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A Guide to the Solar System
By Mark R. Chartrand, Ron Miller
St. Martin's PressCopyright © 1990 St. Martin's Press
All rights reserved.
OBSERVING THE SKY
The zodiac is the region of the sky through which the Sun, Moon, and planets pass, as seen from Earth. It is a band about 30 degrees north and south of the celestial equator, the projection of the Earth's equator into the sky. For simplicity, we pretend that all the stars are attached to a great "celestial sphere" some large, indefinite distance away. These stars, and the patterns of the stars called constellations, are the background against which the planets move.
Following the practice of the ancients, the band of the zodiac is divided into 12 constellations, most of which were thought to represent living creatures. "Zodiac" means "circle of the animals" in Greek. The 12 constellations of the zodiac are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.
A convenient way of locating the planets is to give the name of the constellation of the zodiac they are "in," which really means "in front of." Because of changes in the sky over the past 2,000 years, which altered the orientation of the constellations, today the planets actually travel through more than the 12 classical constellations.
As seen from the Earth, the Sun seems to travel during a year around the zodiac, always along a line or path in the sky called the ecliptic, the plane of the Earth's orbit around the Sun projected out onto the celestial sphere. Because the Earth is tilted over 23 degrees, the ecliptic and the celestial equator are inclined by the same amount. The orbits of the Moon and planets are tilted slightly to the ecliptic, so their paths in the sky do not lie exactly along the ecliptic, but they are always close — except for Pluto.
PLANETARY PHENOMENA refers to the appearance in the sky and the relative positions of planets with respect to the Sun or each other. The angle between the Sun and a planet, as we see them from Earth, is a planet's elongation.
"EVENING STAR" is a term given to any planet that remains in the sky after the Sun has set. Such a planet will be east of the Sun — that is, it will have an easterly elongation.
"MORNING STAR" refers to a planet that is in the sky at dawn. It will be west of the Sun, and so have a westerly elongation. All the planets are at times morning stars and at other times evening stars.
INFERIOR PLANETS — Mercury and Venus — are those with orbits closer to the Sun than Earth's orbit. They are never seen at a great angle from the Sun. When an inferior planet is aligned with the Sun, it is said to be in conjunction. Inferior conjunction ("I" on diagram below) occurs when the planet is on the near side of the Sun, superior conjunction ("S") when it is on the far side of the Sun. The greatest angular distance between an inferior planet and the Sun occurs when our line of sight to the planet is tangent to the planet's orbit, called greatest eastern elongation ("E") when the planet is east of the Sun and greatest western elongation ("W") when it is west of the Sun.
Inferior planets show phases, like the Moon, related to where they are in their orbits. They are at full phase at the time of superior conjunction, half phase at times of greatest elongation, and new phase (hence invisible) at the time of inferior conjunction.
SUPERIORPLANETS–Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto — are those with orbits larger than Earth's, and they may have any elongation. When on the far side of the Sun, as seen from Earth, they are said to be in conjunction ("C" on diagram below). When opposite the Sun, they are in opposition ("O"). When they are at right angles to the Sun, they are said to be in quadrature ("Q"). Such planets have only a narrow range of phases, being almost fully illuminated as seen from Earth at all times.
THE SYNODIC PERIOD of an object is the length of time it takes for the object to return to a previous relative position, such as opposition or conjunction.
VIEWING THE PLANETS For the inferior planets, times of greatest elongation are best for viewing. The superior planets are brightest and visible longest at night around the time of opposition — when the Earth is directly between the planet and the Sun.
Planets are not visible close to the times of conjunctions, for then they are too close to the Sun in the sky.
FINDING THINGS IN THE SKY
What you see depends upon when you look. Sometimes the things you want to see are up in the daytime. The Moon is bright enough to be seen in the daytime, but the stars and planets are overpowered by the brightness of the daytime sky. As the Earth turns, the sky seems to revolve about us, moving west 15 degrees every hour.
To locate things in the sky, you must first know which direction is north. You can use a magnetic compass, or, at night, look for the North Star. For those in the northern hemisphere, the celestial equator will start at the horizon in the east, rise upward and to the right toward the south, reaching its highest point due south, and then decline downward to the right, intersecting the horizon due west, opposite east. All planetary objects will appear within 30 degrees on either side of this invisible celestial path.
AZIMUTH is a measure of direction around the horizon starting at 0 degrees due north, moving clockwise through 90 degrees at east, 180 degrees at south, 270 degrees at west, and 360, or 0, degrees again at north.
ELEVATION of an object is its angle above the horizon, from 0 degrees at the horizon to 90 degrees directly overhead (a point called the zenith). A useful rule of thumb is that your fist, held out at arm's length, is about 10 degrees across.
FINDING CHARTS Each planet in this book has a "finding chart" that shows the planet's path through the zodiac during upcoming years or in the morning and evening skies. (The chart for each planet can be found in the section on "Observing.") Tables give the best times for viewing each planet. For Mercury, this would be the week or so around the dates of greatest elongations; for Venus, several weeks around the greatest elongations; for the other planets, the several months around the times of opposition.
EFFECTS OF THE ATMOSPHERE on what we see in the sky are due to our looking through miles of turbulent air. When we look at objects near the horizon the effects are greater, because we are then looking through more air.
TWINKLING, or scintillation, is caused by air layers slightly bending the light as it passes toward us. Stars, which are so far away they appear as mere points of light, twinkle more than planets do. Planets, which are closer to us, appear as small disks of light; they usually twinkle less. For objects close to the horizon, the air may act like a prism, causing a star or a planet to display flashing colors.
REDDENING occurs because the atmosphere is not equally transparent to all colors of light. As a beam of light travels, the air removes some of its blue light, scattering it in all directions and causing the sky to appear blue. The remaining light reaching your eye looks redder. The more air a light beam travels through, the greater the reddening, which is why the Sun and Moon appear reddish when they are low in the sky. Air pollution may accentuate this effect.
REFRACTION is a bending of the light from an object in the sky, making the object appear slightly higher in the sky than it is. The effect is greatest closer to the horizon, where it seems to flatten the rising or setting Sun and Moon.
THE MOON ILLUSION
The moon illusion is the name given to the optical illusion that the Moon (or Sun) appears larger when it is near the horizon than when it is high in the sky.
This seems to be caused by the fact that, when near the horizon, the Moon is seen near familiar foreground objects, such as trees or buildings or the horizon itself. Because the brain cannot determine how far away the Moon is, it incorrectly interprets what we see. The effect seems most noticeable for the Moon when it is near full phase.
One way to prove that the Moon does not change size is to photograph it rising, and then again, with the same camera and lens, when it is high in the sky. The image of the Moon on the slide or negative will measure the same linear size.CHAPTER 2
SOLAR SYSTEM EXPLORATION
Until the space age, we could only study the universe from Earth with instruments that had to look upward through miles of turbulent, filtering, polluted atmosphere. Now we send our robot observers throughout the solar system.
THE SUN has been examined by several special space observatories near Earth, including the Orbiting Solar Observatory and the Solar Maximum Mission. The distant effects of the solar wind have also been studied by probes called IMP, for Interplanetary Monitoring Platform, and by sensors on space probes heading to other planets. One of the IMP craft was later used to study a comet.
MERCURY has been explored by only one space probe as of 1989, Mariner 10, in 1973 and 1974. Its flight allowed it to pass this innermost planet three times, mapping the surface.
VENUS has been extensively investigated. The U.S. probe Mariner 2 flew by in 1962, revolutionizing our knowledge of this cloud-wrapped world. Since then, other American visits have been made by Mariner 5 in 1967, Mariner 10 in 1974, and Pioneer Venus, which in 1978 orbited the planet and sent a probe into its atmosphere. Most recently the Magellan spacecraft made detailed maps of the surface. By 1989, the Soviet Union had sent 13 spacecraft, all named Venera, to Venus. Some have even landed on the surface and taken a few photographs before the hostile atmosphere of that planet destroyed the probes. These are the only surface photographs we have of any planetary body besides the Moon and Mars.
EARTH, too, has been explored with spacecraft. Meteorological satellites monitor our atmosphere. Closer-orbiting Landsat earth resources satellites have also greatly improved our knowledge of our planet and its biological, geological, and hydrological systems.
THE MOON, the only planetary body besides Earth explored by humans in person, was visited during the American Apollo program from 1969-72. More than 800 pounds of moon rocks were brought back for study. Before that, several U.S. and Soviet probes orbited or landed on the Moon to pave the way. The U.S.S.R. has sent robot landers to gather and bring back rock samples from the surface.
MARS was first photographed close up by the American Mariner 4 in 1965. Three other Mariner craft and several Soviet spacecraft have visited the Red Planet, although the latter were not successful. The climax of Martian investigation came in 1976 when two U.S. Viking spacecraft orbited the planet and sent landing craft down to the surface. These landers operated for years, sending back photographs, soil analyses, and meteorological data.
JUPITER saw the flybys of Pioneer 10 in 1973, Pioneer 11 in 1974, and Voyagers 1 and 2 in 1979. They discovered new satellites and a ring, and photographed and measured details in Jupiter's atmosphere invisible from Earth. The computer-enhanced photographs of Jupiter are some of the most beautiful ever taken.
SATURN was visited by Pioneer 11 in 1979, by Voyager 1 in 1980, and then by Voyager 2 in 1981, after their swings by Jupiter, whose gravity helped get them out to the Ringed Planet. They discovered new moons and myriad rings.
URANUS was visited by Voyager 2 in 1986, after a gravity assist from Saturn. It discovered many new satellites, new rings, and a puzzling tilted magnetic field that has yet to be fully explained.
NEPTUNE was visited by Voyager 2 in August, 1989, the last Voyager encounter before leaving the solar system.
TWO COMETS have been probed. The first was Comet Giacobini-Zinner in 1985, reusing one of the IMP spacecraft, renamed ICE (International Cometary Explorer). Data from ICE helped space scientists from Europe, Japan, and the Soviet Union send probes to Comet Halley.
STARS, NEBULAS, AND GALAXIES as well as planets have been observed by the Orbiting Astronomical Observatory from near-Earth orbit, and by other specialized instruments that can receive kinds of light that never reach the surface of Earth. The latest is the Hubble Space Telescope, which promises to revolutionize our knowledge of the cosmos.CHAPTER 3
FORMATION OF THE SUN AND PLANETS
Our universe began about 15 billion years ago with a tremendous explosion astronomers call the "Big Bang." The lighter elements, mostly hydrogen and helium, were formed in this explosion and spread throughout the universe. Later, most of these gases collected together into large bodies we call the galaxies. Our Milky Way galaxy is one of these, a giant pinwheel of stars, gas, and dust over 100,000 light-years across. Our Sun is one of several hundred billion stars in the Milky Way.
The first stars to form in our Milky Way galaxy, perhaps 10 billion years ago, were almost pure hydrogen and helium, and could not have formed hard, rocky planets because there were no heavier elements. Some of the more massive original stars lived out their lives and exploded as supernovas. In the extreme heat of these explosions, heavier elements such as carbon, oxygen, silicon, and iron were formed and spewed out to enrich the remaining gas.
ABOUT 4.5 BILLION YEARS AGO one such supernova caused a cloud of interstellar gas and dust to condense, and the cloud continued to collapse under the force of its own gravity. As it shrank, it grew hotter and began to spin faster. The inner portion shrank faster, and got even hotter, spinning off and leaving behind smaller orbiting clouds of gas. These smaller clouds condensed to form large lumps of gas with some heavy material at their centers. The central blob of gas collapsed still more, until it was hot enough to cause nuclear fusion, the combining of atoms of hydrogen into helium, which releases energy.
THE BIRTH OF THE SUN, when fusion started, created a blast of radiation that blew the rest of the surrounding gas away from the center. Smaller bodies near the Sun were stripped of their gases, while the more distant ones remained massive enough to keep theirs. Thus the inner planets are small, hard, and have little or no atmosphere; the outer planets are large and mostly primordial gas. The gravitational pulls of these large proto-planets attracted more material, and moreover they were hotter. Thus each of the large outer planets formed extensive satellite systems — with inner satellites that are rocky and dense, and outer moons that are icy and less dense. In the region just beyond proto-Mars, the gravity of proto-Jupiter kept any large bodies from forming, leaving behind the asteroid belt. Comets are probably the remains of material that didn't get caught by the proto-planets.
THE PLANETS and many of their satellites later became hot enough so that they partially or totally melted, letting denser elements sink to form a core and lighter elements float up to form a crust. This heat came from several sources. One source was the heat created by their contraction from a giant sphere of gas to a smaller object. Another was heat from radioactive decay of elements such as uranium and thorium. And a third source was the energy from impacts of smaller bodies falling onto them, converting their energy of motion into heat.
Over eons, some satellites have escaped their planets, others have collided (some re-formed afterwards), and some minor objects have been captured by the major planets to become moons. Thus the solar system was born and evolved to what we see now.CHAPTER 4
Astronomers use the astronomicol unit, a.u., the average distance of approximately 93 million miles between the Earth and the Sun, as a unit of measure. It is almost impossible to show both the sizes of the planets and their orbits to the same scale. If the Sun really were the size shown in the illustration here, the Earth to the same scale would be 4 inches away, and Pluto would be 14 feet away.
Most planet orbits have an inclination within a few degrees of Earth's orbit, called the ecliptic. All the planets orbit in the same direction: Counterclockwise as seen from far above the North Pole of Earth. Planetary orbits are almost circles, except for Pluto. A number describing the amount an orbit differs from a perfect circle is called eccentricity.
The planets, with the exception of Venus and Uranus, all spin on their axes in the same counterclockwise direction. Most of the satellites of the planets revolve in the same way.
Excerpted from Planets by Mark R. Chartrand, Ron Miller. Copyright © 1990 St. Martin's Press. Excerpted by permission of St. Martin's Press.
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