilometers in size can affect the entire globe. When an asteroid of that size impacts the planet large amounts of dust are kicked up into the atmosphere. This dust blocks sunlight from reaching the surface. As a result, crops throughout the planet would die from the lack of sunlight and starvation and disease would overcome much of the population. Even worse are the extremely large asteroids whose impact would block the sun long enough to wipe out the entire population. It was an extremely large asteroid (15 kilometers in size) that caused the extinction of the dinosaurs 65 million years ago.

Cassini – Launched in 1997, the Cassini satellite is scheduled to reach its destination at the planet Saturn by 2004. Onboard the satellite are 12 scientific instruments whose purpose is to make up-close studies of Saturn, its rings, magnetic environment, and its moons. The satellite should remain in orbit of the planet from 2004-2008. It will also release a probe, named Huygens, developed by the European Space Agency to land on the surface of the moon Titan.

Genesis – Scheduled for launch in 2001, the Genesis spacecraft will travel towards the hottest spot in our solar system, the sun itself. After traveling about a million miles towards the sun, the spacecraft will unfold its collectors and sit, for two years, collecting as much of the solar wind as it can. It will then return to Earth with its collection.

Deep Space 1 – Truly a mission meant to test out new technologies. Deep Space 1 was launched in 1998 and orbits Earth. One of its missions was to take close-up studies of an asteroid that came close to Earth. Another possible study may be to perform close-up studies of two comets as they pass by the Earth. Some of the new technologies onboard the spacecraft include an ion propulsion engine, an onboard navigation system using images of the stars, and warning indicators that notify NASA when human intervention is needed to fix problems on the spacecraft.

Deep Space 2 · Mars Microprobe – One of the main objectives of this mission, like the Deep Space 1, is to test out new technology. Rather than put this technology to waste, however, Deep Space 2 will voyage to Mars to perform some scientific studies. Tests will be done to determine if ice exists under the surface of the planet. The mission also plans to determine the thermal properties of the subsurface soil, characterize the atmospheric density, characterize the hardness of the soil, and determine if there is any layering of soil.

Galileo – Launched in 1989, Galileo arrived at the planet Jupiter in 1995. The next two years were spent capturing images and statistics about the planet while also gathering information about Jupiter’s moons. Next, the spacecraft would spend two years studying Jupiter’s moon Europa. After that, it would travel over to Io and explore the large amounts of volcanic activity on the moon.

Lunar Prospector – Launched in 1998, this probe was sent to the moon to try and get a better understanding of its features as well as answer the long-standing question of whether there was truly ice in the craters at the moon’s polar cap.

Mars exploration – There have been a number of missions to Mars, all of which hoping to find some answers to many of the questions held by our small neighbor. Questions such as whether water existed on the planet, whether life ever existed there, and if there is any way we can set up a colony on Mars. These are just some of the missions sent to Mars:

Mars Pathfinder – Last spacecraft to land on the planet to take surface samples.

Mars Global Surveyor – Currently orbiting the planet taking data.

Mars Surveyor 98 – Currently en route to Mars.

Mars Surveyor 2001 – Mission currently under development.

Near-Earth Asteroid Rendezvous (NEAR) – This will be the first satellite ever to orbit an asteroid. Scientist hope that the data discovered during the satellite’s orbit about the asteroid will give us a better understanding of asteroids themselves. Hopefully, with a better understanding of these massive objects we will be able to make better predictions of their behaviors and know sooner whether or not they may impact our planet.

Pioneer 10 – Pioneer 10 was the first satellite to ever cross the asteroid belt. It was launched back in 1972 and provided some of the first information on both Jupiter and Saturn. It also made returned valuable data on the outer reaches of our solar system. It is currently on a path to the constellation of Taurus the bull. It is expected to take two million years for pioneer to reach the first stars in the constellation.

Voyager 1 and 2 – The primary focus of these missions, launched in 1977, is to escape the sun’s influence and return readings from interstellar space. Voyager 1 performed close flybys of Jupiter and Saturn before beginning its journey to the outer reaches of our solar system. Voyager 2 not only performed close flybys of Jupiter and Saturn, but of Uranus and Neptune as well. Voyager 2 is also on a path for the outer reaches of the solar system. The hope of the missions is that by escaping the magnetic influence of the sun and the solar wind, we can get better reading from other stars and galaxies and get a better understanding of what the rest of space is truly like.

Stardust – The Stardust mission, launched in 1999, will be the first mission in history to return extraterrestrial material from outside of the moon’s orbit. The spacecraft is set to rendezvous with the comet Wild 2 in 2004 and obtain samples from it. It will also obtain samples of the interstellar dust streaming into our solar system from the direction of Sagittarius. It will then return with these samples so scientists can study them more closely.

Solar and Heliospheric Observatory (SOHO) – Launched in 1995, SOHO is constantly studying every aspect of the sun, from the core itself to the solar wind. It sits about 1.5 million kilometers away from Earth in the direction of the sun. SOHO is a joint cooperation between the European Space Agency and NASA.

Earth – Moon

Discovered by always known

Date of discovery always known

Mass (kg) 7.4e+22

Radius (km) 1,737

Mean density (gm/cm^3) 3.3

Mean distance from Earth (km) 384,400

Rotational period (days) 27.3

Orbital period (days) 27.3

Mars – Phobos

Discovered by Asaph Hall

Date of discovery 1877

Mass (kg) 1.1e+16

Radius (km)

Mean density (gm/cm^3) 2.0

Mean distance from Mars (km) 9,380

Rotational period (days) 0.32

Orbital period (days) 0.32

Mars – Deimos

Discovered by Asaph Hall

Date of discovery 1877

Mass (kg) 1.8e+15

Radius (km)

Mean density (gm/cm^3) 1.7

Mean distance from Mars (km) 23,460

Rotational period (days) 1.26

Orbital period (days) 1.26

Jupiter – Metis

Discovered by Stephen Synnott

Date of discovery 1979

Mass (kg) 9.6e+16

Equatorial radius (km) 20

Mean density (gm/cm^3) 2.8

Mean distance from Jupiter (km) 127,969

Rotational period (days) ?

Orbital period (days) 0.3

Jupiter – Adrastea

Discovered by D. Jewitt & E. Danielson

Date of discovery 1979

Mass (kg) 1.9e+16

Radius (km)

Mean density (gm/cm^3) 4.5

Mean distance from Jupiter (km) 128,971

Rotational period (days) ?

Orbital period (days) 0.3

Jupiter – Amalthea

Discovered by Edward Emerson Barnard

Date of discovery 1892

Mass (kg) 7.2e+18

Radius (km)

Mean density (gm/cm^3) 1.8

Mean distance from Jupiter (km) 181,300

Rotational period (days) 0.5

Orbital period (days) 0.5

Jupiter – Thebe

Discovered by Stephen Synnott

Date of discovery 1979

Mass (kg) 7.8e+17

Radius (km)

Mean density (gm/cm^3) 1.5

Mean distance from Jupiter (km) 221,895

Rotational period (days) 0.67

Orbital period (days) 0.67

Jupiter – Io

Discovered by Simon Marius & Galileo Galilei

Date of discovery 1610

Mass (kg) 8.9e+22

Radius (km) 1,815

Mean density (gm/cm^3) 3.6

Mean distance from Jupiter (km) 421,600

Rotational period (days) 1.75

Orbital period (days) 1.75

Jupiter – Europa

Discovered by Simon Marius & Galileo Galilei

Date of discovery 1610

Mass (kg) 4.8e+22

Radius (km) 1,569

Mean density (gm/cm^3) 3.0

Mean distance from Jupiter (km) 670,900

Rotational period (days) 3.55

Orbital period (days) 3.55

Jupiter – Ganymede

Discovered by Simon Marius & Galileo Galilei

Date of discovery 1610

Mass (kg) 1.5e+23

Radius (km) 2,631

Mean distance from Jupiter (km) 1,070,000

Rotational period (days) 7.15

Orbital period (days) 7.15

Jupiter – Callisto

Discovered by Simon Marius & Galileo Galilei

Date of discovery 1610

Mass (kg) 1.1e+23

Radius (km) 2,400

Mean density (gm/cm^3) 1.9

Mean distance from Jupiter (km) 1,883,000

Rotational period (days) 16.69

Orbital period (days) 16.69

Jupiter – Leda

Discovered by C. Kowal

Date of discovery 1974

Mass (kg) 5.7e+15

Radius (km) 8

Mean density (gm/cm^3) 2.7

Mean distance from Jupiter (km) 11,094,000

Rotational period (days)

Orbital period (days) 238.72

Jupiter – Himalia

Discovered by C. Perrine

Date of discovery 1904

Mass (kg) 9.6e+18

Radius (km) 93

Mean density (gm/cm^3) 2.8

Mean distance from Jupiter (km) 11,480,000

Rotational period (days) 0.4

Orbital period (days) 250.57

Jupiter – Lysithea

Discovered by S. Nicholson

Date of discovery 1938

Mass (kg) 7.8e+16

Radius (km) 18

Mean density (gm/cm^3) 3.1

Mean distance from Jupiter (km) 11,720,000

Rotational period (days)

Orbital period (days) 259.22

Jupiter – Elara

Discovered by C. Perrine

Date of discovery 1905

Mass (kg) 7.8e+17

Radius (km) 38

Mean density (gm/cm^3) 3.3

Mean distance from Jupiter (km) 11,737,000

Rotational period (days) 0.5

Orbital period (days) 259.65

Jupiter – Ananke

Discovered by S. Nicholson

Date of discovery 1951

Mass (kg) 3.8e+16

Radius (km) 15

Mean density (gm/cm^3) 2.7

Mean distance from Jupiter (km) 21,200,000

Rotational period (days)

Orbital period (days) -631

Jupiter – Carme

Discovered by S. Nicholson

Date of discovery 1938

Mass (kg) 9.6e+16

Radius (km) 20

Mean density (gm/cm^3) 2.8

Mean distance from Jupiter (km) 22,600,000

Rotational period (days)

Orbital period (days) -692

Jupiter – Pasiphae

Discovered by P. Melotte

Date of discovery 1908

Mass (kg) 1.9e+17

Radius (km) 25

Mean density (gm/cm^3) 2.9

Mean distance from Jupiter (km) 23,500,000

Rotational period (days)

Orbital period (days) -735

Jupiter – Sinope

Discovered by S. Nicholson

Date of discovery 1914

Mass (kg) 7.8e+16

Radius (km) 18

Mean density (gm/cm^3) 3.1

Mean distance from Jupiter (km) 23,700,000

Rotational period (days)

Orbital period (days) -758

Saturn – Pan

Discovered by Mark R. Showalter

Date of discovery 1990

Mass (kg)

Radius (km) 9.7

Mean density (gm/cm^3)

Mean distance from Saturn (km) 133,583

Rotational period (days)

Orbital period (days) 0.58

Saturn – Atlas

Discovered by R. Terrile

Date of discovery 1980

Mass (kg)

Radius (km)

Mean density (gm/cm^3)

Mean distance from Saturn (km) 137,640

Rotational period (days)

Orbital period (days) 0.60

Saturn – Prometheus

Discovered by S. Collins & others

Date of discovery 1980

Mass (kg) 2.7e+17

Radius (km)

Mean density (gm/cm^3) 0.7

Mean distance from Saturn (km) 139,350

Rotational period (days)

Orbital period (days) 0.61

Saturn – Pandora

Discovered by S. Collins & others

Date of discovery 1980

Mass (kg) 2.2e+17

Radius (km)

Mean distance from Saturn (km) 141,700

Rotational period (days)

Orbital period (days) 0.63

Saturn – Epimetheus

Discovered by R. Walker

Date of discovery 1980

Mass (kg) 5.6e+17

Radius (km)

Mean density (gm/cm^3) 0.7

Mean distance from Saturn (km) 151,422

Rotational period (days) 0.69

Orbital period (days) 0.69

Saturn – Janus

Discovered by Audouin Dollfus

Date of discovery 1966

Mass (kg) 2.0e+18

Radius (km)

Mean distance from Saturn (km) 151,472

Rotational period (days) 0.69

Orbital period (days) 0.69

Saturn – Mimas

Discovered by William Herschel

Date of discovery 1789

Mass (kg) 3.8e+19

Radius (km) 196

Mean density (gm/cm^3) 1.2

Mean distance from Saturn (km) 185,520

Rotational period (days) 0.94

Orbital period (days) 0.94

Saturn – Enceladus

Discovered by William Herschel

Date of discovery 1789

Mass (kg) 8.4e+19

Radius (km) 250

Mean density (gm/cm^3) 1.2

Mean distance from Saturn (km) 238,020

Rotational period (days) 1.37

Orbital period (days) 1.37

Saturn – Tethys

Discovered by Giovanni Domenico Cassini

Date of discovery 1684

Mass (kg) 7.6e+20

Radius (km) 530

Mean density (gm/cm^3) 1.2

Mean distance from Saturn (km) 294,660

Rotational period (days) 1.89

Orbital period (days) 1.89

Saturn – Telesto (shares Tethys orbit… 60 degrees in front)

Discovered by B. Smith & others

Date of discovery 1980

Mass (kg)

Radius (km)

Mean density (gm/cm^3)

Mean distance from Saturn (km) 294,660

Rotational period (days)

Orbital period (days) 1.89

Saturn – Calypso (shares Tethys orbit… about 60 degrees behind)

Discovered by B. Smith & others

Date of discovery 1980

Mass (kg)

Radius (km)

Mean density (gm/cm^3)

Mean distance from Saturn (km) 294,660

Rotational period (days)

Orbital period (days) 1.89

Saturn – Dione

Discovered by Giovanni Domenico Cassini

Date of discovery 1684

Mass (kg) 1.1e+21

Radius (km) 560

Mean density (gm/cm^3) 1.4

Mean distance from Saturn (km) 377,400

Rotational period (days) 2.74

Orbital period (days) 2.74

Saturn – Helene

Discovered by P. Laques & J. Lecacheus

Date of discovery 1980

Mass (kg)

Radius (km)

Mean density (gm/cm^3)

Mean distance from Saturn (km) 377,400

Rotational period (days)

Orbital period (days) 2.74

Saturn – Rhea

Discovered by Giovanni Domenico Cassini

Date of discovery 1672

Mass (kg) 2.5e+21

Radius (km) 765

Mean density (gm/cm^3) 1.3

Mean distance from Saturn (km) 527,040

Rotational period (days) 4.52

Orbital period (days) 4.52

Saturn – Titan

Discovered by Christiaan Huygens

Date of discovery 1655

Mass (kg) 1.4e+23

Radius (km) 2,575

Mean density (gm/cm^3) 1.9

Mean distance from Saturn (km) 1,221,850

Rotational period (days) 15.95

Orbital period (days) 15.95

Saturn – Hyperion

Discovered by William Cranch Bond

Date of discovery 1848

Mass (kg) 1.8e+19

Radius (km)

Mean density (gm/cm^3) 1.4

Mean distance from Saturn (km) 1,481,000

Rotational period (days) chaotic

Orbital period (days) 21.28

Saturn – Iapetus

Discovered by Giovanni Domenico Cassini

Date of discovery 1671

Mass (kg) 1.9e+21

Radius (km) 730

Mean density (gm/cm^3) 1.2

Mean distance from Saturn (km) 3,561,300

Rotational period (days) 79.33

Orbital period (days) 79.33

Saturn – Phoebe

Discovered by William Henry Pickering

Date of discovery 1898

Mass (kg) 4.0e+18

Radius (km) 110

Mean density (gm/cm^3) 0.7

Mean distance from Saturn (km) 12,952,000

Rotational period (days) 0.4

Orbital period (days) -550.48

Uranus – Cordelia

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 13

Mean density (gm/cm^3)

Mean distance from Uranus (km) 49,750

Rotational period (days)

Orbital period (days) 0.34

Uranus – Ophelia

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 16

Mean density (gm/cm^3)

Mean distance from Uranus (km) 53,760

Rotational period (days)

Orbital period (days) 0.38

Uranus – Bianca

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 22

Mean density (gm/cm^3)

Mean distance from Uranus (km) 59,160

Rotational period (days)

Orbital period (days) 0.43

Uranus – Cressida

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 33

Mean density (gm/cm^3)

Mean distance from Uranus (km) 61,770

Rotational period (days)

Orbital period (days) 0.46

Uranus – Desdemona

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 29

Mean density (gm/cm^3)

Mean distance from Uranus (km) 62,660

Rotational period (days)

Orbital period (days) 0.47

Uranus – Juliet

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 42

Mean density (gm/cm^3)

Mean distance from Uranus (km) 64,360

Rotational period (days)

Orbital period (days) 0.49

Uranus – Portia

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 55

Mean density (gm/cm^3)

Mean distance from Uranus (km) 66,100

Rotational period (days)

Orbital period (days) 0.51

Uranus – Rosalind

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 27

Mean density (gm/cm^3)

Mean distance from Uranus (km) 69,930

Rotational period (days)

Orbital period (days) 0.56

Uranus – Belinda

Discovered by Voyager 2

Date of discovery 1986

Mass (kg)

Equatorial radius (km) 34

Mean density (gm/cm^3)

Mean distance from Uranus (km) 75,260

Rotational period (days)

Orbital period (days) 0.62

Uranus – Puck

Discovered by Stephen Synnott

Date of discovery 1985

Mass (kg)

Equatorial radius (km) 77

Mean density (gm/cm^3)

Mean distance from Uranus (km) 86,010

Rotational period (days)

Orbital period (days) 0.76

Uranus – Miranda

Discovered by Gerard Kuiper

Date of discovery 1948

Mass (kg) 6.33e+19

Equatorial radius (km) 235.8

Mean density (gm/cm^3) 1.15

Mean distance from Uranus (km) 129,780

Rotational period (days) 1.41

Orbital period (days) 1.41

Uranus – Ariel

Discovered by William Lassell

Date of discovery 1851

Mass (kg) 1.3e+21

Equatorial radius (km) 578.9

Mean density (gm/cm^3) 1.6

Mean distance from Uranus (km) 191,240

Rotational period (days) 2.52

Orbital period (days) 2.52

Uranus – Umbriel

Discovered by William Lassell

Date of discovery 1851

Mass (kg) 1.3e+21

Mean density (gm/cm^3) 1.5

Mean distance from Uranus (km) 265,970

Rotational period (days) 4.14

Orbital period (days) 4.14

Uranus – Titania

Discovered by William Herschel

Date of discovery 1787

Mass (kg) 3.5e+21

Equatorial radius (km) 788.9

Mean density (gm/cm^3) 1.7

Mean distance from Uranus (km) 435,840

Rotational period (days) 8.71

Orbital period (days) 8.71

Uranus – Oberon

Discovered by William Herschel

Date of discovery 1787

Mass (kg) 3.0e+21

Equatorial radius (km) 761.4

Mean density (gm/cm^3) 1.6

Mean distance from Uranus (km) 582,600

Rotational period (days) 13.46

Orbital period (days) 13.46

Neptune – Naiad

Discovered by Voyager 2

Date of discovery 1989

Mass (kg)

Equatorial radius (km) 29

Mean density (gm/cm^3)

Mean distance from Neptune (km) 48,000

Rotational period (days)

Orbital period (days) 0.29

Neptune – Thalassa

Discovered by Voyager 2

Date of discovery 1989

Mass (kg)

Equatorial radius (km) 40

Mean density (gm/cm^3)

Mean distance from Neptune (km) 50,000

Rotational period (days)

Orbital period (days) 0.31

Neptune – Despina

Discovered by Voyager 2

Date of discovery 1989

Mass (kg)

Equatorial radius (km) 74

Mean density (gm/cm^3)

Mean distance from Neptune (km) 52,500

Rotational period (days)

Orbital period (days) 0.33

Neptune – Galatea

Discovered by Voyager 2

Date of discovery 1989

Mass (kg)

Equatorial radius (km) 79

Mean density (gm/cm^3)

Mean distance from Neptune (km) 62,000

Rotational period (days)

Orbital period (days) 0.43

Neptune – Larissa

Discovered by Stephen Synnott

Date of discovery 1989

Mass (kg)

Radius (km)

Mean density (gm/cm^3)

Mean distance from Neptune (km) 73,600

Rotational period (days)

Orbital period (days) 0.55

Neptune – Proteus

Discovered by Stephen Synnott

Date of discovery 1989

Mass (kg)

Equatorial radius (km) 200

Mean density (gm/cm^3)

Mean distance from Neptune (km) 117,600

Rotational period (days)

Orbital period (days) 1.12

Neptune – Triton

Discovered by William Lassell

Date of discovery 1846

Mass (kg) 2.1e+22

Equatorial radius (km) 1,350

Mean density (gm/cm^3) 2.1

Mean distance from Neptune (km) 354,800

Rotational period (days) -5.88

Orbital period (days) -5.88

Neptune – Nereid

Discovered by Gerard Kuiper

Date of discovery 1949

Mass (kg)

Equatorial radius (km) 170

Mean density (gm/cm^3)

Mean distance from Neptune (km) 5,513,400

Rotational period (days)

Orbital period (days) 360.14

Pluto – Charon

Discovered by J. Christy

Date of discovery 1978

Mass (kg) 1.9e+21

Equatorial radius (km) 586

Mean density (gm/cm^3) 1.8

Mean distance from Pluto (km) 19,640

Rotational period (days) 6.39

Orbital period (days) 6.39

Mercury

Mass (kg) 3.3e+23

Equatorial radius (km) 2,439.7

Mean density (gm/cm^3) 5.4

Mean distance from the Sun (km) 57,910,000

Rotational period (days) 58.65

Orbital period (days) 87.97

Tilt of axis (degrees) 0

Equatorial surface gravity (m/sec^2) 2.8

Mean surface temperature 179°C

Maximum surface temperature 427°C

Minimum surface temperature -173°C

Atmospheric composition

Helium 42%

Sodium 42%

Oxygen 15%

Other 1%

Venus

Mass (kg) 4.9e+24

Equatorial radius (km) 6,051.8

Mean density (gm/cm^3) 5.3

Mean distance from the Sun (km) 108,200,000

Rotational period (days) -243.02

Orbital period (days) 224.70

Tilt of axis (degrees) 177

Equatorial surface gravity (m/sec^2) 8.9

Mean surface temperature 482°C

Atmospheric composition

Carbon dioxide 96%

Nitrogen 3+%

Trace amounts of: Sulfur dioxide, water vapor, carbon monoxide, argon,

helium, neon, hydrogen chloride, and hydrogen fluoride.

Earth

Mass (kg) 6e+24

Equatorial radius (km) 6,378.14

Mean density (gm/cm^3) 5.5

Mean distance from the Sun (km) 149,600,000

Rotational period (days) 1

Orbital period (days) 365.25

Tilt of axis (degrees) 23.5

Equatorial surface gravity (m/sec^2) 9.8

Mean surface temperature 15°C

Atmospheric composition

Nitrogen 77%

Oxygen 21%

Other 2%

Mars

Mass (kg) 6.4e+23

Equatorial radius (km) 3,397.2

Mean density (gm/cm^3) 3.9

Mean distance from the Sun (km) 227,940,000

Rotational period (days) 1.03

Orbital period (days) 686.98

Tilt of axis (degrees) 25

Equatorial surface gravity (m/sec^2) 3.7

Minimum surface temperature -140°C

Mean surface temperature -63°C

Maximum surface temperature 20°C

Atmospheric composition

Carbon Dioxide (C02) 95.32%

Nitrogen (N2) 2.7%

Argon (Ar) 1.6%

Trace amounts of

Carbon Monoxide (CO)

Neon (Ne)

Krypton (Kr)

Xenon (Xe)

Ozone (O3)

Jupiter

Mass (kg) 1.9e+27

Equatorial radius (km) 71,492

Mean density (gm/cm^3) 1.3

Mean distance from the Sun (km) 778,330,000

Rotational period (days) 0.41

Orbital period (days) 4332.71

Tilt of axis (degrees) 3

Equatorial surface gravity (m/sec^2) 22.9

Mean cloud temperature -121°C

Atmospheric composition

Hydrogen 90%

Helium 10%

Saturn

Mass (kg) 5.7e+26

Equatorial radius (km) 60,268

Mean density (gm/cm^3) 0.7

Mean distance from the Sun (km) 1,429,400,000

Rotational period (hours) 10.23

Orbital period (years) 29.46

Tilt of axis (degrees) 25

Equatorial surface gravity (m/sec^2) 9.1

Mean cloud temperature -125°C

Atmospheric composition

Hydrogen 97%

Helium 3%

Uranus

Discovered by William Herschel

Date of discovery 1781

Mass (kg) 8.7e+25

Equatorial radius (km) 25,559

Mean density (gm/cm^3) 1.29

Mean distance from the Sun (km) 2,870,990,000

Rotational period (hours) -17.9

Orbital period (years) 84.01

Tilt of axis (degrees) 98

Equatorial surface gravity (m/sec^2) 7.8

Mean cloud temperature -193°C

Atmospheric composition

Hydrogen 83%

Helium 15%

Methane 2%

Neptune

Discovered by Johann Gotfried Galle

Date of discovery September 23, 1846

Mass (kg) 1.0e+26

Equatorial radius (km) 24,746

Mean density (gm/cm^3) 1.6

Mean distance from the Sun (km) 4,504,300,000

Rotational period (hours) 16.11

Orbital period (years) 164.79

Tilt of axis (degrees) 28

Equatorial surface gravity (m/sec^2) 11.0

Mean cloud temperature -173°C

Atmospheric composition

Hydrogen 85%

Helium 13%

Methane 2%

Pluto

Discovered by Clyde W. Tombaugh

Date of discovery February 18, 1930

Mass (kg) 1.3e+22

Equatorial radius (km) 1,137

Mean density (gm/cm^3) 2.1

Mean distance from the Sun (km) 5,913,520,000

Rotational period (days) -6.39

Orbital period (years) 248.54

Tilt of axis (degrees) 123

Equatorial surface gravity (m/sec^2) 0.4