Chapter One
Earth's Nearest Neighbors
Astronomy and discoverers
Why does the Sun rise each day and disappear each evening? For much of human history, people believed that the Sun revolved around the Earth. They believed, also, that the Moon, the stars, and the planets also revolve around the Earth. A universe in which a stationary Earth sits at the center and everything else revolves around it is called a geocentric (-geo "Earth"; -centric "centered") universe. Today we are all taught from childhood that the Moon revolves around the Earth and the Earth revolves around the Sun.
The Greek civilization of antiquity flowered for 800 years from about 650 B.C. to A.D. 150 and spawned many famous philosophers. The most influential of the philosophers, Aristotle (384-322 B.C.), espoused a geocentric universe. He pictured the Sun, the Moon, and the five visible planets as being suspended on concentric, hollow spheres that rotate about an imaginary axis extending outward from the two poles of the Earth, with the stars on the outermost sphere. The star sphere had to be outermost because star positions were fixed relative to each other (so they thought), but, day by day, the Sun, Moon, and planets could be seen to move in front of the stars. Beyond the star sphere, and invisible to humans, was the realm of the gods.
A few people in Aristotle's time realized that a geocentric universe is not the only way to explain what is seen. The apparent movement of the star sphere across the sky could also be explained if the stars were fixed and the Earth rotated on its axis once every day. Similarly, the fact that there are seasons could be explained if the Earth revolved in an orbit annually around the Sun. One Greek philosopher in particular, Aristarchus (312-230 B.C.), favored a Sun-centered, or heliocentric (-helio "Sun"; -centric "centered"), system. Aristarchus used two of the branches of mathematics discovered by the Greeks, geometry and trigonometry, to determine the relative sizes of the Sun, Moon, and Earth. His measurements indicated a huge Sun, a small Earth, and a tiny Moon. It did not make sense to Aristarchus that a huge Sun should rotate around a small Earth. He was unable to convince people that his heliocentric hypothesis might be correct, and so the concept of a geocentric universe continued to be widely accepted until the middle of the sixteenth century, more than fifteen hundred years after the death of Aristotle. In fact, belief in a geocentric universe came to be accepted in most Christian and Catholic religions as a divine fact.
The most difficult question that a geocentric universe has to answer concerns the motions of the planets. The five visible planets—Mercury, Venus, Mars, Jupiter, and Saturn—look like stars, but they are stars with a difference because they seem to wander. Indeed, the vary name planets comes from planetai, Greek for wanderers. The paths of the wanderers, measured against the background of fixed stars, are a little odd. They move a bit farther east each evening, but periodically they slow down and briefly reverse direction before once again resuming their eastward motion (they look as though they reverse direction, but in fact they do not). (In fact, the planet's are at their farthest points in orbit and just simply swing around, finishing the orbit.) The temporary reversal of direction is known as retrograde motion.
When Nicolaus Copernicus (1473-1543 A.D.) was a student at the University of Bologna, Italy, in the 1490s, he read a Latin translation of the Almagest (published by Claudius Ptolemy about A.D. 150) and decided that the heliocentric system of Aristarchus was more attractive than the geocentric system of Aristotle and Ptolemy. Copernicus recognized that the retrograde motions of planets could be explained in a heliocentric system as a result of differences between the time it takes the Earth to orbit the Sun and the time it takes for any other planet to orbit the Sun. Furthermore, Copernicus suggested that because Mars has a larger retrograde motion than Jupiter or Saturn, it must be the closest of the three planets to the Earth, while Saturn, with the smallest motion, must be the most distant. He was right.
Copernicus sowed the seeds that finally separated science from religion and spawned the continuing scientific revolution that has shaped the society in which we live today. It is especially noteworthy that modern science has its roots in astronomical studies and in particular in studies of the motions of the Earth and planets.
When the ideas of Copernicus were published in 1543, they convinced most, but not all. In 1572, with funds from King Frederick II of Denmark, Tycho Brahe (1546-1601) built the first modern astronomical observatory on the Danish island of Hven, and he named it Uraniborg, or Castle of the Heavens. Telescopes had not been invented yet so all measurements were taken with the naked eye. Tycho's measurements of planetary positions, made in part to prove Copernicus wrong, were by far the most accurate made up to that time.
Frederick II died in 1588 and Tycho, disliked by Frederick's successor, fell from achieving substantial funds. In 1597 Tycho moved to Prague and while there he hired a young German mathematician, Johannes Kepler (1571-1630), to do his astronomical calculations. After Tycho died, Kepler continued to have access to his numerous measurements of planetary positions. Kepler, unlike Tycho, thought Copernicus must be right, and he also gave a great deal of thought to a problem Copernicus had not gone over—what is the nature of the force that keeps the planets moving around the Sun? Why do they revolve in orbits instead of moving in straight lines out into space?
Kepler suggested that a mysterious force must reside in the Sun and have a greater effect on closer objects. Today we know that the force is gravity, but in Kepler's day gravity was an unknown concept. Kepler thought that it was magnetism at first that might be that force. Kepler could not make planetary positions calculated from circular orbits agree with Tycho's measurements. Eventually, Kepler tried calculating the position of Mars based on an elliptical orbit.
Kepler discovered three laws that describe planetary motions:
1. The law of ellipses. The orbit of each planet is an ellipse with the Sun at one focus.
2. The law of equal areas. A line drawn from a planet to the Sun sweeps out equal areas in equal times. One consequence of the law of equal areas is that orbital speeds are not uniform but instead change in regular ways. A planet (or any object for that matter) moves rapidly when close to the Sun and slowly when far away from the Sun.
3. The law of orbital harmony. For any planet, the square of the orbital in years is proportional to the cube of the planet's average distance from the Sun. The period is the time a planet takes to make one complete revolution around the Sun. (For instance, the period of the Earth is 365.24219 days ;x.) This third law, which describes what Kepler considered to be a cosmic harmony among the planets, can be expressed as p²=kd³ where p is the period, k is a constant, and d is the average distance between the planet and the Sun.
The force that Kepler had misidentified as magnetism was actually gravity. Isaac Newton (1642-1727), through his own insight, discovered one of the universal laws of nature, the law of gravitation, which states that every body in the universe attracts every other body.
On to the real astronomical information you need to know...
The solar system consists of the Sun, nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto), a vast number of small rocky bodies called asteroids (or minor planets), millions of comets (such as P/Encke which passes by Earth once every 3.3 years, and P/Halley "Halley's Comet" which passes by Earth once every 76 years), innumerable small fragments of rock and dust called meteors or meteoroids (known popularly as "shooting stars"), and 61 known moons. All of the objects in the solar system move through space in smooth, regular orbits, held in place by gravitational forces. The planets, asteroids, comets, and meteoroids orbit the Sun, while the moons orbit the planets.
Go to the links below and be sure to read those before you move on.
¬==];:::::::›The Birth of the Solar
System
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Planetary Evolution
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The Terrestrial Planets
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The Jovian Planets
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Summary of Chapter One
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Chapter One Test
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On To Chapter Two
Back
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